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Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo The asaphid trilobite fauna: Its rise and fall in Baltica Helje Pärnaste a,⁎, Jan Bergström b,1 a Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia b Department of Palaeozoology, Swedish Museum of Natural History, P.O. Box 5007, SE-104 05 Stockholm, Sweden a r t i c l e i n f o a b s t r a c t Article history: Received 11 February 2013 Received in revised form 4 June 2013 Accepted 5 June 2013 Available online 14 June 2013 Keywords: Ordovician Baltica Trilobites Functional morphology Biodiversity Sea-level The Baltica terrane is well known as the Asaphus Province of the Ordovician World. Being situated in inter-mediate latitudes and surrounded by fairly wide oceans, Baltica had a benthic fauna that developed in a rel-atively endemic direction. The most common trilobites were the asaphids. The present study discusses how they appeared, evolved in various environments around Baltoscandia and finally disappeared. The total range of the asaphids based on data from 381 species is estimated as extending from the Furongian, latest Cambrian to the end Ordovician. The remnant Cambrian Olenid Fauna, which was adapted to black bituminous shale facies, survived into the Tremadocian. This Cambrian fauna includes the first asaphids (Promegalaspides, Niobella, ?Eoasaphus). With a shift to lighter-coloured and carbonatic sediments in the middle Tremadocian, this fauna was replaced by the immigrating Ceratopyge Fauna. The latter contains the first five widely distrib-uted asaphid genera Promegalaspides, Niobe, Niobella, Niobina, and Asaphellus. As carbonate sedimentation became more widespread in many areas from the Floian onwards, the asaphids increased in number and formed a stabilised Asaphid Fauna during the Dapingian. In Baltoscandia a gradual development of endemism related to facies belts is observed during the Darriwilian. Records of the mud-related niobines, Megistaspis and Ptychopyge faunas, together with some other benthic macrofaunas are not found beyond the boundary of the Kunda/Aseri regional stages (middle Darriwilian). Their disappearance may be due to a catastrophic event. In contrast Asaphus s.l. survived this interval. Drift of Baltica towards lower latitudes and fluctuations in sea-level influenced the Asaphid Fauna leading to its gradual collapse by late Sandbian time. Influx of the islotelines during the early Katian gave way to a set of illaenimorph species found together with corals that are related to a reefal environment. This morph disappears prior to the Hirnantian glaciation. © 2013 Elsevier B.V. All rights reserved. 1. Introduction In 1966, H. B. Whittington gave a superbly illustrated overview on the phylogeny and distribution of the Ordovician trilobites in his presidential address to the Paleontological Society (Whittington, 1966). The phylogeny of families was drawn following their parallel development in different regions, pointing out geographic and climatic influence. He demonstrated that the Tremadoc (then being included within the Late Cambrian) trilobites are widely distributed worldwide, while successive faunas are endemic or with limited distribution. He pointed out that the greatest change ever in trilobite faunas occurred shortly before or during Tremadoc time when the majority of ancient trilobite families vanished and a small relatively short-lived group together with the earliestmembers of the dominant Ordovician families appeared. These include six geographically recognised faunas defined for the Tremadoc, followed by several more including an Asaphid Fauna during post-Tremadoc times. The Asaphid Fauna is characteristic of the Early Ordovician of Baltoscandia. This term was already used earlier by Whittington (1963) to distinguish the Baltic province from that of North America and Siberia. In these regions the bathyurids and asaphids co-occur along with totally different elements. The Baltoscandian elements were mentioned as those figured in works by Tjernvik (1956), Jaanusson (1953a,b, 1956) and Henningsmoen (1960) but a comprehensive list was lacking. In our study we review all repre-sentatives of the family Asaphidae (see for systematics Fortey and Chatterton, 1988; Jell and Adrain, 2003). The trilobite records from the Ural Mountains and the islands to the north, Vaygach and Novaya Zemlya, which form a further continuation of the chain in the north reflect some similarity with Baltoscandia, includ-ing occurrence of some common asaphids (Balashova, 1961, 1967; Burskiy, 1970; Ancygin, 2001; for more detail see Bergström et al., in press; Pärnaste and Bergström, in press). Following this appearance the entire Baltica terrane is considered to represent the Asaphid province in the Arenig and Llanvirn (Whittington and Hughes, 1972) or in modern terms in the Floian, Dapingian and Darriwilian ages (see for global stratig-raphy e.g. Bergström et al., 2009). A southern mid-latitudeMegistaspidine Realm to include Baltica was introduced by Adrain and others (Adrain et al., 2004) in their newglobal analysis for theOrdovician radiation of tri-lobites, because specifically the megistaspidine asaphids are endemic to this region (Cocks and Fortey, 1982, 1990). A family Megistaspididae ⁎ Corresponding author. Tel.: +372 620 30 10; fax: +372 620 30 11. E-mail address: helje@gi.ee (H. Pärnaste). 1 Professor Emeritus Jan Bergström passed away in November 2012. 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.06.007
H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 65 was erected by Balashova (1976) covering the previously established subfamily Isotelinae in sense of Jaanusson (in Moore, 1959) to em-brace four subfamilies Megistaspidinae, Asaphellinae, Hoekaspinae, and Thysanopyginae using diagnostic characters different fromthose cho-sen by Jaanusson. The systematics of this group is still lacking so we continue to use the term Asaphid Fauna as first introduced, especially since the subfamily Megistaspidinae in sense of Balashova (1976) includ-ed the following genera (Megistaspis, Paramegistaspis, Megistaspidella, Ogygitoides, Stenorhachis, Plesiomegalaspis, Paramegalaspis, Ekeraspis, Dolerasaphus, Kayseraspis, Nerudaspis, Rhinoferus, Hunnebergia) of which some are also known outside of Baltica, i.e. are not endemic to Baltica. The aim of this contribution is to reveal the dynamics of distribution and development of different groups of asaphids,which prevailed during the first half of the Ordovician in Baltica. For thatwe (1) compare the tri-lobite succession in 13 regions across Baltoscandia throughout 15 inter-vals from the beginning of the Ordovician to the end of the Darriwilian, when the asaphidswere flourishing; (2) review the trilobite occurrences in the Ural Mountains together with Vaygach and Novaya Zemlya—the islands in the north–west (in present-day terms) on the other side of Baltica; (3) detect the distribution of asaphid subfamilies in different regions and time intervals in Baltoscandia; (4) analyse the major events in the diversification of the asaphids until the demise of the family at the end of Ordovician. 2. Geological setting 2.1. Baltica Plate—boundaries and palaeogeographical position Baltica formed a discrete continental block before the Caledonian collisions (Fig. 1A; see also, e.g., Torsvik, 1998; Cocks and Torsvik, 2002, 2005). A series of Caledonian nappes, the lowest of which con-tains fossils of Baltic and Avalonian origin and the uppermost those of Laurentian origin (Neuman and Bruton, 1974; Bruton and Harper, 1981, 1988; Bruton et al., 1989) have been transported eastwards from off-shore Norway (Ebbestad, 1999, fig. 2; Harper et al., 2009; Lamminen et al., 2011, fig. 11), and the evidence for litho- and biofacies in these areas is largely destroyed. Magmatic rocks included in some nappes bear evidence of the emergence of island arcs off the coast in Fig. 1. A. Palaeogeographical reconstruction showing change in position of Baltica from the beginning of the Tremadocian to the beginning of the Sandbian (Map generated using the T.H. Torsvik's GIS-oriented software from2009, BugPlates: linking biogeography and palaeogeography). Grey circle marks the ocean circulation in cool sea surface temperatures suggested by current circulation inmodern oceans, and thewide line indicates the winter–summer amplitude. B. The confacies (bio-lithofacies) belts of Baltoscandia modified from Jaanusson (1982a).
66 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 the late Arenig (Bruton and Bockelie, 1982; Harper et al., 2008; Andersen et al., 2012). At the Ural border, part of the original continental plate is thought to have been lost through subduction under the Magnitogorsk volcanic island arc and the Siberian plate, thus indicating an example of continental subduction (Matte, 1998; Brown, 2009; Puchkov, 2009). In the south-west, marginal parts of the Baltica Plate appear to have been lost through major strike-slip fault movements along the Tornquist–Teisseyre zone, which separates areas with contrasting thick-nesses of the crust and different Ordovician faunas. In the Ordovician, long before this happened, the Uralian borderwas being subjected to ex-tensional strain associated with volcanism. Baltica had a long journey before it arrived at its present position, with its centre around 55° North. At the beginning of the Tremadoc (earliest Ordovician) the centre was around 55° South (Fig. 1A) while at the end of the Darriwilian (Fig. 2; part of the Middle Ordovician), the position was around 40° South (e.g., Cocks and Torsvik, 2002, 2005, fig. 6). The Ordovician palaeogeographic maps of the western part of the Baltica Plate reveal a series of facies belts (Jaanusson, 1976, 1982a; Pärnaste et al., 2013). The most offshore, and most deepwater, belt is known as the Scanian Confacies Belt (Fig. 1B). It widens in the west, extending as an embayment towards the Oslo Region (Bruton et al., 2010), which has been regarded to represent the next shore-ward belt. The next inward belt is the Central Baltoscandian Confacies Belt. In Ölandian times an orogenic foredeep started to develop between this belt and the Caledonian Orogenic Belt. The Central Baltoscandian Confacies Belt extends into the East Baltic area as the Livonian Tongue. The most shoreward zone consists of the North Estonian Belt and its counterpart to the south, the Lithuanian Confacies Belt. A somewhat deeper channel in between these two appears to extend to the centre of Baltica as the Moscow Basin. Part of the succession in this basin is rich in graptolites (Dmitrovskaya, 1989). In some levels there is also an impoverished Central Baltoscandian type trilobite fauna (Bergström et al., in press). 2.2. Sedimentation and stratigraphical framework A general characteristic of the Ordovician deposits in the east Baltic is that sedimentation was extremely slow—in the Central Scandinavian Confacies Belt the thickness of the entire Ordovician is only some 75–150 m in general, whereas it reaches some 200 m in the Scanian Confacies Belt (Jaanusson, 1982a). In reality the sedimentation rate must have been evenmore remarkable. The Kundan Orthoceratite lime-stone has some 5 cm thick beds separated by leaf-thin clayey lamina. The beds have shells in all possible orientations, indicating very rapid sedimentation (Bohlin, 1949, fig. 7). The most impressive examples of trilobites embedded at an angle to bedding are fromthe North Estonian Confacies Belt (Dronov, 2005a; Pärnaste, 2006, fig. 4)where sedimenta-tion of a single bed maywell have taken place in a daywith the ensuing bed being desposited some50,000 years later (Bohlin, 1949; Lindström, 1963, 1979; Dronov, 2005a). Someminor erosion is probably part of the picture. The Oslo Region forms an exception. Notable variations in facies and thickness, the latter reaching up to some 1000 m (Bjørlykke, 1974; Bruton and Owen, 1982;Owen et al., 1990; Bruton et al., 2010), indicat-ing exceptional relief conditions and perhaps synsedimentary tectonic movements. Thus the development suggests that the Oslo Region should be interpreted as a local mixed zone rather than a stable sedi-mentary confacies belt (Braithwaite et al., 1995). A general, westward shallowing, within the Oslo Region (Størmer, 1967; Bockelie, 1978; Bruton et al., 2010; Hansen et al., 2011) points in the same direction. For the present study, the Norwegian trilobite data is treated together as representing one belt, however, that of the allochthonous belts of non-Baltic origin are left out. Aluoja Valaste Hunderum Langevoja Vääna Saka Kukruse Uhaku Lasnamägi Kunda Volkhov Billingen Hunneberg Varangu Pakerort CΙΙ CΙc CΙb γ β α γ β α γ β α BΙΙΙ BΙΙ BΙ AΙΙΙ AΙΙ Sa1 Dw2 Dw1 Dp3 Dp2 Dp1 Fl3 Fl2 Fl1 Tr3 Tr2 Tr1 Nemagraptus gracilis Hustedograptus teretiusculus Amorphognathus tvaerensis Pygodus anserinus Pygodus serra Pseudoamplexograptus distichus Eoplacognathus pseudoplanus Yangtzeplacognathus crassus Lenodus variabilis Lenodus antivariabilis Baltoniodus norrlandicus Paroistodus originalis Baltoniodus navis Baltoniodus triangularis Oepikodus evae Prioniodus elegans Paroistodus proteus Paltodus deltifer Cordylodus spp. Nicholsonograptus fasciculatus Holmograptus spinosus Holmograptus lentus ?Corymbograptus retroflexus Undulograptus sinicus Arienigraptus zhejiangensis A.dumosus / P.manubriatus Isograptus sp.2 / M.schmalenseei Isograptus victoriae Isograptus lunatus Baltograptus minutus Baltograptus sp. cf. B.deflexus Baltograptus vacillans Cymatograptus protobalticus Tertragraptus phyllograptoides Hunnegraptus copiosus Araneograptus murrayi Kiaerograptus supremus Adelograptus tenellus Rhabdinopora spp. Baltoscandian Regional Stages Substages Global Stages Global Series Stage slices Brittish Series Trilobite Zones and Beds Conodont Zones Graptolite Zones Asaphus sulevi / Megistaspis gigas Megistaspis obtusicauda Asaphus vicarius Asaphus raniceps Asaphus expansus Asaphus lepidurus / Megistaspis limbata Asaphus broeggeri / Megistaspis simon Megistaspis polyphemus Megistaspis estonica Megalaspides dalecarlicus Megistaspis aff. estonica Megistaspis planilimbata Megistaspis armata Ceratopyge acicularis Shumardia pusilla Boeckaspis spp. Bed Tremadoc Arenig Llanvirn Öland Series Iru Subseries Ontika Subseries Kiaerograptus kiaeri CΙa Xenasaphus spp. Bed Asaphus ornatus / A. bottnicus Bed Asaphus kowalewskii Asaphus intermedius Asaphus cornutus Asaphus latisegmentatus Viru Series Aseri Eoplacognathus suecicus Pterograptus elegans Dw3 Lower Ordovician Middle Ordovician Tremadocian Floian Dapingian Darriwilian Fig. 2. Correlation chart for the lower half of the Ordovician in Baltoscandia. The correlation of biozones compiled from (Bergström et al., 2002; Pärnaste, 2006; Bergström and Löfgren, 2009; Hansen et al., 2011; Maletz and Ahlberg, 2011; Pärnaste and Viira, 2012; Pärnaste et al., 2013), following the revised global standard (Bergström et al., 2009). The graptolites in the Lower Tremadoc (Tr1) represent ‘superzones’; for a finer zonation, see Egenhoff et al. (2010).
H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 67 A stratigraphic gap locally covering much of the lower and middle Tremadoc succession in parts of Baltoscandia (Fig. 3) has been interpreted as an indication of a regression in the earliest Ordovician, with limited extension of the sea (e.g. Erdtmann, 1982). However, there is pronounced variation in the development of the Alum Shale Formation, indicating local erosion or non-deposition rather than uplift. An exception may be the transition from the lower Tremadoc Alum Shale to the middle Tremadoc Bjørkåsholmen Formation (Ceratopyge Limestone), which seems to be complete only in the Oslo Region (Jaanusson, 1982a). Terrigenous clastic sediments are found in this level in Dalarna (Jaanusson, 1982b). The Alum Shale is black, fissile and bituminous with scattered limestone concretions. The middle Cambrian to lowest Ordovician Alum Shale has a mineral composition of 50–65% quartz, 5–15% feldspar and 10–15% muscovite (Pedersen, 1989, p. 156). The quartz is very fine-grained. The muscovite may have a mixed detrital and diagenetic origin. The shale owes its black col-our to organic material. In Scania (southernmost Sweden) it has been strongly heated and occurs as kerogen with a content of TOC (Total Organic Carbon) that averages 9% (Pedersen, 1989, fig. 11). It contains remains of unicellular and larger algae. In the Central Scandinavian Confacies Belt the organic content is high enough to produce oil. Sedimentation was slow and the composition and the preservation of algal remains indicate a dysoxic–anoxic depositional environment. The evidence for burrowing activity is poor. There is some indication that trilobites come and go abruptly depending on environmental changes such as anoxia (summarised by Pedersen, 1989; see also Clarkson et al., 1998; Eriksson and Terfelt, 2007). The Alum Shale is succeeded by a thin succession of black to grey shale and limestone (the former Ceratopyge Shale and Limestone) now known as the Bjørkåsholmen Formation (Owen et al., 1990; Ebbestad, 1999) except in Öland (Fig. 3; Frisk and Ebbestad, 2008). The limestone is locally rich in glauconite. In the North Estonian Confacies Belt, the lower Tremadocian (AII; Fig. 3) consists of sandstone and alum shale of the Kallavere and Türisalu Formations, whereas the middle Tremadocian Scanian Belt Central Baltoscandian Belt North Estonian Belt Lanna limestone Persnäs lst Källa lst Folkeslunda limestone sandstones limestones Seby limestone Skärlöv limestone D Gillberga Fm Horns Udde Fm Bruddesta Fm Bruddesta Fm Köpingsklint Fm Djupvik Fm glauconite hiatus oolites Veltsy Fm Valim Fm Porogi Fm Doboviki Fm Simonkovo Sinjavino Kandle Fm / Kandle Formations Sillaoru Formation Toila Formation Toila Formation Leetse Formation Pakri Fm Varangu Formation v v v a specific discontinuity surface with deep borings siltstones shales v v v v v v v v v v v v v Trilobite Zones and Beds NW Estonia Ingria NE Estonia Scania Oslo Region Väster-götland Öster-götland Närke Dalarna Öland Gävle Region Sularp Formation Almelund Shale Vollen Fm Dalby limestone Huk Formation Komstad Limestone ? Tøyen Shale Formation Hagastrand Member Ryd Formation Bjørkåsholmen Formation Incipiens lst Alum Shale Formation Alum Shale Formation Ceratopyge Shale Dictyonema Shale C Latorp limestone Orthoceratite limestone lithology Türisalu Formation Kallavere Formation A. sulevi/ /M. gigas M. obtusicauda A. vicarius A. raniceps A. expansus A. lepidurus/ /≈M. limbata A. broeggeri/ /M. simon M. polyphemus M. estonica M. dalecarlicus M. aff. estonica M. planilimbata M. armata C. acicularis S. (C.) pusilla Boeckaspis spp. CΙc CΙb BΙΙΙγ BΙΙΙβ BΙΙΙα BΙΙγ BΙΙβ BΙΙα BΙγ BΙβ BΙα2 BΙα1 AΙΙ Holen limestone Viivikonna Fm Kõrgekallas Fm Väo Fm CΙa Obukhovo / Loobu Fms CΙΙ Xenasaphus devexus Bed Asaphus ornatus Bed A. kowalewskii A. intermedius A. cornutus A. latisegmentatus Dalby limestone Furudal limestone Segerstad limestone Gullhögen Fm Skövde lst Våmb limestone Elnes Formation Killeröd Fm Tjällsten lst AΙΙΙb AΙΙΙa Fig. 3. Distribution of main lithologies in Baltoscandia, with generally deeper facies to the left, shallower to the right. Many small breaks in the sequence are not indicated on the diagram except for the lower Tremadocian level. This compilation resulted from numerous papers describing the sections with trilobites of the Öland Series (e.g. Nielsen, 1995; Pärnaste et al., 2013). Added are the data of the Viru Series (Jaanusson, 1982a; Bergström et al., 2002; Pålsson et al., 2002; Ivantsov, 2003; Dronov, 2005b; Hansen, 2009; Pärnaste and Popp, 2011).
68 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 (AIII) consists of shale and glauconitic silt- or sandstone of the Varangu Formation. The sedimentary rocks overlying the Alum Shale and the sed-iments with the Ceratopyge Fauna, are shales, mudstones and limestones with shelly faunas present in various confacies belts (Jaanusson, 1976, 1982a), whilst most offshore, in the Scanian Confacies Belt, the latest Tremadocian to the latest Darriwilian Tøyen and Almelund shales are rich in graptolites. The middle belt is the Central Baltoscandian Confacies Belt. The dominant facies is a bedded limestone, the middle part of which is known as the Orthoceratite Limestone. The lower part, up to the base of the Asaphus expansus Zone (Figs. 2, 3) is typically calcilutitic, without observable grains, whereas the upper Kundan part is a fine-grained calcarenitic indicative of a higher energy depositional environment (Jaanusson, 1982b, p. 19; Nielsen, 1995). The shift is abrupt. Authigenic silicateminerals, where present in lower strata, is invariably glauconite with shifts to chamosite only in intervals where the rock is lutitic. This is the pattern all over Baltoscandia (Jaanusson, 1982b, p. 19). In the Central Baltoscandian Belt the Kundan limestone turns red towards the top but is grey as it extends into southern Norway as the Huk Formation and further as a tongue, the Komstad Limestone, into the Scanian Confacies Belt. In southern Norway the Kundan portion has limestone with shale interbeds. The development in the wide western belt was influenced by Caledonian tectonics and is not known in detail because of metamor-phism. Turbidites in allochthonous lower nappes in Jämtland (the Caledonian Fore-deep Confacies Belt; Fig. 1B) become thicker west-wards in the Volkhovian(?) and Kundan (Jaanusson and Karis, 1982) indicating rising land further west. In Hardangervidda in southwest Norway, the source of quartzitic sediments is thought to be in a ‘Telemark Land’ to the south, whereas the sea became deeper in a north–west di-rection (Rasmussen et al., 2011). A limestone (Bjørnaskalle Formation) in the lower Darriwilian contains deformed trilobites, brachiopods and conodonts that confirm the Baltoscandian affinity (Bruton et al., 1989). In the Iapetus Ocean further west there were island arcs of which remains are now preserved in strongly metamorphosed lower nappe now considered part of a microcontinent which was detached from the edge of the Baltic platform and then later obducted onto it (Harper et al., 2008; Rasmussen et al., 2011, figs. 1, 4; Andersen et al., 2012). 3. Material and methods The knowledge of the Ordovician trilobite faunas of Baltoscandia is notably uneven. Whereas some trilobite faunas and particular trilobite groups have been studied in recent years, others have not been studied for more than a century, if at all. The last overview of all Swedish faunas was by Angelin (1854). East Baltic faunas were reviewed by Schmidt (e.g., 1881, 1885, 1901, 1904, 1906, 1907), and their taxonomic revi-sions over the years were listed by Bruton et al. (1997). More recent reviews of the Lower Ordovician asaphid faunas include contributions by Tjernvik (1956), Henningsmoen (1960), Balashova (1976), Nielsen (1995), Ebbestad (1999), Hoel (1999), Ivantsov (2003, 2004), Pärnaste (2006), Hansen (2009), and Stein and Bergström (2010). A complete list of the regional Ölandian Series (Tremadocian to mid Darriwilian) trilobites from the whole of Baltoscandia is producedwith some revision of systematics as well as the vertical and horizontal distribution. In all, the occurrences of over 400 species (of which a third are asaphids) belonging to about 125 genera over thirteen regions are listed from 15 trilobite biozones (Pärnaste et al., 2013). Herein the data set is extended to cover the following zones to the end of Ordovician. In most cases the data is presented by separate biozones with a few exceptions concerning the Kunda Regional Stage to leave better option for comparison with the other faunal groups. The Kundan stage has long been divided into the Asaphus expansus, A. ‘raniceps’, Megistaspis obtusicauda and M. gigas zones. The two latter are very poor in trilobite species and the zonal species are rather unique to the Central Scandinavian Belt,which means that no detailed correlation is possible. Asaphus ‘raniceps’ was recently revised (Stein and Bergström, 2010). It was found that the true A. raniceps is a good marker of the base of the zone, but is replaced by A. vicarius for much of the zonal range. We have combined here the A. raniceps and A. vicarius zones (Fig. 2) to fit with the Valaste Regional Substage (BIIIß). The uppermost, Aluoja Sub-stage (BIIIγ) is divided into several trilobite zones following the evolu-tionary appearance of the asaphines as is defined by Ivantsov (2003) in the St. Petersburg region, north-eastern Russia. This region in the vicinities of the Russian part of the Baltic-Ladoga Glint is called Ingria, or Ingermanland that was designated as the province of St. Petersburg at Schmidt's time (e.g. Schmidt, 1881), and the name is used herein to denote this district briefly. The preservation of beds there indicates a higher rate of sedimentation than in northern Estonia (cf. e.g. Ivantsov, 2003, fig. 2; Rasmussen et al., 2009). Faunal zones have yet to be recognised for the area and we refer to the older zonational scheme as used in Sweden and Norway for this part of the succession and the succeeding Aseri Regional Stage (e.g. Jaanusson, 1953a, 1982a; Wandås, 1984). No trilobite zones are defined above the Aseri stage, where the less formal units (beds) are named. Thus the Asaphus ornatus and A. bottnicus bed correlates with the Lasnamägi Regional Stage (CIb), and a bed with Xenasaphus with the Uhaku stage (CIc), forming the upper part of the Darriwilian Stage (Fig. 2; Ivantsov, 2003). By Sandbian times the diversity of the asaphids had decreased drastically and fromthis peri-od regional stage names are used herein and the resolution of our data is restricted to each individual stage. In all 381 asaphid species ranging from the Latest Cambrian (Furongian) to the end Ordovician are included in our data sets. Regional data sets are mainly literature-based and vary from ran-domly collected localities in older literature to detailed bed by bed col-lections as listed by various authors (e.g. Nielsen, 1995; Ebbestad, 1999; Hansen, 2009). Thus, the absence of records in some regions may be either because collections are lacking or that the pertinent sediments are not exposed or are absent. Nevertheless, where present, regional distribution of different groups of asaphids allows recognition of the particular sedimentary environments these groups favoured. The Ural border of Baltica is less known even if brief comparisons of lists of trilobite taxa recorded from the areas around Baltica suggest some similarity between east and west (e.g. Cocks and Fortey, 1998). The records of trilobites of the Ural Mountains date back to 1933 and 1948 when A.F. Lesnikova and V.N. Weber described the collections of N.E. Chernysheva and others. We have reviewed all the taxa available in the taxonomic works (e.g. Weber, 1948; Balashova, 1961; Burskiy, 1970; Ancygin, 1973, 1977, 1978, 1991, 2001), with some revision based on available photographs and descriptions (Bergström et al., in press; Pärnaste and Bergström, in press). In all approximately 220 species belonging to 105 genera ranging from the Tremadocian to the Darriwilian are listed in an extended Ural area together with Pay-Khoy and Vaygach in the north, extending south to theMugodzhars in Aktyuby Oblast, north-western Kazakhstan. In this study we follow the stratigraphical data of Ancygin (2001). 4. Discussions and results 4.1. Systematics of asaphids common in Baltica The family Asaphidae encompasses asaphine trilobites with librigenae separated anteriorly by a median suture, with the posterior margin of the hypostoma varying from pointed to deeply notched, with the Panderian organs developed as notches or separate openings. The family Asaphidae has been classified in several ways into subfamilies (e.g. Jaanusson in Moore, 1959; Balashova, 1976; Fortey, 1980; Fortey and Chatterton, 1988).Wefollow herein Fortey's grouping into three sub-families— asaphines, isotelines and niobines. As the ogygiocaridines are closely related to the niobines and they both preferred deeper water en-vironments in this study we have treated their occurrence data together (Section 4.3.1). In our previous study on Baltoscandian trilobites of the Ölandian Series (Pärnaste et al., 2013) we revised the outdated names.
H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 69 There are a number of superfluous generic names, not the least in Russian literature, which do not have any obvious phylogenetic meaning and blur relationships. We have therefore retained Megistaspis, Asaphus and Ptychopyge in a broader and supposedlymore ‘natural’ sense but separate Niobe and Niobella since both genera seemto define conservative lineages starting in the Cambrian. In Table 1, numerous diversification events can be traced via the number of species rather than genera. Below (Section 4.3) we discuss details of functional morphology in each subfamily and evaluate their distribution across the Baltoscandian palaeobasin. 4.2. First asaphids of the Baltica The earliest possible asaphid representative in Baltoscandia is a rare Eoasaphus superstes known from one incomplete specimen in the Furongian, Jiangshanian Stage of Sweden (Terfelt et al., 2011). In the Furongian uppermost stage (Stage 10), Promegalaspides (Promegalaspides) and Niobella are rare in Baltoscandia but are common globally. Two Baltoscandian species, Promegalaspides (P.) kinnekullensis and P. (P.) pelturae are present in Baltoscandia and in Siberia, where they occur just above the lower boundary of Stage 10 in the Khos-Nelege reference section, a potential candidate for the GSSP of Stage 10 (Lazarenko et al., 2011). This occurrence indicates a close link between the terranes of Baltoscandia and Siberia at the end of the Cambrian. Promegalaspides later becomes more common in the middle Tremadocian of Baltoscandia (Figs. 2, 3), with the occurrence of Promegalaspides (Borogothus) intactus and P. (B.) stenorachis (Ebbestad, 1999, Fig. 15) forms where the cephalon and pygidium become more elongated thus resembling isotelines rather than niobines (cf. Tjernvik, 1956; Hoel, 1999). Jaanusson (in Moore, 1959) believed the latter to be the case and erected the new sub-family Promegalaspidinae. Different from all the other asaphids Promegalaspides possesses the posteriormost thoracic pleura ending in a long spine. The palaeogeographic origin of this genus remains unclear as it dates back to the beginning of the Cambrian Age 10 in two terranes. In addition to occurrences in Baltoscandia and Siberia it is also known fromthe early Tremadocian Kidryas Age of the Uralian border of Baltica (Aktyuby Oblast, north-western Kazakhstan; Balashova, 1961; Ancygin, 2001). Promegalaspides is also recorded from the late Tremadocian and younger beds of the Montagne Noire region, Armorica (Thoral, 1935; Hoel, 1999), Siberia, and Gornaya Shoria. The other early Baltic asaphid, Niobella, is already cosmopolitan in the late Cambrian of Laurentia and around Gondwana: Avalonia, Armorica, Turkey, South China, and Australia (see for references, Pärnaste and Bergström, in press). The cos-mopolitan genus Asaphellus, is recorded from the middle Tremadocian Ceratopyge beds of the Oslo Region (Owen et al., 1990; Pärnaste et al., 2013), but as it has never been published estimation of this record is not possible. 4.3. Regional distribution of morphologically different groups In describing nileid trilobites, Fortey (1986, fig. 12) discussed several morphotypes interpreted as being adapted to different environments as shown by the patterns of terraced lines on the exoskeleton. Later Fortey and Owens (1999) discussed the shape of the exoskeleton and modifi-cations allowing hypotheses to be formulated on the life style and feed-ing habits of these creatures. Often the form of the hypostome, its connection to the dorsal exoskeleton and the space between the glabel-la and hypostome are used when suggesting their feeding strategies. Recent discoveries of various types of digestive system and its position in the trilobite body (Chatterton et al., 1994; Lerosey-Aubril et al., 2011; Fatka et al., 2013) question the direct relation between the size of the stomach and space between the glabella and hypostome. Asaphids in general have been considered to belong to the fast-moving and detriti-vore low-level epifauna, but they may have muchwider diversification. We test here preferences of the morphologically different Ordovician asaphids inhabiting various areas on the Baltoscandian shelf. In a broad sense the Baltoscandian asaphids can be subdivided into four major groups following the general morphology of their body-plan and characters diagnostic for assignment to different subfamilies (Fig. 4). 4.3.1. Niobines Niobines possess a relatively flat dorsal shield rounded, both anteri-orly and posteriorly, with a well-defined axial furrow over its shield Table 1 List of asaphid genera showing the number of taxa by trilobite zones and regional stages. AII AIIIa AIIIb BIα1 BIα2 BIβ BIγ BIIα BIIβ BIIγ BIIIα BIIIβ BIIIγ CIa CIb CIc CII CIII-DI DII DIII E FIa FIb-FIc Niobinae 1 5 4 7 6 2 3 7 8 6 3 4 5 6 7 1 Platypeltoides 1 1 Niobe 1 1 2 1 1 2 3 3 2 Niobella 3 1 3 5 2 3 5 5 2 Niobina 1 Lapidaria 1 1 Gog 1 1 1 1 Ogygiocaris 2 5 6 7 1 Isotelinae 2 4 6 13 4 8 7 21 13 18 16 1 1 5 Promegalaspides 2 1 1 Megalaspides 1 7 1 Hunnebergia 1 Megistaspis 2 4 6 3 8 7 21 13 17 15 Homalopyge 1 1 Isotelus 1 1 2 “Brachyaspis” 3 Asaphinae 2 2 8 9 15 22 19 19 30 14 16 10 10 4 0 1 0 3 Proasaphus 2 1 Onchometopus 1 1 1 Asaphus 1 1 11 12 10 22 6 7 6 7 3 1 2 Pseudasaphus 3 5 3 2 4 1 Ogmasaphus 2 2 1 3 2 2 Pseudomegalaspis 1 2 1 Ptychopyge 8 7 13 11 4 Pseudobasilicus 2 2 3 1 1 1 1 Opsimasaphus ? Number of species 0 1 7 8 13 21 8 19 23 44 41 40 39 35 20 23 11 10 4 0 2 1 7 Data in bold emphases a total number of the species included to the group of genera below.
70 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 (Fig. 4A). They have deep, wide, pleural furrows, and a slightly concave or convex moderate border area. The most prominent terrace ridges on the border area and hypostome appear aligned transversely to the axis. The terrace lines on the frontal part of the shield, including on the hypo-stome, are oriented with the steepest slope facing anteriorly while on the pygidium facing posteriorly (cf. Nielsen, 1995, Figs. 129, 131). If these were connected with sensory organs to detect current flow as suggested by Miller (1975), then their well-defined expression may indicate functioning in relatively quiet flow conditions. Alternatively, if acting as stabilisers in locomotion (Fortey, 1986), then the lines could well prevent back and forth sliding when crawling on the muddy bot-tom. They have the most prominent macula, a possible visual organ, ever seen on Baltoscandian trilobites. The eyes of Niobella are large but not high, tilted anteriorly and affording downward vision to see in front of the head and to the sides (cf. Ebbestad, 1999, Fig. 49A–C). Gog and Niobe have relatively smaller eyes (cf. Nielsen, 1995, Figs. 129C, 142A). In Ogygiocaris the eyes are horizontal and raised above the gla-bella on a small palpebral lobe so that the visual surface affords vision both laterally and dorsally (cf. eyes: Hansen, 2009, pl. 2, Fig. 11, pl. 3, Fig. 1; and macula: pl. 2, Fig. 9). The palaeoecological study by Ebbestad on abundance of various tri-lobites within the Tremadocian Bjørkasholmen Formation of the Oslo Region, shows that the spinous Ceratopyge (cf. Ebbestad, 1999, Fig. 56) is replaced by the robust Niobe and Niobella (cf. Ebbestad, 1999, Figs. 11–12). This replacement corresponds to a shift in abundance and a change from a compact limestone to thinner beds with interbedded shales suggesting slightly deeper water conditions. The detrended cor-respondence analysis of the Huk Formation samples places Niobella to the deeper water but Niobe to the shallower water habitat (Nielsen, 1995, Figs. 49, 51) but a log of the Slemmestad section shows Niobe in the shaley part of the section (Nielsen, 1995, Fig. 48). From a range chart of the occurrences of trilobites in the Lynna section, Ingria (Hansen and Nielsen, 2003, Fig. 3) it is evident that Niobella disappears when silty clay beds are replaced by carbonates in the upper part of the Asaphus lepidurus Zone. It would appear that the niobines seem to have preferred the muddier bottom or deeper water. 16 14 12 10 8 BIIIγ CIb Asaphus s.l. 12 6 10 4 2 6 4 2 8 A B 6 4 BIα1 BIβ BIIβ BIIIα BIIIγ CIb C D 6 4 2 8 2 BIIIα BIIIα BIIIγ CIb Isotelinae Group B Niobinae Ptychopyge s.l. AII AIII BIα2 BIγ BIIα BIIγ BIIIβ CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia AII BIα1 AIII BIβ BIα2 BIγ BIIβ BIIα BIIIα BIIγ BIIIγ BIIIβ CIb CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia AII BIα1 AIII BIβ BIα2 BIγ BIIβ BIIα BIIγ BIIIβ CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia AII BIα1 AIII BIβ BIα2 BIγ BIIβ BIIα BIIγ BIIIβ CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia Fig. 4. 3D graphs with distribution of various groups of asaphids over the basin from the lower Tremadocian to the upper Darriwilian (x-axis) ordered in following fields: (A) Niobinae; (B) Asaphus s.l., (C) Ptychopyge s.l., (D) Isotelinae Group B. Thirteen regions in y-axis follow geographic order similar to Fig. 3, reflecting more nearshore conditions apart from 0. Number of species increases in z-axis. The sketches next to the 3D graphs show general shape of the exoskeleton (in lateral view) and the hypostome (in ventral view).
H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 71 The distribution map of the species belonging to Niobella, Niobe, Niobina, Lapidaria, Gog, and Ogygiocaris (Fig. 4A) exhibits relatively homogenous dispersal over the region and times with a few low peaks. If one compares the distribution in terms of lithostratigraphy (Fig. 3) then niobines appear first in the regions where carbonate sed-imentation starts, such as the outer western shelf belts during the Tremadocian, extending to the first maximum in the Hunnebergian of the western and central belts. It is not known if they reached the eastern belt as here there are only high enegry sandstones lacking fossils with carbonate skeletons. The second peak is recognisable in the upper Dapingian, Megistaspis simon and M. limbata zones of the Komstad limestone Formation in the Scania and Oslo Region, followed by a shift into the eastern belt in the lower Darriwilian, Asaphus expansus and A. raniceps zoneswith a maximumin the latter. If niobines are the indicators of the deeper waters, then their appearance on the shallow shelf marks a sea-level rise during the early Darriwilian and a low stand before that. Alternatively, if niobines were adapted to be a mud-dependant inhabitant, their richness in the eastern belts indicates a sedimentary source for the mudwith iron-rich oolites. Sturesson et al. (2005) suggested these early Darriwilian oolite beds might be connected with an influx of volcanic debris. Taking all regions into consideration together shows richness peaks of niobines at the mid Floian Megalaspides dalecarlicus Zone, latest Dapingian Asaphus lepidurus / Megistaspis limbata Zone, and finally in late Darriwilian times (Table 1). The youngest peak corresponds to the arrival of the ogygiocarines in the Oslo Region in the Aseri to Uhaku interval during late Darriwilian times. They also reached Jämtland and the Livonian Tongue region (i.e. Latvia in Fig. 4A; Lt in Fig. 1B). They are absent in the shallow shelf of the North Estonian Belt. 4.3.2. Asaphines The asaphines (excluding Ptychopyge and closely allied genera) are a groupwith arched exoskeleton, possessing a rather short pygidium and cephalon; deep axial furrow; pygidiumwith narrowaxis and ill-defined pleural ribs and is slightly convex at the border area; cranidium with very limited frontal area ahead of glabella and a deep posterior border furrow (Fig. 4B). The exoskeleton possesses a well developed terrace line pattern on the pleural field, the strongest following the direction of the pleural ribs. The elevated and stalked eyes vary in size with the visual surface, allowing an all round vision in the horizontal plane (cf. Ivantsov, 2003, pl. 13). When less elevated, the palpebral lobe is rather strongly tilted, downwards anteriorly, allowing anterior vision (cf. Ivantsov, 2003, pl. 19, Figs. 2, 8a, pl. 21, Figs. 6, 12; Nielsen, 1995, Fig. 49A–C; Hansen, 2009, pl. 6, Fig. 2). Hypostoma possess a rather short middle body and a prominent forked posterior border. The anteri-or wings are designed for strenghtening the attachment to the inside of the glabella (cf. Ivantsov, 2003, text-Fig. 20, pl. 21, Fig. 3). The thickened inner surface of the forks is covered by a series of fine raised ridges which Ivantsov (2003, pl. 21, Figs. 5, 22) interpreted as being ‘adapted for grinding large food particles and thus being predators feeding on the animal organisms found in the upper layer of bottom sediment’. The same structures were interpreted by Fortey and Owens (1999) as being used for breaking down the body wall of prey before ingestion into the stomach, and thus the trilobites with such hypostomes are interpreted as predators (see also Hegna, 2010). Asaphus specimens bur-ied in tunnel systems ascribed to Thalassinoides, have been described from the Holen Limestone in Jämtland, Sweden (Cherns et al., 2006) suggesting that the trilobites lived on a soft seafloor (Gibb et al., 2010). The following asaphines are included in our distribution overview (Fig. 4B): Proasaphus, Asaphus, Asaphus (Onchometopus), Asaphus (Neoasaphus), Asaphus (Subasaphus), Ogmasaphus, Pseudomegalaspis, Plectasaphus, Xenasaphus, and Pseudasaphus including Volkhovites, Valdaites, Baltiites, Pseudoasaphoides, Dubovikites, Pseudoasaphinus, Leningradiites, Estoniites (Balashova, 1976; Ivantsov, 2003). A very rare Proasaphus, known only from cranidia and pygidia, doc-uments the first occurrence of this group in two regions (Närke and North Estonian Belt) in the mid Floian (Pärnaste, 2006; Pärnaste and Viira, 2012). By the Dapingian, an increase in the number of asaphines occurred over the entire region, but was most diversified in the North Estonian Belt. The first maximum rise marks the start of the Kunda Regional Stage, and the second and most prominent rise, the Aseri Stage. The latter iswell known for an extremely well preserved trilobite fauna in Vilpovitsy quarry, St. Petersburg region,where numerous com-plete specimens of very attractive forms occur. Among these is Asaphus kowalewskii with an extraordinary long palpebral lobe and stalked eye reaching well above the cephalon. Following the Aseri Stage, the num-ber of Asaphus s.l. species falls to more than a third and the number of all asaphines by half (Table 1). By Oandu times Baltica had moved to thewarm, temperate climate zone (Fig. 1A) and fromthen on asaphines were shortly to vanish. Most probably the phacopine trilobites with a similar body shape but much better vision abilities took over their niche on the shallow shelf of the Baltoscandian basin. It is of interest that during the Aseri Age, the height of eyes increased in certain asaphids which Balashova (1953) and later Ivantsov (2003) recognised for an evolutionary line containing Asaphus koltukovi koltukovi–A. k. tumidus–A. punctatus–A. intermedius–A. kowalewskii. Suggesting that this morphological trend was related to the gradual change of the environment towards an increase in the accumulation of mud on the seafloor itmight reflect a slow sea-level rise or local sub-sidence of the basin. Some of the above Ingrian species including A. punctatus and A. kowalewski have been found on the Pakri peninsula in north-western Estonia more than 500 km from Ingria. On the Estonian peninsula and nearby islands, the Aseri beds are extremely thin being no more than a few decimetres thick (Orviku, 1940) while Ivantsov (2003) records thicknesses of 6–7 m in Ingria. On Pakri the iron oolitic limestone of the Aserian Kandle Formation (Fig. 3) consists of several distinct beds each separated by discontinuity surfaces and yielding complete unrolled trilobites (at least one per square metre and laying at different angles to the bedding) a situation most unusual for the North Estonian near shore belt where disarticulated remains are most common. How can this be explained? If the elevation of the eyes was caused by a sediment-related trend, it could hardly be explained as happening in similar terms over such an extended area. Thus, it seems that there must have been some connection between north-western Estonia and Ingria and that the trilobites migrated be-tween these areas independent of basinal bottom conditions. The other option could be that they were carried there from time to time, perhaps as buried components within partly lithified mudmasses moved by events such as big storms (Lindström, 1979) or tsunamis following vol-canic activities (see for volcanic activities in e.g. Sturesson et al., 2005). The trilobites were supposedly enrolled mainly to protect their body from enemies. Thus, the bed with numerous complete unenrolled spec-imens points to a situation of sudden burial of slightly lithified sediment containing borrowed-in trilobites (see above Section 4.3.2). 4.3.3. Ptychopyge s.l. We have chosen to treat Ptychopyge s.l. separately from the other asaphines because of the characteristic hypostoma of the former, which has a relatively long middle body, ending in a forked posterior border, bearing exsagittally prominent terrace lines, and its relatively small, rounded shoulders thus bringing the hypostoma close to the dor-sal shield (cf. Balashova, 1964, pl. 9; Balashova, 1976, pl. 16). Conse-quently Ptychopyge s.l. has a relatively flat exoskeleton with a wide concave border of both cephalon and pygidium. The cephalon has a narrow glabella and small eyes close to each other. In our database for Ptychopyge s.l. we also include Pseudobasilicus and Pseudobasiliella, which share similar characters and are most probably descendant from Ptychopyge. Ptychopygids are especially diversified (cf. Balashova, 1964) in clay-ey limestones connected to the Volkhovian mudmound structures in Ingria (e.g. Dronov and Ivantsov, 1994; Tolmacheva et al., 2003) and Estonia (Pärnaste, unpublished data), but also within limestones of the
72 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 more off-shore facies of the Livonian Tongue in Latvia and the Central Baltoscandian Confacies Belt in Västergötland. The ptychopygids togeth-er with the other trilobites found in mudmounds around Ingria, extend to the off-shore western belts such as in the Cyclopyge stigmata limestone of Bornholm (Poulsen, 1965; Nielsen, 1995). Ptychopygids of the west-ern belts are often mentioned without identification of the species (cf. Pärnaste et al., 2013, Table S5). The geographical distribution of this group can be detected (Fig. 4C), but the diversity picture is incomplete. In the Lynna section of Ingria, Asaphus and Ptychopyge s.l. succeeded each other as the most dominant genera through the upper part of the Volkhov and lower part of the Kunda regional stages (Hansen and Nielsen, 2003). The changeover largely coincides with the shift of the glauconite content in the sediments. Hansen and Nielsen (2003) sug-gested that this shift was connected to a sea-level change and that the higher glauconite content and the higher abundance of Asaphus indicate environments corresponding to lowstands, and the higher abundance of Ptychopyge with highstands. They describe several sea-level varia-tions within this interval but our data resolution is too low to trace these fluctuations. The peak in the easternmost area is greater at the A. lepidurus level than at the A. expansus level. This peak is probably exaggerated as a result of extreme taxonomic splitting by Balashova (1964, 1976) but it does indicate the high variability of this group during that time. Ptychopyge s.l. becomes extinct at the Kunda–Aseri boundary together with Megistaspis (Table 1). A closely related genus Pseudobasilicus, appears (cf. the latest rise in Fig. 4C) in the muddy sediments of the Elnes Formation in the Oslo area (Hansen, 2009), and later spreads over the shelf in the Central Baltoscandian and North Estonian belts until the earliest Katian. The youngest representative of this genus Pseudobasilicus kegelensis,is found in the lowermost peripheral beds of the carbonate mounds of the Vasalemma Formation of the Keila Stage (Hints et al., 2004). These mounds continue into the Oandu Stage and contain the earliest heliolitid corals recorded in Baltoscandia (Mõtus and Zaika, 2012). This interval corresponds to when the Baltic terrane was positioned in lower latitudes with warmer waters unfavourable to the asaphid trilobites which gradually disappeared forever. 4.3.4. Isotelines Isotelines are in general relatively large elongated asaphids with a broad concave border; often with weakly defined glabella and pygidial pleurae best seen on the internal mould. Jaanusson (in Moore, 1959) divided them into A and B groups based on the presence or absence of the posterior notch of the hypostoma. GroupA, genera possess a forked hypostome similar to the Asaphinae and Ptychopyge-group, and it includes the rare Baltoscandian genera Megalaspides, Lannacus, Homalopyge, Isotelus, and probably Brachyaspis (Table 1). Of these, Megalaspides is very similar to the Ptychopyge s.l. (e.g. Ptychopyge plautini) but outwardly differs in having a faintly expressed glabella, although when seen in the right light, it resembles that of the ptychopygids. The typematerial needs to be examined before this can be confirmed. Isotelus? robustus is an illaenimorph with its ill-defined axial region, but as an isoteline it has a forked hypostome, long effaced pygidial rhachis reaching close to the posterior border, and Panderian openings on the thoracic pleurae and librigena. This species appears in accumulationswithin the Pirgu carbonate sediments together with colonies of an early heliolitine coral, Protoheliolites (Liang et al., 2013). Group B (Fig. 4D) has no posterior notch on the hypostome and the border is usually very short (cf. Schmidt, 1906, pl. 5, Figs. 6–7) and often pointed medially. Included in this group are the Baltoscandian genera Asaphellus, Hunnebergia, Megistaspis including Megistaspis (Ekeraspis), Megistaspis (Heraspis), Megistaspis (Megalaspidella), Megistaspis (Paramegistaspis), and Megistaspis (Rhinoferus). Except for Asaphellus this group encompasses the Baltoscandianmembers ofMegistaspidinae pars sensu Balashova (1976). They first occur in the Bjørkasholmen For-mation in the western belts, and are recorded in first but somewhat younger carbonate accumulations in the eastern belts (Fig. 4D). The megistaspine trilobites are extremely abundant in the upper Floian and lower Dapingian limestones, especially in the North Estonian Confacies Belt, where successive accumulations of these trilobites occur (Pärnaste, 2003, 2006). The late Floian so called ‘trilobite-graveyards’ contain nearly monospecific accumulations of disarticulated remains of Megistaspis (Paramegistaspis) estonica. Nielsen's study (1995) revealed that Megistaspis is rather abundant throughout the Dapingian though rates of abundance fluctuate throughout the sections (cf. Nielsen, 1995, Figs. 44–48) and can be correlated with changes in sea-level. The Megistaspis (Megistaspis) biofacies was defined by Nielsen (1995) as containing 20–35% of the entire assemblage, and as representing the low-est sea-level even far off-shore (Nielsen, 1995, Figs. 47, 49). Our data also confirms that the most diversified Megistaspis fauna occurs in near shore areas in the eastern belts with the highest peak at the Asaphus lepidurus level on Öland and in Ingria. The number ofmegistaspine species remains stable in these regions until their sudden disappearance and extinction at the boundary betweentheKunda andAseri regional stages, in the middle of the Darriwilian Stage (Table 1). The megistaspine species differ from asaphines in having a wider geographical range and higher diversity. This group of asaphids is peculiar in having a characteristic broad bor-der area (e.g. Schmidt, 1906; pls. 5–7) that may well have aided move-ment along the bottom, either hard or soft or it could have provided extra buoyancy during swimming or sliding over the substrate. The mid-dle portion of the cephalon is vaulted and exhibits several morphological trends. Some cephala are rather flat and smooth with glabellar furrows nearly effaced (subgenera Paramegistaspis, Megistaspidella), others have prominent glabellar furrows especially adjacent to the axial furrow (subgenera Megistaspis, Rhinoferus) while a medial glabellar ridge de-velops and is directed towards the occipital ring (Rhinoferus hyorrhina group), or a prominent hump is present on the anterior part of the gla-bella (Rhinoferus gibba group). The preglabellar field carries distinctive features including a ridge extending from a shallow, concave, area. The hypostoma has a slightly notched or smooth posterior margin and large vaulted anterior wings which result in a large space below the gla-bella. This space could have been filled with a relatively large stomach implying that they were particle feeders. Their eyes are large like those of the asaphines suggesting life in a similar environment with possibly low light inhabited by large numbers of predators such as cephalopods. Judging from the high variability of the morphology of this group it is obvious that theywere highly specified and their disappearance may well have been caused by a sudden change in the environment, disappearance of certain food chain elements or a catastrophic event. It is no coincidence that the period of extinction corresponds to the wide distribution of tuff beds in Sweden and Estonia (Sturesson, 1992a,b; Sturesson et al., 2005) and a rapid changeover in trilobite faunas (Aru, 1990; Rõõmusoks, 1997; Pärnaste et al., 2009), accompanied by extinction of the lingulid and rhynchonellid brachiopods (Sturesson et al., 2005). This event coin-cides with a positive shift in the isotopic curve starting from the late Kunda Stage and reaching to the Aseri Stage in Baltoscandia (Meidla et al., 2004; Kaljo et al., 2007) reflecting a cooling period (Rasmussen et al., 2009; Ainsaar et al., 2010). A trend of exoskeletal enlargement is obvious in megistaspines and asaphines (e.g. Megistaspis rudis, Megistaspis gigas, Asaphus platyurus etc.) during this period. Giant trilobites occur at the high southern latitudes on themargin of Gondwana and are considered to be a cold-water adaptation such as those of the Iberian, Valongo Forma-tion. This fauna is of middle Darriwilian age (Didymograptus artus grapto-lite biozone, Gutiérrez-Marco et al., 2009) and correlative to the late Kunda and Aseri faunas in Baltoscandia. 4.4. Biodiversity pattern of Asaphidae in Baltica The biodiversity curve (Fig. 5), presenting a number of species for each stratigraphical unit exhibits the major evolutionary trends and events of the Asaphidae in Baltica during the Ordovician Period.
4.4.1. Furongian appearance H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 73 The first asaphids to appearwere representatives of the Niobinae (see Section 4.2), that were related to the latest Cambrian faunas in Siberia. Niobe and Promegalaspides were cosmopolitan during the Tremadocian, reaching the Uralian side of Baltica and spreading over wide areas at the western edge of Gondwana in cool and temperate climates. 4.4.2. Mid Tremadocian–mid Floian Rise The first members of all the major lineages of asaphids appeared to-gether with the Ceratopyge Fauna (Varangu Stage, AIII; Table 1, Fig. 5). The gradual rise of the biodiversity curve includes a small diversification of niobines, development of the first megistaspines that show different morphologies in their general body shape, and appearance of the first asaphines. Fifty species are known from this period with twenty-one of them from the uppermost mid Floian Megalaspides dalecarlicus Zone. This time-frame also includes development of the Asaphid Fauna on the Uralian side of Baltica. Niobe, Niobella, Gog, Asaphellus, and Promegalaspides are represented by a few species from the Southern Urals, and the species of Niobella, Asaphellus, and Megalaspides from Pay-Khoy, north of the Urals in the Tremadocian and the lower Floian beds (Ancygin, 2001; Bergström et al., in press). 4.4.3. Late Floian Fall The late Floian, Megistaspis estonica level, illustrates a sudden drop to only eight species. This minimum was also recognised by Hammer (2003), but not by Adrain et al. (2004), who used larger basic units. A corresponding decrease is recorded in the Baltoscandian brachiopod faunas (Schmitz et al., 2008) and this decline coincides with a period of very low rates of carbonate sedimentation. The thickest limestone suc-cession containing numerous beds is just over onemetre in theNärke re-gion, Sweden (Tjernvik, 1956, Figs. 10, 12; Lindström, 1963), or less than one metre in the Popovka and Lava sections in north-eastern Russia (Pärnaste, 2006). In the other regions thicknesses are much lower and in underlying carbonate beds even more so. Thus, decrease in diversity must have been the result of other factors. Maybe this was a time when the Baltic area located in the region of the ocean current circulation influenced by the winter–summer amplitude (Fig. 1A) which was per-haps less suitable for these trilobites. These terms may explain also the poor rate of carbonate accumulation. The Uralian trilobites Niobella and Megalaspides do not occur above the upper Floian in the Pay-Khoy and Vaygach areas, to the north, and no asaphids are recorded in the south (Bergström et al., in press; Pärnaste and Bergström, in press), which may be due to the drop in sea-level globally (Basal Whiterock Lowstand of Nielsen (2004). 4.4.4. Dapingian Rise with Late Dapingian Peak The boundary between the Floian and Dapingian stages marks the first appearance of the Ptychopyge-group (see Section 4.3.3), which may be derived from Megalaspides (Megalaspides). The first Asaphus, Asaphus broeggeri, shares intermediate characters with Ptychopyge and may have evolved from a common ancestor. The most sudden burst in diversity coincides with an extension of carbonate sedimentation throughout the basin during latest Dapingian times. Both Megistaspis and Ptychopyge, ac-quire various morphologies over the entire shelf area, especially in the shallow water of the North Estonian Confacies Belt (see Section 4.3.4) and in all, forty-four asaphids are recorded from the latest Dapingian. The extension of Megistaspis and appearance of the first Asaphus, is recorded from this period in the Pay-Khoy and Vaygach areas, north of the Urals. The late Dapingian Peak in the trilobite biodiversity curve is shown by Hammer (2003), but is not visible on the normalised trilobite curve of Adrain et al. (2004), based on the same data. Brachiopods and other shelly fossils show no rise in the uppermost Volkhov, but this can be recognised in the succeeding Kunda Stage (Sturesson et al., 2005, fig. 5; Schmitz et al., 2008, fig. 3). The trilobite peak is highlighted by radia-tion of the isotelines over the entire shelf (Fig. 4). Number of species Asaphinae Isotelinae Tr1 Tr2 Tr3 Fl1 Fl2 Fl3 Dp1-2 Dp3 Dw1 Dw2 Dw3 Sa1 Sa2 Ka1 Ka2-3 Ka4-5 Hi1-2 Tremadocian Floian Dapingian Darriwilian Sandbian Katian Hi 50 45 40 35 30 25 20 15 10 5 50 45 40 35 30 25 20 15 10 5 0 Niobinae 0 Asaphidae Niobinae Isotelinae Asaphinae AII AII AIIIa AIIIa AIIIb AIIIb BIα1 BIα1 BIα2 BIα2 BIβ BIβ BIγ BIγ BIIα BIIα BIIβ BIIβ BIIγ BIIγ BIIIα BIIIα BIIIβ BIIIβ BIIIγ BIIIγ CIa CIa CIb CIb CIc CIc CII CII CIII-DI CIII-DI DII DII E E FIa FIa FIb-FIc FIb-FIc FII FII A B Fig. 5. A–B. Graphs showing number of asaphid taxa throughout the entire Ordovician.
74 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 4.4.5. Early–mid Darriwilian High The zone of Asaphus expansus and the succeeding two zones of the Kunda Regional Stage contain fewer species of asaphids but the genera are the same as in the underlying stage. In addition to these early gen-era, several newcomers appear, including the short-lived Homalopyge, which occurs in the near shore facies on Öland and in the North Esto-nian Belt. This isoteline may be related to a Laurentian stock. Of other genera, Ogmasaphus and Pseudobasilicus appear in the Oslo Region during the latest Kunda Age and are probably of Baltoscandian origin. Ogygiocaris is recorded also from the Darriwilian Stage in the north of the Urals (Ancygin, 2001; Bergström et al., in press) and arrives in deeper water environments, including the Oslo Region at this time. The occurrence of Megistaspis elongata s.l. on both sides of Baltica reflects the exchange of faunas around this time. In Baltoscandia Megistaspis elongata is common in the lower Darriwilian, Asaphus expansus Zone (Nielsen, 1995), but its close relative M. pseudolimbata occurs in the upper Dapingian, Asaphus lepidurus/Megistaspis limbata Zone (Balashova, 1976; Pärnaste et al., 2013; Table S8). These two periods place a time limit for the possible connection between the two sides of Baltica. The Uralian species, Megistaspis aff. elongata (cf. Burskiy, 1970) ismost similar tothetype species fromtheA. expansus Zone in Baltoscandia and if this is the case, it would indicate that both areas were in contact during the early Darriwilian. The connection might well have been via the Moscow Basin where these asaphids have been recorded from boreholes (Dmitrovskaya, 1989; Bergström et al., in press) or during a sea-level rise in the latest Volkhovian, i.e. latest Dapingian (Hints et al., 2010, fig. 6). 4.4.6. Kunda/Aseri Turnover (mid Darriwilian Turnover) The richness of species remains stable during the Kunda/Aseri transition but a remarkable faunal turnover occurs (Table 1). The Aserian interval was favourable for a rapid trilobite evolution, including for the Asaphus s.l. group. There are twenty-two (sub)species recognised from the shallow shelf of Ingria. Other asaphid genera contain only a few species and Niobe and the megistaspines disappear at the Kunda/Aseri boundary corresponding to that between the Öland and Viru Series (Fig. 2). A similar pattern cannot be recognised in other groups although brachiopod diversity is drastically reduced (e.g. Hints and Harper, 2003, fig. 3; Sturesson et al., 2005, fig. 5). 4.4.7. Post-Aserian Fall In the development of the Asaphid Fauna during the next period a slow decline is observed leading up to its extinction towards the end of the Ordovician. In detail, this period can be subdivided into the following three diversity events: 4.4.7.1. Late Darriwilian Fall. The number of Asaphus s.l. species falls by at least a half by the beginning of the late Darriwilian. Ogygiocaris becomes most diversified during the late Darriwilian, and Pseudomegalaspis disap-pears at the boundary with the Sandbian. 4.4.7.2. Sandbian Fall. A further reduction of the Asaphid Fauna, by one half, occurs at the base of the Kukruse Stage, which otherwise is a time of high diversity within the Baltoscandian trilobite faunas (cf. e.g. Hammer, 2003; Pärnaste et al., 2009; Pärnaste and Popp, 2011) related to the massive accumulation of algae in the ‘Oil Shale’ beds of the Viivikonna Formation in the North Estonian Belt, thus forming a rich food supply for the shelly faunas including the trilobites. Subsequent to the Kukruse Stage Pseudasaphus disappears from the near-shore areas, and Ogygiocaris, from neighbouring off-shore areas. Ogmasaphus and Pseudobasilicus disappear higher in the succession. Isotelus has been recorded also fromthe approximately contemporane-ous sediments in Pay-Khoy, Arctic Russia on the Uralian side of the Baltica Plate (Owens and Fortey, 2009). 4.4.7.3. Katian gradual extinction. Fromthe Oandu Age onwards only a very few asaphid taxa inhabited the near-shore facies of the Baltoscandian Basin at a timewhen the Baltic terrane was located at lower latitudes with a warmer climate and this together with a change in sedimentation type was unfavourable for this group of trilobites. Isotelus, represented by a few species from the Rakvere Age onwards, may represent immigration from Laurentia or from Pay-Khoy. By the end of the Ordovician all asaphids have become extinct. 5. Conclusions The earliest Baltic asaphids appear in marginal areas of the Baltoscandian palaeobasin during the latest Cambrian. The first to appear is an obscure genus Eoasaphus, later followed by Niobella and Promegalaspides known also from Siberia, indicating a possible connection with the Baltic area. Niobella and Niobe were cosmopoli-tan during the Tremadocian and were among those of the Ceratopyge Fauna to survive the Ceratopyge Extinction Event. The Baltoscandian asaphids are foundwithin slightly differing sets of genera to the North and South of the Uralian side of Baltica during the Tremadocian and early Floian, but in the late Floian the genera are observed to be in common only with the northern area (in today's terms). In the latest Dapingian or early Darriwilian a species level con-nection by Megistaspis elongata s.l. marks a close connection between the two sides of Baltica, which might be related to the sea-level rise. The Asaphid Fauna is far from uniform. Four morphologically differ-ent groups with diverse feeding habits and locomotion evolved, leading to different patterns of distribution over the shelf of the Baltoscandian Basin. The dynamics of radiation and extinction varies between these dif-ferent groups and was determined by various environmental changes. From the Floian onwards carbonate sedimentation extended over broad areas of the basin and numbers of asaphids gradually increased. The late Tremadocian to Dapingian includes a gradual rise of niobines and the appearance of the first megistaspines showing novel morphol-ogies in body shape. In the middle of this rise one abrupt fall (the late Floian Fall) occurs, corresponding to a drop in sea-level, with a subse-quent recovey of faunas containing new forms. The Dapingian Rise endswith a sudden burst involving diversification of the ptychopygines and occurrence of the megistaspines all over the shelf. The succeeding Darriwilian High is characterised by the growth of a stabilised asaphid fauna with various morphotypes sharing the same space in the basin. Destabilisation of the faunas, probably resulting from a catastrophic event, occurs in the transition of the Öland and Viru series (Kunda and Aseri stages). Three morphotypes, adapted to a soft substrate, van-ish suddenly in the mid Darriwilian Turnover and from then on, loss is stepwise until the beginning of the Hirnantian. A group of asaphines with ill-defined axial regions, with a morphol-ogy resembling the illaenids, may have been adapted to a life as preda-tors inmud-mounds and reefs in a similarmanner to the illaenids. These illaenimorph asaphids appear twice: in the late Kunda, and Rakvere stages, and are the last asaphids occurring in Baltoscandia. They become extinct by the Hirnantian. Acknowledgements The inferences of this paper are the fruit of research done in inten-sive cooperation during the two years before Jan Bergström's unfortu-nate death in November 2012. The results were presented at the IGC in Brisbane, Australia in August 2012. Helje Pärnaste acknowledges support from the Estonian Research Council (grants ETF8054 and SF0140020s08). The present study is a contribution to the Interna-tional Geoscience Programme project 591 ‘The Early to Middle Paleo-zoic Revolution’. I thank two referees and guest editor Dr. Kathleen Histon for their valuable notes, which improved the manuscript, and I am especially thankful to Professor emeritus David L. Bruton for his incredible contribution in correcting the language.
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Trilobita

  • 1. Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo The asaphid trilobite fauna: Its rise and fall in Baltica Helje Pärnaste a,⁎, Jan Bergström b,1 a Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia b Department of Palaeozoology, Swedish Museum of Natural History, P.O. Box 5007, SE-104 05 Stockholm, Sweden a r t i c l e i n f o a b s t r a c t Article history: Received 11 February 2013 Received in revised form 4 June 2013 Accepted 5 June 2013 Available online 14 June 2013 Keywords: Ordovician Baltica Trilobites Functional morphology Biodiversity Sea-level The Baltica terrane is well known as the Asaphus Province of the Ordovician World. Being situated in inter-mediate latitudes and surrounded by fairly wide oceans, Baltica had a benthic fauna that developed in a rel-atively endemic direction. The most common trilobites were the asaphids. The present study discusses how they appeared, evolved in various environments around Baltoscandia and finally disappeared. The total range of the asaphids based on data from 381 species is estimated as extending from the Furongian, latest Cambrian to the end Ordovician. The remnant Cambrian Olenid Fauna, which was adapted to black bituminous shale facies, survived into the Tremadocian. This Cambrian fauna includes the first asaphids (Promegalaspides, Niobella, ?Eoasaphus). With a shift to lighter-coloured and carbonatic sediments in the middle Tremadocian, this fauna was replaced by the immigrating Ceratopyge Fauna. The latter contains the first five widely distrib-uted asaphid genera Promegalaspides, Niobe, Niobella, Niobina, and Asaphellus. As carbonate sedimentation became more widespread in many areas from the Floian onwards, the asaphids increased in number and formed a stabilised Asaphid Fauna during the Dapingian. In Baltoscandia a gradual development of endemism related to facies belts is observed during the Darriwilian. Records of the mud-related niobines, Megistaspis and Ptychopyge faunas, together with some other benthic macrofaunas are not found beyond the boundary of the Kunda/Aseri regional stages (middle Darriwilian). Their disappearance may be due to a catastrophic event. In contrast Asaphus s.l. survived this interval. Drift of Baltica towards lower latitudes and fluctuations in sea-level influenced the Asaphid Fauna leading to its gradual collapse by late Sandbian time. Influx of the islotelines during the early Katian gave way to a set of illaenimorph species found together with corals that are related to a reefal environment. This morph disappears prior to the Hirnantian glaciation. © 2013 Elsevier B.V. All rights reserved. 1. Introduction In 1966, H. B. Whittington gave a superbly illustrated overview on the phylogeny and distribution of the Ordovician trilobites in his presidential address to the Paleontological Society (Whittington, 1966). The phylogeny of families was drawn following their parallel development in different regions, pointing out geographic and climatic influence. He demonstrated that the Tremadoc (then being included within the Late Cambrian) trilobites are widely distributed worldwide, while successive faunas are endemic or with limited distribution. He pointed out that the greatest change ever in trilobite faunas occurred shortly before or during Tremadoc time when the majority of ancient trilobite families vanished and a small relatively short-lived group together with the earliestmembers of the dominant Ordovician families appeared. These include six geographically recognised faunas defined for the Tremadoc, followed by several more including an Asaphid Fauna during post-Tremadoc times. The Asaphid Fauna is characteristic of the Early Ordovician of Baltoscandia. This term was already used earlier by Whittington (1963) to distinguish the Baltic province from that of North America and Siberia. In these regions the bathyurids and asaphids co-occur along with totally different elements. The Baltoscandian elements were mentioned as those figured in works by Tjernvik (1956), Jaanusson (1953a,b, 1956) and Henningsmoen (1960) but a comprehensive list was lacking. In our study we review all repre-sentatives of the family Asaphidae (see for systematics Fortey and Chatterton, 1988; Jell and Adrain, 2003). The trilobite records from the Ural Mountains and the islands to the north, Vaygach and Novaya Zemlya, which form a further continuation of the chain in the north reflect some similarity with Baltoscandia, includ-ing occurrence of some common asaphids (Balashova, 1961, 1967; Burskiy, 1970; Ancygin, 2001; for more detail see Bergström et al., in press; Pärnaste and Bergström, in press). Following this appearance the entire Baltica terrane is considered to represent the Asaphid province in the Arenig and Llanvirn (Whittington and Hughes, 1972) or in modern terms in the Floian, Dapingian and Darriwilian ages (see for global stratig-raphy e.g. Bergström et al., 2009). A southern mid-latitudeMegistaspidine Realm to include Baltica was introduced by Adrain and others (Adrain et al., 2004) in their newglobal analysis for theOrdovician radiation of tri-lobites, because specifically the megistaspidine asaphids are endemic to this region (Cocks and Fortey, 1982, 1990). A family Megistaspididae ⁎ Corresponding author. Tel.: +372 620 30 10; fax: +372 620 30 11. E-mail address: helje@gi.ee (H. Pärnaste). 1 Professor Emeritus Jan Bergström passed away in November 2012. 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.06.007
  • 2. H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 65 was erected by Balashova (1976) covering the previously established subfamily Isotelinae in sense of Jaanusson (in Moore, 1959) to em-brace four subfamilies Megistaspidinae, Asaphellinae, Hoekaspinae, and Thysanopyginae using diagnostic characters different fromthose cho-sen by Jaanusson. The systematics of this group is still lacking so we continue to use the term Asaphid Fauna as first introduced, especially since the subfamily Megistaspidinae in sense of Balashova (1976) includ-ed the following genera (Megistaspis, Paramegistaspis, Megistaspidella, Ogygitoides, Stenorhachis, Plesiomegalaspis, Paramegalaspis, Ekeraspis, Dolerasaphus, Kayseraspis, Nerudaspis, Rhinoferus, Hunnebergia) of which some are also known outside of Baltica, i.e. are not endemic to Baltica. The aim of this contribution is to reveal the dynamics of distribution and development of different groups of asaphids,which prevailed during the first half of the Ordovician in Baltica. For thatwe (1) compare the tri-lobite succession in 13 regions across Baltoscandia throughout 15 inter-vals from the beginning of the Ordovician to the end of the Darriwilian, when the asaphidswere flourishing; (2) review the trilobite occurrences in the Ural Mountains together with Vaygach and Novaya Zemlya—the islands in the north–west (in present-day terms) on the other side of Baltica; (3) detect the distribution of asaphid subfamilies in different regions and time intervals in Baltoscandia; (4) analyse the major events in the diversification of the asaphids until the demise of the family at the end of Ordovician. 2. Geological setting 2.1. Baltica Plate—boundaries and palaeogeographical position Baltica formed a discrete continental block before the Caledonian collisions (Fig. 1A; see also, e.g., Torsvik, 1998; Cocks and Torsvik, 2002, 2005). A series of Caledonian nappes, the lowest of which con-tains fossils of Baltic and Avalonian origin and the uppermost those of Laurentian origin (Neuman and Bruton, 1974; Bruton and Harper, 1981, 1988; Bruton et al., 1989) have been transported eastwards from off-shore Norway (Ebbestad, 1999, fig. 2; Harper et al., 2009; Lamminen et al., 2011, fig. 11), and the evidence for litho- and biofacies in these areas is largely destroyed. Magmatic rocks included in some nappes bear evidence of the emergence of island arcs off the coast in Fig. 1. A. Palaeogeographical reconstruction showing change in position of Baltica from the beginning of the Tremadocian to the beginning of the Sandbian (Map generated using the T.H. Torsvik's GIS-oriented software from2009, BugPlates: linking biogeography and palaeogeography). Grey circle marks the ocean circulation in cool sea surface temperatures suggested by current circulation inmodern oceans, and thewide line indicates the winter–summer amplitude. B. The confacies (bio-lithofacies) belts of Baltoscandia modified from Jaanusson (1982a).
  • 3. 66 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 the late Arenig (Bruton and Bockelie, 1982; Harper et al., 2008; Andersen et al., 2012). At the Ural border, part of the original continental plate is thought to have been lost through subduction under the Magnitogorsk volcanic island arc and the Siberian plate, thus indicating an example of continental subduction (Matte, 1998; Brown, 2009; Puchkov, 2009). In the south-west, marginal parts of the Baltica Plate appear to have been lost through major strike-slip fault movements along the Tornquist–Teisseyre zone, which separates areas with contrasting thick-nesses of the crust and different Ordovician faunas. In the Ordovician, long before this happened, the Uralian borderwas being subjected to ex-tensional strain associated with volcanism. Baltica had a long journey before it arrived at its present position, with its centre around 55° North. At the beginning of the Tremadoc (earliest Ordovician) the centre was around 55° South (Fig. 1A) while at the end of the Darriwilian (Fig. 2; part of the Middle Ordovician), the position was around 40° South (e.g., Cocks and Torsvik, 2002, 2005, fig. 6). The Ordovician palaeogeographic maps of the western part of the Baltica Plate reveal a series of facies belts (Jaanusson, 1976, 1982a; Pärnaste et al., 2013). The most offshore, and most deepwater, belt is known as the Scanian Confacies Belt (Fig. 1B). It widens in the west, extending as an embayment towards the Oslo Region (Bruton et al., 2010), which has been regarded to represent the next shore-ward belt. The next inward belt is the Central Baltoscandian Confacies Belt. In Ölandian times an orogenic foredeep started to develop between this belt and the Caledonian Orogenic Belt. The Central Baltoscandian Confacies Belt extends into the East Baltic area as the Livonian Tongue. The most shoreward zone consists of the North Estonian Belt and its counterpart to the south, the Lithuanian Confacies Belt. A somewhat deeper channel in between these two appears to extend to the centre of Baltica as the Moscow Basin. Part of the succession in this basin is rich in graptolites (Dmitrovskaya, 1989). In some levels there is also an impoverished Central Baltoscandian type trilobite fauna (Bergström et al., in press). 2.2. Sedimentation and stratigraphical framework A general characteristic of the Ordovician deposits in the east Baltic is that sedimentation was extremely slow—in the Central Scandinavian Confacies Belt the thickness of the entire Ordovician is only some 75–150 m in general, whereas it reaches some 200 m in the Scanian Confacies Belt (Jaanusson, 1982a). In reality the sedimentation rate must have been evenmore remarkable. The Kundan Orthoceratite lime-stone has some 5 cm thick beds separated by leaf-thin clayey lamina. The beds have shells in all possible orientations, indicating very rapid sedimentation (Bohlin, 1949, fig. 7). The most impressive examples of trilobites embedded at an angle to bedding are fromthe North Estonian Confacies Belt (Dronov, 2005a; Pärnaste, 2006, fig. 4)where sedimenta-tion of a single bed maywell have taken place in a daywith the ensuing bed being desposited some50,000 years later (Bohlin, 1949; Lindström, 1963, 1979; Dronov, 2005a). Someminor erosion is probably part of the picture. The Oslo Region forms an exception. Notable variations in facies and thickness, the latter reaching up to some 1000 m (Bjørlykke, 1974; Bruton and Owen, 1982;Owen et al., 1990; Bruton et al., 2010), indicat-ing exceptional relief conditions and perhaps synsedimentary tectonic movements. Thus the development suggests that the Oslo Region should be interpreted as a local mixed zone rather than a stable sedi-mentary confacies belt (Braithwaite et al., 1995). A general, westward shallowing, within the Oslo Region (Størmer, 1967; Bockelie, 1978; Bruton et al., 2010; Hansen et al., 2011) points in the same direction. For the present study, the Norwegian trilobite data is treated together as representing one belt, however, that of the allochthonous belts of non-Baltic origin are left out. Aluoja Valaste Hunderum Langevoja Vääna Saka Kukruse Uhaku Lasnamägi Kunda Volkhov Billingen Hunneberg Varangu Pakerort CΙΙ CΙc CΙb γ β α γ β α γ β α BΙΙΙ BΙΙ BΙ AΙΙΙ AΙΙ Sa1 Dw2 Dw1 Dp3 Dp2 Dp1 Fl3 Fl2 Fl1 Tr3 Tr2 Tr1 Nemagraptus gracilis Hustedograptus teretiusculus Amorphognathus tvaerensis Pygodus anserinus Pygodus serra Pseudoamplexograptus distichus Eoplacognathus pseudoplanus Yangtzeplacognathus crassus Lenodus variabilis Lenodus antivariabilis Baltoniodus norrlandicus Paroistodus originalis Baltoniodus navis Baltoniodus triangularis Oepikodus evae Prioniodus elegans Paroistodus proteus Paltodus deltifer Cordylodus spp. Nicholsonograptus fasciculatus Holmograptus spinosus Holmograptus lentus ?Corymbograptus retroflexus Undulograptus sinicus Arienigraptus zhejiangensis A.dumosus / P.manubriatus Isograptus sp.2 / M.schmalenseei Isograptus victoriae Isograptus lunatus Baltograptus minutus Baltograptus sp. cf. B.deflexus Baltograptus vacillans Cymatograptus protobalticus Tertragraptus phyllograptoides Hunnegraptus copiosus Araneograptus murrayi Kiaerograptus supremus Adelograptus tenellus Rhabdinopora spp. Baltoscandian Regional Stages Substages Global Stages Global Series Stage slices Brittish Series Trilobite Zones and Beds Conodont Zones Graptolite Zones Asaphus sulevi / Megistaspis gigas Megistaspis obtusicauda Asaphus vicarius Asaphus raniceps Asaphus expansus Asaphus lepidurus / Megistaspis limbata Asaphus broeggeri / Megistaspis simon Megistaspis polyphemus Megistaspis estonica Megalaspides dalecarlicus Megistaspis aff. estonica Megistaspis planilimbata Megistaspis armata Ceratopyge acicularis Shumardia pusilla Boeckaspis spp. Bed Tremadoc Arenig Llanvirn Öland Series Iru Subseries Ontika Subseries Kiaerograptus kiaeri CΙa Xenasaphus spp. Bed Asaphus ornatus / A. bottnicus Bed Asaphus kowalewskii Asaphus intermedius Asaphus cornutus Asaphus latisegmentatus Viru Series Aseri Eoplacognathus suecicus Pterograptus elegans Dw3 Lower Ordovician Middle Ordovician Tremadocian Floian Dapingian Darriwilian Fig. 2. Correlation chart for the lower half of the Ordovician in Baltoscandia. The correlation of biozones compiled from (Bergström et al., 2002; Pärnaste, 2006; Bergström and Löfgren, 2009; Hansen et al., 2011; Maletz and Ahlberg, 2011; Pärnaste and Viira, 2012; Pärnaste et al., 2013), following the revised global standard (Bergström et al., 2009). The graptolites in the Lower Tremadoc (Tr1) represent ‘superzones’; for a finer zonation, see Egenhoff et al. (2010).
  • 4. H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 67 A stratigraphic gap locally covering much of the lower and middle Tremadoc succession in parts of Baltoscandia (Fig. 3) has been interpreted as an indication of a regression in the earliest Ordovician, with limited extension of the sea (e.g. Erdtmann, 1982). However, there is pronounced variation in the development of the Alum Shale Formation, indicating local erosion or non-deposition rather than uplift. An exception may be the transition from the lower Tremadoc Alum Shale to the middle Tremadoc Bjørkåsholmen Formation (Ceratopyge Limestone), which seems to be complete only in the Oslo Region (Jaanusson, 1982a). Terrigenous clastic sediments are found in this level in Dalarna (Jaanusson, 1982b). The Alum Shale is black, fissile and bituminous with scattered limestone concretions. The middle Cambrian to lowest Ordovician Alum Shale has a mineral composition of 50–65% quartz, 5–15% feldspar and 10–15% muscovite (Pedersen, 1989, p. 156). The quartz is very fine-grained. The muscovite may have a mixed detrital and diagenetic origin. The shale owes its black col-our to organic material. In Scania (southernmost Sweden) it has been strongly heated and occurs as kerogen with a content of TOC (Total Organic Carbon) that averages 9% (Pedersen, 1989, fig. 11). It contains remains of unicellular and larger algae. In the Central Scandinavian Confacies Belt the organic content is high enough to produce oil. Sedimentation was slow and the composition and the preservation of algal remains indicate a dysoxic–anoxic depositional environment. The evidence for burrowing activity is poor. There is some indication that trilobites come and go abruptly depending on environmental changes such as anoxia (summarised by Pedersen, 1989; see also Clarkson et al., 1998; Eriksson and Terfelt, 2007). The Alum Shale is succeeded by a thin succession of black to grey shale and limestone (the former Ceratopyge Shale and Limestone) now known as the Bjørkåsholmen Formation (Owen et al., 1990; Ebbestad, 1999) except in Öland (Fig. 3; Frisk and Ebbestad, 2008). The limestone is locally rich in glauconite. In the North Estonian Confacies Belt, the lower Tremadocian (AII; Fig. 3) consists of sandstone and alum shale of the Kallavere and Türisalu Formations, whereas the middle Tremadocian Scanian Belt Central Baltoscandian Belt North Estonian Belt Lanna limestone Persnäs lst Källa lst Folkeslunda limestone sandstones limestones Seby limestone Skärlöv limestone D Gillberga Fm Horns Udde Fm Bruddesta Fm Bruddesta Fm Köpingsklint Fm Djupvik Fm glauconite hiatus oolites Veltsy Fm Valim Fm Porogi Fm Doboviki Fm Simonkovo Sinjavino Kandle Fm / Kandle Formations Sillaoru Formation Toila Formation Toila Formation Leetse Formation Pakri Fm Varangu Formation v v v a specific discontinuity surface with deep borings siltstones shales v v v v v v v v v v v v v Trilobite Zones and Beds NW Estonia Ingria NE Estonia Scania Oslo Region Väster-götland Öster-götland Närke Dalarna Öland Gävle Region Sularp Formation Almelund Shale Vollen Fm Dalby limestone Huk Formation Komstad Limestone ? Tøyen Shale Formation Hagastrand Member Ryd Formation Bjørkåsholmen Formation Incipiens lst Alum Shale Formation Alum Shale Formation Ceratopyge Shale Dictyonema Shale C Latorp limestone Orthoceratite limestone lithology Türisalu Formation Kallavere Formation A. sulevi/ /M. gigas M. obtusicauda A. vicarius A. raniceps A. expansus A. lepidurus/ /≈M. limbata A. broeggeri/ /M. simon M. polyphemus M. estonica M. dalecarlicus M. aff. estonica M. planilimbata M. armata C. acicularis S. (C.) pusilla Boeckaspis spp. CΙc CΙb BΙΙΙγ BΙΙΙβ BΙΙΙα BΙΙγ BΙΙβ BΙΙα BΙγ BΙβ BΙα2 BΙα1 AΙΙ Holen limestone Viivikonna Fm Kõrgekallas Fm Väo Fm CΙa Obukhovo / Loobu Fms CΙΙ Xenasaphus devexus Bed Asaphus ornatus Bed A. kowalewskii A. intermedius A. cornutus A. latisegmentatus Dalby limestone Furudal limestone Segerstad limestone Gullhögen Fm Skövde lst Våmb limestone Elnes Formation Killeröd Fm Tjällsten lst AΙΙΙb AΙΙΙa Fig. 3. Distribution of main lithologies in Baltoscandia, with generally deeper facies to the left, shallower to the right. Many small breaks in the sequence are not indicated on the diagram except for the lower Tremadocian level. This compilation resulted from numerous papers describing the sections with trilobites of the Öland Series (e.g. Nielsen, 1995; Pärnaste et al., 2013). Added are the data of the Viru Series (Jaanusson, 1982a; Bergström et al., 2002; Pålsson et al., 2002; Ivantsov, 2003; Dronov, 2005b; Hansen, 2009; Pärnaste and Popp, 2011).
  • 5. 68 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 (AIII) consists of shale and glauconitic silt- or sandstone of the Varangu Formation. The sedimentary rocks overlying the Alum Shale and the sed-iments with the Ceratopyge Fauna, are shales, mudstones and limestones with shelly faunas present in various confacies belts (Jaanusson, 1976, 1982a), whilst most offshore, in the Scanian Confacies Belt, the latest Tremadocian to the latest Darriwilian Tøyen and Almelund shales are rich in graptolites. The middle belt is the Central Baltoscandian Confacies Belt. The dominant facies is a bedded limestone, the middle part of which is known as the Orthoceratite Limestone. The lower part, up to the base of the Asaphus expansus Zone (Figs. 2, 3) is typically calcilutitic, without observable grains, whereas the upper Kundan part is a fine-grained calcarenitic indicative of a higher energy depositional environment (Jaanusson, 1982b, p. 19; Nielsen, 1995). The shift is abrupt. Authigenic silicateminerals, where present in lower strata, is invariably glauconite with shifts to chamosite only in intervals where the rock is lutitic. This is the pattern all over Baltoscandia (Jaanusson, 1982b, p. 19). In the Central Baltoscandian Belt the Kundan limestone turns red towards the top but is grey as it extends into southern Norway as the Huk Formation and further as a tongue, the Komstad Limestone, into the Scanian Confacies Belt. In southern Norway the Kundan portion has limestone with shale interbeds. The development in the wide western belt was influenced by Caledonian tectonics and is not known in detail because of metamor-phism. Turbidites in allochthonous lower nappes in Jämtland (the Caledonian Fore-deep Confacies Belt; Fig. 1B) become thicker west-wards in the Volkhovian(?) and Kundan (Jaanusson and Karis, 1982) indicating rising land further west. In Hardangervidda in southwest Norway, the source of quartzitic sediments is thought to be in a ‘Telemark Land’ to the south, whereas the sea became deeper in a north–west di-rection (Rasmussen et al., 2011). A limestone (Bjørnaskalle Formation) in the lower Darriwilian contains deformed trilobites, brachiopods and conodonts that confirm the Baltoscandian affinity (Bruton et al., 1989). In the Iapetus Ocean further west there were island arcs of which remains are now preserved in strongly metamorphosed lower nappe now considered part of a microcontinent which was detached from the edge of the Baltic platform and then later obducted onto it (Harper et al., 2008; Rasmussen et al., 2011, figs. 1, 4; Andersen et al., 2012). 3. Material and methods The knowledge of the Ordovician trilobite faunas of Baltoscandia is notably uneven. Whereas some trilobite faunas and particular trilobite groups have been studied in recent years, others have not been studied for more than a century, if at all. The last overview of all Swedish faunas was by Angelin (1854). East Baltic faunas were reviewed by Schmidt (e.g., 1881, 1885, 1901, 1904, 1906, 1907), and their taxonomic revi-sions over the years were listed by Bruton et al. (1997). More recent reviews of the Lower Ordovician asaphid faunas include contributions by Tjernvik (1956), Henningsmoen (1960), Balashova (1976), Nielsen (1995), Ebbestad (1999), Hoel (1999), Ivantsov (2003, 2004), Pärnaste (2006), Hansen (2009), and Stein and Bergström (2010). A complete list of the regional Ölandian Series (Tremadocian to mid Darriwilian) trilobites from the whole of Baltoscandia is producedwith some revision of systematics as well as the vertical and horizontal distribution. In all, the occurrences of over 400 species (of which a third are asaphids) belonging to about 125 genera over thirteen regions are listed from 15 trilobite biozones (Pärnaste et al., 2013). Herein the data set is extended to cover the following zones to the end of Ordovician. In most cases the data is presented by separate biozones with a few exceptions concerning the Kunda Regional Stage to leave better option for comparison with the other faunal groups. The Kundan stage has long been divided into the Asaphus expansus, A. ‘raniceps’, Megistaspis obtusicauda and M. gigas zones. The two latter are very poor in trilobite species and the zonal species are rather unique to the Central Scandinavian Belt,which means that no detailed correlation is possible. Asaphus ‘raniceps’ was recently revised (Stein and Bergström, 2010). It was found that the true A. raniceps is a good marker of the base of the zone, but is replaced by A. vicarius for much of the zonal range. We have combined here the A. raniceps and A. vicarius zones (Fig. 2) to fit with the Valaste Regional Substage (BIIIß). The uppermost, Aluoja Sub-stage (BIIIγ) is divided into several trilobite zones following the evolu-tionary appearance of the asaphines as is defined by Ivantsov (2003) in the St. Petersburg region, north-eastern Russia. This region in the vicinities of the Russian part of the Baltic-Ladoga Glint is called Ingria, or Ingermanland that was designated as the province of St. Petersburg at Schmidt's time (e.g. Schmidt, 1881), and the name is used herein to denote this district briefly. The preservation of beds there indicates a higher rate of sedimentation than in northern Estonia (cf. e.g. Ivantsov, 2003, fig. 2; Rasmussen et al., 2009). Faunal zones have yet to be recognised for the area and we refer to the older zonational scheme as used in Sweden and Norway for this part of the succession and the succeeding Aseri Regional Stage (e.g. Jaanusson, 1953a, 1982a; Wandås, 1984). No trilobite zones are defined above the Aseri stage, where the less formal units (beds) are named. Thus the Asaphus ornatus and A. bottnicus bed correlates with the Lasnamägi Regional Stage (CIb), and a bed with Xenasaphus with the Uhaku stage (CIc), forming the upper part of the Darriwilian Stage (Fig. 2; Ivantsov, 2003). By Sandbian times the diversity of the asaphids had decreased drastically and fromthis peri-od regional stage names are used herein and the resolution of our data is restricted to each individual stage. In all 381 asaphid species ranging from the Latest Cambrian (Furongian) to the end Ordovician are included in our data sets. Regional data sets are mainly literature-based and vary from ran-domly collected localities in older literature to detailed bed by bed col-lections as listed by various authors (e.g. Nielsen, 1995; Ebbestad, 1999; Hansen, 2009). Thus, the absence of records in some regions may be either because collections are lacking or that the pertinent sediments are not exposed or are absent. Nevertheless, where present, regional distribution of different groups of asaphids allows recognition of the particular sedimentary environments these groups favoured. The Ural border of Baltica is less known even if brief comparisons of lists of trilobite taxa recorded from the areas around Baltica suggest some similarity between east and west (e.g. Cocks and Fortey, 1998). The records of trilobites of the Ural Mountains date back to 1933 and 1948 when A.F. Lesnikova and V.N. Weber described the collections of N.E. Chernysheva and others. We have reviewed all the taxa available in the taxonomic works (e.g. Weber, 1948; Balashova, 1961; Burskiy, 1970; Ancygin, 1973, 1977, 1978, 1991, 2001), with some revision based on available photographs and descriptions (Bergström et al., in press; Pärnaste and Bergström, in press). In all approximately 220 species belonging to 105 genera ranging from the Tremadocian to the Darriwilian are listed in an extended Ural area together with Pay-Khoy and Vaygach in the north, extending south to theMugodzhars in Aktyuby Oblast, north-western Kazakhstan. In this study we follow the stratigraphical data of Ancygin (2001). 4. Discussions and results 4.1. Systematics of asaphids common in Baltica The family Asaphidae encompasses asaphine trilobites with librigenae separated anteriorly by a median suture, with the posterior margin of the hypostoma varying from pointed to deeply notched, with the Panderian organs developed as notches or separate openings. The family Asaphidae has been classified in several ways into subfamilies (e.g. Jaanusson in Moore, 1959; Balashova, 1976; Fortey, 1980; Fortey and Chatterton, 1988).Wefollow herein Fortey's grouping into three sub-families— asaphines, isotelines and niobines. As the ogygiocaridines are closely related to the niobines and they both preferred deeper water en-vironments in this study we have treated their occurrence data together (Section 4.3.1). In our previous study on Baltoscandian trilobites of the Ölandian Series (Pärnaste et al., 2013) we revised the outdated names.
  • 6. H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 69 There are a number of superfluous generic names, not the least in Russian literature, which do not have any obvious phylogenetic meaning and blur relationships. We have therefore retained Megistaspis, Asaphus and Ptychopyge in a broader and supposedlymore ‘natural’ sense but separate Niobe and Niobella since both genera seemto define conservative lineages starting in the Cambrian. In Table 1, numerous diversification events can be traced via the number of species rather than genera. Below (Section 4.3) we discuss details of functional morphology in each subfamily and evaluate their distribution across the Baltoscandian palaeobasin. 4.2. First asaphids of the Baltica The earliest possible asaphid representative in Baltoscandia is a rare Eoasaphus superstes known from one incomplete specimen in the Furongian, Jiangshanian Stage of Sweden (Terfelt et al., 2011). In the Furongian uppermost stage (Stage 10), Promegalaspides (Promegalaspides) and Niobella are rare in Baltoscandia but are common globally. Two Baltoscandian species, Promegalaspides (P.) kinnekullensis and P. (P.) pelturae are present in Baltoscandia and in Siberia, where they occur just above the lower boundary of Stage 10 in the Khos-Nelege reference section, a potential candidate for the GSSP of Stage 10 (Lazarenko et al., 2011). This occurrence indicates a close link between the terranes of Baltoscandia and Siberia at the end of the Cambrian. Promegalaspides later becomes more common in the middle Tremadocian of Baltoscandia (Figs. 2, 3), with the occurrence of Promegalaspides (Borogothus) intactus and P. (B.) stenorachis (Ebbestad, 1999, Fig. 15) forms where the cephalon and pygidium become more elongated thus resembling isotelines rather than niobines (cf. Tjernvik, 1956; Hoel, 1999). Jaanusson (in Moore, 1959) believed the latter to be the case and erected the new sub-family Promegalaspidinae. Different from all the other asaphids Promegalaspides possesses the posteriormost thoracic pleura ending in a long spine. The palaeogeographic origin of this genus remains unclear as it dates back to the beginning of the Cambrian Age 10 in two terranes. In addition to occurrences in Baltoscandia and Siberia it is also known fromthe early Tremadocian Kidryas Age of the Uralian border of Baltica (Aktyuby Oblast, north-western Kazakhstan; Balashova, 1961; Ancygin, 2001). Promegalaspides is also recorded from the late Tremadocian and younger beds of the Montagne Noire region, Armorica (Thoral, 1935; Hoel, 1999), Siberia, and Gornaya Shoria. The other early Baltic asaphid, Niobella, is already cosmopolitan in the late Cambrian of Laurentia and around Gondwana: Avalonia, Armorica, Turkey, South China, and Australia (see for references, Pärnaste and Bergström, in press). The cos-mopolitan genus Asaphellus, is recorded from the middle Tremadocian Ceratopyge beds of the Oslo Region (Owen et al., 1990; Pärnaste et al., 2013), but as it has never been published estimation of this record is not possible. 4.3. Regional distribution of morphologically different groups In describing nileid trilobites, Fortey (1986, fig. 12) discussed several morphotypes interpreted as being adapted to different environments as shown by the patterns of terraced lines on the exoskeleton. Later Fortey and Owens (1999) discussed the shape of the exoskeleton and modifi-cations allowing hypotheses to be formulated on the life style and feed-ing habits of these creatures. Often the form of the hypostome, its connection to the dorsal exoskeleton and the space between the glabel-la and hypostome are used when suggesting their feeding strategies. Recent discoveries of various types of digestive system and its position in the trilobite body (Chatterton et al., 1994; Lerosey-Aubril et al., 2011; Fatka et al., 2013) question the direct relation between the size of the stomach and space between the glabella and hypostome. Asaphids in general have been considered to belong to the fast-moving and detriti-vore low-level epifauna, but they may have muchwider diversification. We test here preferences of the morphologically different Ordovician asaphids inhabiting various areas on the Baltoscandian shelf. In a broad sense the Baltoscandian asaphids can be subdivided into four major groups following the general morphology of their body-plan and characters diagnostic for assignment to different subfamilies (Fig. 4). 4.3.1. Niobines Niobines possess a relatively flat dorsal shield rounded, both anteri-orly and posteriorly, with a well-defined axial furrow over its shield Table 1 List of asaphid genera showing the number of taxa by trilobite zones and regional stages. AII AIIIa AIIIb BIα1 BIα2 BIβ BIγ BIIα BIIβ BIIγ BIIIα BIIIβ BIIIγ CIa CIb CIc CII CIII-DI DII DIII E FIa FIb-FIc Niobinae 1 5 4 7 6 2 3 7 8 6 3 4 5 6 7 1 Platypeltoides 1 1 Niobe 1 1 2 1 1 2 3 3 2 Niobella 3 1 3 5 2 3 5 5 2 Niobina 1 Lapidaria 1 1 Gog 1 1 1 1 Ogygiocaris 2 5 6 7 1 Isotelinae 2 4 6 13 4 8 7 21 13 18 16 1 1 5 Promegalaspides 2 1 1 Megalaspides 1 7 1 Hunnebergia 1 Megistaspis 2 4 6 3 8 7 21 13 17 15 Homalopyge 1 1 Isotelus 1 1 2 “Brachyaspis” 3 Asaphinae 2 2 8 9 15 22 19 19 30 14 16 10 10 4 0 1 0 3 Proasaphus 2 1 Onchometopus 1 1 1 Asaphus 1 1 11 12 10 22 6 7 6 7 3 1 2 Pseudasaphus 3 5 3 2 4 1 Ogmasaphus 2 2 1 3 2 2 Pseudomegalaspis 1 2 1 Ptychopyge 8 7 13 11 4 Pseudobasilicus 2 2 3 1 1 1 1 Opsimasaphus ? Number of species 0 1 7 8 13 21 8 19 23 44 41 40 39 35 20 23 11 10 4 0 2 1 7 Data in bold emphases a total number of the species included to the group of genera below.
  • 7. 70 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 (Fig. 4A). They have deep, wide, pleural furrows, and a slightly concave or convex moderate border area. The most prominent terrace ridges on the border area and hypostome appear aligned transversely to the axis. The terrace lines on the frontal part of the shield, including on the hypo-stome, are oriented with the steepest slope facing anteriorly while on the pygidium facing posteriorly (cf. Nielsen, 1995, Figs. 129, 131). If these were connected with sensory organs to detect current flow as suggested by Miller (1975), then their well-defined expression may indicate functioning in relatively quiet flow conditions. Alternatively, if acting as stabilisers in locomotion (Fortey, 1986), then the lines could well prevent back and forth sliding when crawling on the muddy bot-tom. They have the most prominent macula, a possible visual organ, ever seen on Baltoscandian trilobites. The eyes of Niobella are large but not high, tilted anteriorly and affording downward vision to see in front of the head and to the sides (cf. Ebbestad, 1999, Fig. 49A–C). Gog and Niobe have relatively smaller eyes (cf. Nielsen, 1995, Figs. 129C, 142A). In Ogygiocaris the eyes are horizontal and raised above the gla-bella on a small palpebral lobe so that the visual surface affords vision both laterally and dorsally (cf. eyes: Hansen, 2009, pl. 2, Fig. 11, pl. 3, Fig. 1; and macula: pl. 2, Fig. 9). The palaeoecological study by Ebbestad on abundance of various tri-lobites within the Tremadocian Bjørkasholmen Formation of the Oslo Region, shows that the spinous Ceratopyge (cf. Ebbestad, 1999, Fig. 56) is replaced by the robust Niobe and Niobella (cf. Ebbestad, 1999, Figs. 11–12). This replacement corresponds to a shift in abundance and a change from a compact limestone to thinner beds with interbedded shales suggesting slightly deeper water conditions. The detrended cor-respondence analysis of the Huk Formation samples places Niobella to the deeper water but Niobe to the shallower water habitat (Nielsen, 1995, Figs. 49, 51) but a log of the Slemmestad section shows Niobe in the shaley part of the section (Nielsen, 1995, Fig. 48). From a range chart of the occurrences of trilobites in the Lynna section, Ingria (Hansen and Nielsen, 2003, Fig. 3) it is evident that Niobella disappears when silty clay beds are replaced by carbonates in the upper part of the Asaphus lepidurus Zone. It would appear that the niobines seem to have preferred the muddier bottom or deeper water. 16 14 12 10 8 BIIIγ CIb Asaphus s.l. 12 6 10 4 2 6 4 2 8 A B 6 4 BIα1 BIβ BIIβ BIIIα BIIIγ CIb C D 6 4 2 8 2 BIIIα BIIIα BIIIγ CIb Isotelinae Group B Niobinae Ptychopyge s.l. AII AIII BIα2 BIγ BIIα BIIγ BIIIβ CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia AII BIα1 AIII BIβ BIα2 BIγ BIIβ BIIα BIIIα BIIγ BIIIγ BIIIβ CIb CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia AII BIα1 AIII BIβ BIα2 BIγ BIIβ BIIα BIIγ BIIIβ CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia AII BIα1 AIII BIβ BIα2 BIγ BIIβ BIIα BIIγ BIIIβ CIa CIc Bornholm Scania Oslo Region Västergötland Östergötland Närke Dalarna Jämtland Öland Gävle region Latvia Ingria North Estonia Fig. 4. 3D graphs with distribution of various groups of asaphids over the basin from the lower Tremadocian to the upper Darriwilian (x-axis) ordered in following fields: (A) Niobinae; (B) Asaphus s.l., (C) Ptychopyge s.l., (D) Isotelinae Group B. Thirteen regions in y-axis follow geographic order similar to Fig. 3, reflecting more nearshore conditions apart from 0. Number of species increases in z-axis. The sketches next to the 3D graphs show general shape of the exoskeleton (in lateral view) and the hypostome (in ventral view).
  • 8. H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 71 The distribution map of the species belonging to Niobella, Niobe, Niobina, Lapidaria, Gog, and Ogygiocaris (Fig. 4A) exhibits relatively homogenous dispersal over the region and times with a few low peaks. If one compares the distribution in terms of lithostratigraphy (Fig. 3) then niobines appear first in the regions where carbonate sed-imentation starts, such as the outer western shelf belts during the Tremadocian, extending to the first maximum in the Hunnebergian of the western and central belts. It is not known if they reached the eastern belt as here there are only high enegry sandstones lacking fossils with carbonate skeletons. The second peak is recognisable in the upper Dapingian, Megistaspis simon and M. limbata zones of the Komstad limestone Formation in the Scania and Oslo Region, followed by a shift into the eastern belt in the lower Darriwilian, Asaphus expansus and A. raniceps zoneswith a maximumin the latter. If niobines are the indicators of the deeper waters, then their appearance on the shallow shelf marks a sea-level rise during the early Darriwilian and a low stand before that. Alternatively, if niobines were adapted to be a mud-dependant inhabitant, their richness in the eastern belts indicates a sedimentary source for the mudwith iron-rich oolites. Sturesson et al. (2005) suggested these early Darriwilian oolite beds might be connected with an influx of volcanic debris. Taking all regions into consideration together shows richness peaks of niobines at the mid Floian Megalaspides dalecarlicus Zone, latest Dapingian Asaphus lepidurus / Megistaspis limbata Zone, and finally in late Darriwilian times (Table 1). The youngest peak corresponds to the arrival of the ogygiocarines in the Oslo Region in the Aseri to Uhaku interval during late Darriwilian times. They also reached Jämtland and the Livonian Tongue region (i.e. Latvia in Fig. 4A; Lt in Fig. 1B). They are absent in the shallow shelf of the North Estonian Belt. 4.3.2. Asaphines The asaphines (excluding Ptychopyge and closely allied genera) are a groupwith arched exoskeleton, possessing a rather short pygidium and cephalon; deep axial furrow; pygidiumwith narrowaxis and ill-defined pleural ribs and is slightly convex at the border area; cranidium with very limited frontal area ahead of glabella and a deep posterior border furrow (Fig. 4B). The exoskeleton possesses a well developed terrace line pattern on the pleural field, the strongest following the direction of the pleural ribs. The elevated and stalked eyes vary in size with the visual surface, allowing an all round vision in the horizontal plane (cf. Ivantsov, 2003, pl. 13). When less elevated, the palpebral lobe is rather strongly tilted, downwards anteriorly, allowing anterior vision (cf. Ivantsov, 2003, pl. 19, Figs. 2, 8a, pl. 21, Figs. 6, 12; Nielsen, 1995, Fig. 49A–C; Hansen, 2009, pl. 6, Fig. 2). Hypostoma possess a rather short middle body and a prominent forked posterior border. The anteri-or wings are designed for strenghtening the attachment to the inside of the glabella (cf. Ivantsov, 2003, text-Fig. 20, pl. 21, Fig. 3). The thickened inner surface of the forks is covered by a series of fine raised ridges which Ivantsov (2003, pl. 21, Figs. 5, 22) interpreted as being ‘adapted for grinding large food particles and thus being predators feeding on the animal organisms found in the upper layer of bottom sediment’. The same structures were interpreted by Fortey and Owens (1999) as being used for breaking down the body wall of prey before ingestion into the stomach, and thus the trilobites with such hypostomes are interpreted as predators (see also Hegna, 2010). Asaphus specimens bur-ied in tunnel systems ascribed to Thalassinoides, have been described from the Holen Limestone in Jämtland, Sweden (Cherns et al., 2006) suggesting that the trilobites lived on a soft seafloor (Gibb et al., 2010). The following asaphines are included in our distribution overview (Fig. 4B): Proasaphus, Asaphus, Asaphus (Onchometopus), Asaphus (Neoasaphus), Asaphus (Subasaphus), Ogmasaphus, Pseudomegalaspis, Plectasaphus, Xenasaphus, and Pseudasaphus including Volkhovites, Valdaites, Baltiites, Pseudoasaphoides, Dubovikites, Pseudoasaphinus, Leningradiites, Estoniites (Balashova, 1976; Ivantsov, 2003). A very rare Proasaphus, known only from cranidia and pygidia, doc-uments the first occurrence of this group in two regions (Närke and North Estonian Belt) in the mid Floian (Pärnaste, 2006; Pärnaste and Viira, 2012). By the Dapingian, an increase in the number of asaphines occurred over the entire region, but was most diversified in the North Estonian Belt. The first maximum rise marks the start of the Kunda Regional Stage, and the second and most prominent rise, the Aseri Stage. The latter iswell known for an extremely well preserved trilobite fauna in Vilpovitsy quarry, St. Petersburg region,where numerous com-plete specimens of very attractive forms occur. Among these is Asaphus kowalewskii with an extraordinary long palpebral lobe and stalked eye reaching well above the cephalon. Following the Aseri Stage, the num-ber of Asaphus s.l. species falls to more than a third and the number of all asaphines by half (Table 1). By Oandu times Baltica had moved to thewarm, temperate climate zone (Fig. 1A) and fromthen on asaphines were shortly to vanish. Most probably the phacopine trilobites with a similar body shape but much better vision abilities took over their niche on the shallow shelf of the Baltoscandian basin. It is of interest that during the Aseri Age, the height of eyes increased in certain asaphids which Balashova (1953) and later Ivantsov (2003) recognised for an evolutionary line containing Asaphus koltukovi koltukovi–A. k. tumidus–A. punctatus–A. intermedius–A. kowalewskii. Suggesting that this morphological trend was related to the gradual change of the environment towards an increase in the accumulation of mud on the seafloor itmight reflect a slow sea-level rise or local sub-sidence of the basin. Some of the above Ingrian species including A. punctatus and A. kowalewski have been found on the Pakri peninsula in north-western Estonia more than 500 km from Ingria. On the Estonian peninsula and nearby islands, the Aseri beds are extremely thin being no more than a few decimetres thick (Orviku, 1940) while Ivantsov (2003) records thicknesses of 6–7 m in Ingria. On Pakri the iron oolitic limestone of the Aserian Kandle Formation (Fig. 3) consists of several distinct beds each separated by discontinuity surfaces and yielding complete unrolled trilobites (at least one per square metre and laying at different angles to the bedding) a situation most unusual for the North Estonian near shore belt where disarticulated remains are most common. How can this be explained? If the elevation of the eyes was caused by a sediment-related trend, it could hardly be explained as happening in similar terms over such an extended area. Thus, it seems that there must have been some connection between north-western Estonia and Ingria and that the trilobites migrated be-tween these areas independent of basinal bottom conditions. The other option could be that they were carried there from time to time, perhaps as buried components within partly lithified mudmasses moved by events such as big storms (Lindström, 1979) or tsunamis following vol-canic activities (see for volcanic activities in e.g. Sturesson et al., 2005). The trilobites were supposedly enrolled mainly to protect their body from enemies. Thus, the bed with numerous complete unenrolled spec-imens points to a situation of sudden burial of slightly lithified sediment containing borrowed-in trilobites (see above Section 4.3.2). 4.3.3. Ptychopyge s.l. We have chosen to treat Ptychopyge s.l. separately from the other asaphines because of the characteristic hypostoma of the former, which has a relatively long middle body, ending in a forked posterior border, bearing exsagittally prominent terrace lines, and its relatively small, rounded shoulders thus bringing the hypostoma close to the dor-sal shield (cf. Balashova, 1964, pl. 9; Balashova, 1976, pl. 16). Conse-quently Ptychopyge s.l. has a relatively flat exoskeleton with a wide concave border of both cephalon and pygidium. The cephalon has a narrow glabella and small eyes close to each other. In our database for Ptychopyge s.l. we also include Pseudobasilicus and Pseudobasiliella, which share similar characters and are most probably descendant from Ptychopyge. Ptychopygids are especially diversified (cf. Balashova, 1964) in clay-ey limestones connected to the Volkhovian mudmound structures in Ingria (e.g. Dronov and Ivantsov, 1994; Tolmacheva et al., 2003) and Estonia (Pärnaste, unpublished data), but also within limestones of the
  • 9. 72 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 more off-shore facies of the Livonian Tongue in Latvia and the Central Baltoscandian Confacies Belt in Västergötland. The ptychopygids togeth-er with the other trilobites found in mudmounds around Ingria, extend to the off-shore western belts such as in the Cyclopyge stigmata limestone of Bornholm (Poulsen, 1965; Nielsen, 1995). Ptychopygids of the west-ern belts are often mentioned without identification of the species (cf. Pärnaste et al., 2013, Table S5). The geographical distribution of this group can be detected (Fig. 4C), but the diversity picture is incomplete. In the Lynna section of Ingria, Asaphus and Ptychopyge s.l. succeeded each other as the most dominant genera through the upper part of the Volkhov and lower part of the Kunda regional stages (Hansen and Nielsen, 2003). The changeover largely coincides with the shift of the glauconite content in the sediments. Hansen and Nielsen (2003) sug-gested that this shift was connected to a sea-level change and that the higher glauconite content and the higher abundance of Asaphus indicate environments corresponding to lowstands, and the higher abundance of Ptychopyge with highstands. They describe several sea-level varia-tions within this interval but our data resolution is too low to trace these fluctuations. The peak in the easternmost area is greater at the A. lepidurus level than at the A. expansus level. This peak is probably exaggerated as a result of extreme taxonomic splitting by Balashova (1964, 1976) but it does indicate the high variability of this group during that time. Ptychopyge s.l. becomes extinct at the Kunda–Aseri boundary together with Megistaspis (Table 1). A closely related genus Pseudobasilicus, appears (cf. the latest rise in Fig. 4C) in the muddy sediments of the Elnes Formation in the Oslo area (Hansen, 2009), and later spreads over the shelf in the Central Baltoscandian and North Estonian belts until the earliest Katian. The youngest representative of this genus Pseudobasilicus kegelensis,is found in the lowermost peripheral beds of the carbonate mounds of the Vasalemma Formation of the Keila Stage (Hints et al., 2004). These mounds continue into the Oandu Stage and contain the earliest heliolitid corals recorded in Baltoscandia (Mõtus and Zaika, 2012). This interval corresponds to when the Baltic terrane was positioned in lower latitudes with warmer waters unfavourable to the asaphid trilobites which gradually disappeared forever. 4.3.4. Isotelines Isotelines are in general relatively large elongated asaphids with a broad concave border; often with weakly defined glabella and pygidial pleurae best seen on the internal mould. Jaanusson (in Moore, 1959) divided them into A and B groups based on the presence or absence of the posterior notch of the hypostoma. GroupA, genera possess a forked hypostome similar to the Asaphinae and Ptychopyge-group, and it includes the rare Baltoscandian genera Megalaspides, Lannacus, Homalopyge, Isotelus, and probably Brachyaspis (Table 1). Of these, Megalaspides is very similar to the Ptychopyge s.l. (e.g. Ptychopyge plautini) but outwardly differs in having a faintly expressed glabella, although when seen in the right light, it resembles that of the ptychopygids. The typematerial needs to be examined before this can be confirmed. Isotelus? robustus is an illaenimorph with its ill-defined axial region, but as an isoteline it has a forked hypostome, long effaced pygidial rhachis reaching close to the posterior border, and Panderian openings on the thoracic pleurae and librigena. This species appears in accumulationswithin the Pirgu carbonate sediments together with colonies of an early heliolitine coral, Protoheliolites (Liang et al., 2013). Group B (Fig. 4D) has no posterior notch on the hypostome and the border is usually very short (cf. Schmidt, 1906, pl. 5, Figs. 6–7) and often pointed medially. Included in this group are the Baltoscandian genera Asaphellus, Hunnebergia, Megistaspis including Megistaspis (Ekeraspis), Megistaspis (Heraspis), Megistaspis (Megalaspidella), Megistaspis (Paramegistaspis), and Megistaspis (Rhinoferus). Except for Asaphellus this group encompasses the Baltoscandianmembers ofMegistaspidinae pars sensu Balashova (1976). They first occur in the Bjørkasholmen For-mation in the western belts, and are recorded in first but somewhat younger carbonate accumulations in the eastern belts (Fig. 4D). The megistaspine trilobites are extremely abundant in the upper Floian and lower Dapingian limestones, especially in the North Estonian Confacies Belt, where successive accumulations of these trilobites occur (Pärnaste, 2003, 2006). The late Floian so called ‘trilobite-graveyards’ contain nearly monospecific accumulations of disarticulated remains of Megistaspis (Paramegistaspis) estonica. Nielsen's study (1995) revealed that Megistaspis is rather abundant throughout the Dapingian though rates of abundance fluctuate throughout the sections (cf. Nielsen, 1995, Figs. 44–48) and can be correlated with changes in sea-level. The Megistaspis (Megistaspis) biofacies was defined by Nielsen (1995) as containing 20–35% of the entire assemblage, and as representing the low-est sea-level even far off-shore (Nielsen, 1995, Figs. 47, 49). Our data also confirms that the most diversified Megistaspis fauna occurs in near shore areas in the eastern belts with the highest peak at the Asaphus lepidurus level on Öland and in Ingria. The number ofmegistaspine species remains stable in these regions until their sudden disappearance and extinction at the boundary betweentheKunda andAseri regional stages, in the middle of the Darriwilian Stage (Table 1). The megistaspine species differ from asaphines in having a wider geographical range and higher diversity. This group of asaphids is peculiar in having a characteristic broad bor-der area (e.g. Schmidt, 1906; pls. 5–7) that may well have aided move-ment along the bottom, either hard or soft or it could have provided extra buoyancy during swimming or sliding over the substrate. The mid-dle portion of the cephalon is vaulted and exhibits several morphological trends. Some cephala are rather flat and smooth with glabellar furrows nearly effaced (subgenera Paramegistaspis, Megistaspidella), others have prominent glabellar furrows especially adjacent to the axial furrow (subgenera Megistaspis, Rhinoferus) while a medial glabellar ridge de-velops and is directed towards the occipital ring (Rhinoferus hyorrhina group), or a prominent hump is present on the anterior part of the gla-bella (Rhinoferus gibba group). The preglabellar field carries distinctive features including a ridge extending from a shallow, concave, area. The hypostoma has a slightly notched or smooth posterior margin and large vaulted anterior wings which result in a large space below the gla-bella. This space could have been filled with a relatively large stomach implying that they were particle feeders. Their eyes are large like those of the asaphines suggesting life in a similar environment with possibly low light inhabited by large numbers of predators such as cephalopods. Judging from the high variability of the morphology of this group it is obvious that theywere highly specified and their disappearance may well have been caused by a sudden change in the environment, disappearance of certain food chain elements or a catastrophic event. It is no coincidence that the period of extinction corresponds to the wide distribution of tuff beds in Sweden and Estonia (Sturesson, 1992a,b; Sturesson et al., 2005) and a rapid changeover in trilobite faunas (Aru, 1990; Rõõmusoks, 1997; Pärnaste et al., 2009), accompanied by extinction of the lingulid and rhynchonellid brachiopods (Sturesson et al., 2005). This event coin-cides with a positive shift in the isotopic curve starting from the late Kunda Stage and reaching to the Aseri Stage in Baltoscandia (Meidla et al., 2004; Kaljo et al., 2007) reflecting a cooling period (Rasmussen et al., 2009; Ainsaar et al., 2010). A trend of exoskeletal enlargement is obvious in megistaspines and asaphines (e.g. Megistaspis rudis, Megistaspis gigas, Asaphus platyurus etc.) during this period. Giant trilobites occur at the high southern latitudes on themargin of Gondwana and are considered to be a cold-water adaptation such as those of the Iberian, Valongo Forma-tion. This fauna is of middle Darriwilian age (Didymograptus artus grapto-lite biozone, Gutiérrez-Marco et al., 2009) and correlative to the late Kunda and Aseri faunas in Baltoscandia. 4.4. Biodiversity pattern of Asaphidae in Baltica The biodiversity curve (Fig. 5), presenting a number of species for each stratigraphical unit exhibits the major evolutionary trends and events of the Asaphidae in Baltica during the Ordovician Period.
  • 10. 4.4.1. Furongian appearance H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 73 The first asaphids to appearwere representatives of the Niobinae (see Section 4.2), that were related to the latest Cambrian faunas in Siberia. Niobe and Promegalaspides were cosmopolitan during the Tremadocian, reaching the Uralian side of Baltica and spreading over wide areas at the western edge of Gondwana in cool and temperate climates. 4.4.2. Mid Tremadocian–mid Floian Rise The first members of all the major lineages of asaphids appeared to-gether with the Ceratopyge Fauna (Varangu Stage, AIII; Table 1, Fig. 5). The gradual rise of the biodiversity curve includes a small diversification of niobines, development of the first megistaspines that show different morphologies in their general body shape, and appearance of the first asaphines. Fifty species are known from this period with twenty-one of them from the uppermost mid Floian Megalaspides dalecarlicus Zone. This time-frame also includes development of the Asaphid Fauna on the Uralian side of Baltica. Niobe, Niobella, Gog, Asaphellus, and Promegalaspides are represented by a few species from the Southern Urals, and the species of Niobella, Asaphellus, and Megalaspides from Pay-Khoy, north of the Urals in the Tremadocian and the lower Floian beds (Ancygin, 2001; Bergström et al., in press). 4.4.3. Late Floian Fall The late Floian, Megistaspis estonica level, illustrates a sudden drop to only eight species. This minimum was also recognised by Hammer (2003), but not by Adrain et al. (2004), who used larger basic units. A corresponding decrease is recorded in the Baltoscandian brachiopod faunas (Schmitz et al., 2008) and this decline coincides with a period of very low rates of carbonate sedimentation. The thickest limestone suc-cession containing numerous beds is just over onemetre in theNärke re-gion, Sweden (Tjernvik, 1956, Figs. 10, 12; Lindström, 1963), or less than one metre in the Popovka and Lava sections in north-eastern Russia (Pärnaste, 2006). In the other regions thicknesses are much lower and in underlying carbonate beds even more so. Thus, decrease in diversity must have been the result of other factors. Maybe this was a time when the Baltic area located in the region of the ocean current circulation influenced by the winter–summer amplitude (Fig. 1A) which was per-haps less suitable for these trilobites. These terms may explain also the poor rate of carbonate accumulation. The Uralian trilobites Niobella and Megalaspides do not occur above the upper Floian in the Pay-Khoy and Vaygach areas, to the north, and no asaphids are recorded in the south (Bergström et al., in press; Pärnaste and Bergström, in press), which may be due to the drop in sea-level globally (Basal Whiterock Lowstand of Nielsen (2004). 4.4.4. Dapingian Rise with Late Dapingian Peak The boundary between the Floian and Dapingian stages marks the first appearance of the Ptychopyge-group (see Section 4.3.3), which may be derived from Megalaspides (Megalaspides). The first Asaphus, Asaphus broeggeri, shares intermediate characters with Ptychopyge and may have evolved from a common ancestor. The most sudden burst in diversity coincides with an extension of carbonate sedimentation throughout the basin during latest Dapingian times. Both Megistaspis and Ptychopyge, ac-quire various morphologies over the entire shelf area, especially in the shallow water of the North Estonian Confacies Belt (see Section 4.3.4) and in all, forty-four asaphids are recorded from the latest Dapingian. The extension of Megistaspis and appearance of the first Asaphus, is recorded from this period in the Pay-Khoy and Vaygach areas, north of the Urals. The late Dapingian Peak in the trilobite biodiversity curve is shown by Hammer (2003), but is not visible on the normalised trilobite curve of Adrain et al. (2004), based on the same data. Brachiopods and other shelly fossils show no rise in the uppermost Volkhov, but this can be recognised in the succeeding Kunda Stage (Sturesson et al., 2005, fig. 5; Schmitz et al., 2008, fig. 3). The trilobite peak is highlighted by radia-tion of the isotelines over the entire shelf (Fig. 4). Number of species Asaphinae Isotelinae Tr1 Tr2 Tr3 Fl1 Fl2 Fl3 Dp1-2 Dp3 Dw1 Dw2 Dw3 Sa1 Sa2 Ka1 Ka2-3 Ka4-5 Hi1-2 Tremadocian Floian Dapingian Darriwilian Sandbian Katian Hi 50 45 40 35 30 25 20 15 10 5 50 45 40 35 30 25 20 15 10 5 0 Niobinae 0 Asaphidae Niobinae Isotelinae Asaphinae AII AII AIIIa AIIIa AIIIb AIIIb BIα1 BIα1 BIα2 BIα2 BIβ BIβ BIγ BIγ BIIα BIIα BIIβ BIIβ BIIγ BIIγ BIIIα BIIIα BIIIβ BIIIβ BIIIγ BIIIγ CIa CIa CIb CIb CIc CIc CII CII CIII-DI CIII-DI DII DII E E FIa FIa FIb-FIc FIb-FIc FII FII A B Fig. 5. A–B. Graphs showing number of asaphid taxa throughout the entire Ordovician.
  • 11. 74 H. Pärnaste, J. Bergström / Palaeogeography, Palaeoclimatology, Palaeoecology 389 (2013) 64–77 4.4.5. Early–mid Darriwilian High The zone of Asaphus expansus and the succeeding two zones of the Kunda Regional Stage contain fewer species of asaphids but the genera are the same as in the underlying stage. In addition to these early gen-era, several newcomers appear, including the short-lived Homalopyge, which occurs in the near shore facies on Öland and in the North Esto-nian Belt. This isoteline may be related to a Laurentian stock. Of other genera, Ogmasaphus and Pseudobasilicus appear in the Oslo Region during the latest Kunda Age and are probably of Baltoscandian origin. Ogygiocaris is recorded also from the Darriwilian Stage in the north of the Urals (Ancygin, 2001; Bergström et al., in press) and arrives in deeper water environments, including the Oslo Region at this time. The occurrence of Megistaspis elongata s.l. on both sides of Baltica reflects the exchange of faunas around this time. In Baltoscandia Megistaspis elongata is common in the lower Darriwilian, Asaphus expansus Zone (Nielsen, 1995), but its close relative M. pseudolimbata occurs in the upper Dapingian, Asaphus lepidurus/Megistaspis limbata Zone (Balashova, 1976; Pärnaste et al., 2013; Table S8). These two periods place a time limit for the possible connection between the two sides of Baltica. The Uralian species, Megistaspis aff. elongata (cf. Burskiy, 1970) ismost similar tothetype species fromtheA. expansus Zone in Baltoscandia and if this is the case, it would indicate that both areas were in contact during the early Darriwilian. The connection might well have been via the Moscow Basin where these asaphids have been recorded from boreholes (Dmitrovskaya, 1989; Bergström et al., in press) or during a sea-level rise in the latest Volkhovian, i.e. latest Dapingian (Hints et al., 2010, fig. 6). 4.4.6. Kunda/Aseri Turnover (mid Darriwilian Turnover) The richness of species remains stable during the Kunda/Aseri transition but a remarkable faunal turnover occurs (Table 1). The Aserian interval was favourable for a rapid trilobite evolution, including for the Asaphus s.l. group. There are twenty-two (sub)species recognised from the shallow shelf of Ingria. Other asaphid genera contain only a few species and Niobe and the megistaspines disappear at the Kunda/Aseri boundary corresponding to that between the Öland and Viru Series (Fig. 2). A similar pattern cannot be recognised in other groups although brachiopod diversity is drastically reduced (e.g. Hints and Harper, 2003, fig. 3; Sturesson et al., 2005, fig. 5). 4.4.7. Post-Aserian Fall In the development of the Asaphid Fauna during the next period a slow decline is observed leading up to its extinction towards the end of the Ordovician. In detail, this period can be subdivided into the following three diversity events: 4.4.7.1. Late Darriwilian Fall. The number of Asaphus s.l. species falls by at least a half by the beginning of the late Darriwilian. Ogygiocaris becomes most diversified during the late Darriwilian, and Pseudomegalaspis disap-pears at the boundary with the Sandbian. 4.4.7.2. Sandbian Fall. A further reduction of the Asaphid Fauna, by one half, occurs at the base of the Kukruse Stage, which otherwise is a time of high diversity within the Baltoscandian trilobite faunas (cf. e.g. Hammer, 2003; Pärnaste et al., 2009; Pärnaste and Popp, 2011) related to the massive accumulation of algae in the ‘Oil Shale’ beds of the Viivikonna Formation in the North Estonian Belt, thus forming a rich food supply for the shelly faunas including the trilobites. Subsequent to the Kukruse Stage Pseudasaphus disappears from the near-shore areas, and Ogygiocaris, from neighbouring off-shore areas. Ogmasaphus and Pseudobasilicus disappear higher in the succession. Isotelus has been recorded also fromthe approximately contemporane-ous sediments in Pay-Khoy, Arctic Russia on the Uralian side of the Baltica Plate (Owens and Fortey, 2009). 4.4.7.3. Katian gradual extinction. Fromthe Oandu Age onwards only a very few asaphid taxa inhabited the near-shore facies of the Baltoscandian Basin at a timewhen the Baltic terrane was located at lower latitudes with a warmer climate and this together with a change in sedimentation type was unfavourable for this group of trilobites. Isotelus, represented by a few species from the Rakvere Age onwards, may represent immigration from Laurentia or from Pay-Khoy. By the end of the Ordovician all asaphids have become extinct. 5. Conclusions The earliest Baltic asaphids appear in marginal areas of the Baltoscandian palaeobasin during the latest Cambrian. The first to appear is an obscure genus Eoasaphus, later followed by Niobella and Promegalaspides known also from Siberia, indicating a possible connection with the Baltic area. Niobella and Niobe were cosmopoli-tan during the Tremadocian and were among those of the Ceratopyge Fauna to survive the Ceratopyge Extinction Event. The Baltoscandian asaphids are foundwithin slightly differing sets of genera to the North and South of the Uralian side of Baltica during the Tremadocian and early Floian, but in the late Floian the genera are observed to be in common only with the northern area (in today's terms). In the latest Dapingian or early Darriwilian a species level con-nection by Megistaspis elongata s.l. marks a close connection between the two sides of Baltica, which might be related to the sea-level rise. The Asaphid Fauna is far from uniform. Four morphologically differ-ent groups with diverse feeding habits and locomotion evolved, leading to different patterns of distribution over the shelf of the Baltoscandian Basin. The dynamics of radiation and extinction varies between these dif-ferent groups and was determined by various environmental changes. From the Floian onwards carbonate sedimentation extended over broad areas of the basin and numbers of asaphids gradually increased. The late Tremadocian to Dapingian includes a gradual rise of niobines and the appearance of the first megistaspines showing novel morphol-ogies in body shape. In the middle of this rise one abrupt fall (the late Floian Fall) occurs, corresponding to a drop in sea-level, with a subse-quent recovey of faunas containing new forms. The Dapingian Rise endswith a sudden burst involving diversification of the ptychopygines and occurrence of the megistaspines all over the shelf. The succeeding Darriwilian High is characterised by the growth of a stabilised asaphid fauna with various morphotypes sharing the same space in the basin. Destabilisation of the faunas, probably resulting from a catastrophic event, occurs in the transition of the Öland and Viru series (Kunda and Aseri stages). Three morphotypes, adapted to a soft substrate, van-ish suddenly in the mid Darriwilian Turnover and from then on, loss is stepwise until the beginning of the Hirnantian. A group of asaphines with ill-defined axial regions, with a morphol-ogy resembling the illaenids, may have been adapted to a life as preda-tors inmud-mounds and reefs in a similarmanner to the illaenids. These illaenimorph asaphids appear twice: in the late Kunda, and Rakvere stages, and are the last asaphids occurring in Baltoscandia. They become extinct by the Hirnantian. Acknowledgements The inferences of this paper are the fruit of research done in inten-sive cooperation during the two years before Jan Bergström's unfortu-nate death in November 2012. The results were presented at the IGC in Brisbane, Australia in August 2012. Helje Pärnaste acknowledges support from the Estonian Research Council (grants ETF8054 and SF0140020s08). The present study is a contribution to the Interna-tional Geoscience Programme project 591 ‘The Early to Middle Paleo-zoic Revolution’. I thank two referees and guest editor Dr. Kathleen Histon for their valuable notes, which improved the manuscript, and I am especially thankful to Professor emeritus David L. Bruton for his incredible contribution in correcting the language.
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