Characteristics Oldest known craniates Date back to the middle of Ordovician until the end of Devonian Entire body was covered with bony dermal armour Bony shield head covering Lacked jaws Most lacked paired fins Mostly 2-3 cm long; a few reached up to 2 m long Heterocercal tail
Probably were slow, bottom-dwelling animals Have median fins located down the midline of their backs Gills were located in pouches Cartilaginous internal skeletons A paired of side flaps aid in steering Mouth served to obtain oxygen and retain bits of food and was permanently open Gills were used exclusively for respiration
Used muscular gill pouch to pull in small and slow moving prey
According to E.A. Stensiö – paleontologist at the University of Stockholm Head skeleton of cephalaspis (one species of an ostracoderm) was more or less flattened denticle-covered body shield with four dorsal apertures Two dorsal apertures accommodated a pair of upward-staring eyes One accommodated the median or pineal eye Another accommodated a small, anterior opening that was a single naris from which a nasohypophyseal duct led to an olfactory sac and beyond
lateral fieldnostril eye Dorsal field Pineal foramen
The body shield turned along its lateral edges Beneath the gills the body covering were tile like scales A small mouth that is lined by gills opens to the oropharyngeal chamber is found between the anterior edge of the shield and the scales Curved row external gill slits extends from the corners of the mouth to the caudal margin of the head shield
Head contained an endoskeleton of endochondral bone and considerable cartilage
Acanthodians? Class of Extinct species Share features with Bony and Cartilaginous fishes In form they resemble sharks, but theirepidermis was covered by tiny rhomboid platelets likethe scales of holosteans.Oldest jawed fishesHolosteans – Bony fishes (Gars, Bowfins)
Acanthodians Like Ostracoderms, their head and body were protected by a dermal armor of bony plates and scales
Acanthodians Acanthodians did have cartilaginous skeletons But their fins are bony based with a dentine spine Most Acanthodians have heterocercal caudal fins. Silurian to Permian Period. Marine/Freshwater -Devonian Period: Freshwater species became dominant
Orders Climatiiformes had shoulder armor and many small sharp spines Ischnacanthiformes Had teeth fused to the jawAcanthodiformes filter feeders, with no teeth in the jaw, but long gill rakers.
Spiny Sharks they were superficially shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail - hence, "spiny sharks". However, they were not sharks. Skeleton consisted of bone and cartilage. They had a large operculum.
Brief History Despite being called "spiny sharks," acanthodians predate sharks. They evolved in the sea at the beginning of the Silurian Period, some 50 million years before the first sharks appeared. Later the acanthodians colonized fresh waters, and thrived in the rivers and lakes during the Devonian Period. But the first bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in Permian times (approximately 250 MYA).
Many paleonthologists consider that the acanthodians were close to the ancestors of the bony fishes. Although their interior skeletons were made of cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales. Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle. Others developed a bony flap over the gill openings analogous to the operculum in later bony fishes.
TAXONOMY Domain: Eukarya Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Placodermi Order: Arthrodira Order Antiarchi Order Rhenanida Order Petalichthyida Order Acanthothoraci Order Ptyctodontida
Placoderms Greek word “tablet & skin” referring to their heavy armoured bony plates an extinct class of armored fishes Placoderms evolved from agnathan (jawless) fishes Abundant in the fresh waters of the Devonian era First to evolve true jaws First animals to exhibit internalize egg fertilization
Distinct Characteristics layer of segmented armor across the head and thorax 2 paired fins Depending on species, the remainder of the body would be scaled or naked. sharpened points of bone which protrude from the head to grind their food.
Order Arthrodira Best known placoderms A heavy dermal shield covered the head and gill region and another covered much of the trunk 2 shield meet in a movable joint Remainder of the body: covered with smaller bony scales or,in late species, was naked. 2 pairs upper jaw tooth plates
shark-like bodies Some very large – e.g. Dunkleosteus and Gorgonichthys to 6 - 9 m The gigantic superpreditor Dunkleosteus terrelli
Gorgonichthys sp. Reconstruction of placoderm Gorgonichthys, Cleveland Shale - copyright John Long, Museum Victoria Gorgonichthys sp.
Order Antiarchi Were small placoderms reach a length of 15-20 feet but majority were shorter Exhibit atypical pectoral fins & dorsal eyes Have flattened ventral surface Suggested to be bottom Bothriolepis Canadensi feeders Have eyes on top of their heads Pectoral fins are enclosed in bony appendages
Order RhenanidaCharacteristics of Rhenanids: reduction of the thoracic armour Body is covered by small tesserae (small square of stones) dorso-ventrally flattened shape enlarged pectoral fins known from marine environments and probably pursued a ray-like benthic lifestyle Rhenanida, Gemuendina sp.
Order Petalichthyida Closely related to arthrodires Rarely exceeds a meter They are freshwater bottom dwellers Unlike the Arthrodires, they are bottom-feeding fish Flattened fish with short trunk and long, spine-like plates. The nostrils and the anterior part of the head shield around the orbits, is covered by a number of tiny scales, Lunaspis sp. as is the long trunk.
Order Acanthothoraci Generalized by a stout spine emanating from the median dorsal plate Some acanthothoracids are similar to rays, expanded pectoral fins and flattened appearance Long head shields, very shortened trunk Brindabellaspis stensioi With eyes and nares pointing upwards, suggesting a bottom dweller
Order Ptyctodontida have enlongate bodies whip-like tails reduced head thoracic armor Exhibits sexual dimorphism males of one ptyctodontid, Rhamphod Rhamphodopis opis, have clasper-like intromittent organs similar to those found sharks and their kins
SUMMARYOrder Arthrodira (Arthrodires)– jointed neck–movable joint between the head and the thoracic bodyOrder Antiarchi (Antiarchs)– pectoral fins are enclosed in bony tubes (pectoral appendages)Order Rhenanida(Rhenanids)– enlarged pectoral fins– body covered by unfused scalesOrder Petalichthyida (Petalichthyids)– typified by their splayed (spread out) finsOrder Acanthothoraci– spiny trunkOrder Ptyctodontida– some species exhibit sexual dimorphism– has beak-like mouth
DifferencesOstracoderms Acanthodians Placoderms-oldest known -oldest jawed -First animals tovertebrate fishes exhibit-jawless fishes -their skeleton internalize egg-lacked paired consist of bone fertilizationfins and cartilage -armored fishes with jaws
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