DIFFERENTIATED APPENDAGES IN ISOXYS ILLUMINATE ORIGIN OF VENTRAL APPENDAGES ARTHROPODIZATION

1. DIFFERENTIATED APPENDAGES IN ISOXYS ILLUMINATE ORIGIN OF ARTHROPODIZATION
Caixia Zhang1,2 , Yu Liu1,2*, Javier Ortega-Hernández3*, Joanna M. Wolfe3, Changfei Jin1,2, Huijuan Mai1,2, Xianguang Hou1,2, Jin Guo1,2,4, Dayou Zhai1,2*
1Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming, PRC.
2MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan University, Kunming, PRC.
3Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, USA.
4Management Committee of the Chengjiang Fossil Site World Heritage, Chengjiang, China.
Introduction
 One pair of frontal appendages (raptorial
or antenna-form);
 One pair of stalked eyes;
 ≥ 11 pairs of uniform biramous
appendages (exopods with setae or
lamellae);
 Paired, lobate-to-subrectangular midgut;
 Telson flaps, a symmetrical fan-like
structure.
Isoxys auritus
Isoxys chilhoweanus
Walott, 1890
Isoxys actangulus
Main Materials
Isoxys curvirostratus : YKLP 16260b Isoxys sp. of Vannier et al. (2009) : CFM 00047
Williams et al., 1996;
García-Bellido et al., 2009b;
Fu et al., 2014.
Methods
Zeiss Xradia 520 Versa
X-ray microscope
Keyence VHX 6000
stereomicroscope
Drishti 2.4
5 mm 5 mm

2. Figure 1 Appendicular morphology of Isoxys curvirostratus (YKLP 16260a,b) from the early Cambrian (Stage 3) Chengjiang in South China. a,
Tomographic model. b, Detail of tomographic model. c, Model with carapace digitally removed. d, First trunk appendage pair showing
morphology of endopod (purple) and exopod (blue). e, Second appendage pair. f, Third appendage pair. g, Fourth appendage pair.
Abbreviations: an, the nth ventral appendage; as, anterior spine; en, endopod; es, eye stalk; ex, exopod; ey, eye; fa, frontal appendage; pes, the
paired spines on each endite; pps, paired posterior spines on endopod; ps, posterior cardinal spine of carapace; sp, spines. Scale bars: a, 5 mm; b,
c, 2.5 mm; d, h, i, 1 mm; e–g, 0.5 mm.
YKLP 16260
a b c
d e f g h i
as
ps
ey
fa
a1
a4
a7
a10
a14
a1
a4
en
ex
a1 a2(r) a3(l) a4 a7 a8
en
ex
en
pps
sp
pes

3. Figure 2 Appendicular morphology of Isoxys sp.. a, Complete specimen photographed under reflected light. b, Tomographic model of a. c,
Magnification of b. d, Exopod from left fourth appendage showing elongate shaft and paddle-shaped lamellae. e, Right fourth appendage
pair showing protopod (yellow), endopod (purple) and exopod (blue). f, Rotated 180 degrees of e. Abbreviations: dg, digestive gland; la,
lamellae of exopod; pr, protopod; tr, trunk. Scale bars: a, b, 5 mm; c, 2.5 mm; d–f, 1 mm.
CFM 00047

4. Other Soft Parts
Figure 3 Other soft body evidences from Isoxys in Chengjiang biota. a, a1, YKLP 16261a. a, Isoxys curvirostratus. a1, Magnification of
unarthrodized trunk and paddle-shaped lamellae in exopod. b, b1, YKLP 16266. b, Isoxys cf. curvirostratus with well-preserved frontal
appendages. b1, Detail of elongate terminal claw showing short endites. Abbreviations: ltp, lateral processes of telson. Scale bars: a, b, 5
mm; a1, b1, 1 mm.
a1
b1
a
b
a1
b1
tr
tlp as
a1
fa
ey
a10
a1
4
a5
tr
e
x e
n
ps as
ey
fa

5. Figure 4 Morphological reconstruction and
phylogenetic position of Isoxys curvirostratus. a,
Lateral view. b, Detail of the frontal appendage. c,
Morphology of anterior batch of biramous
appendages (a1–a4). d, Morphology of posterior
batch of biramous appendages (a5–a14). e.
Simplified strict consensus of maximum
parsimony under implied weights. f. Simplified
strict consensus of maximum parsimony under
equal weights. g. Majority rule consensus tree
retrieved with Bayesian inference. h. Treespace
analysis comparing the distribution of topologies
favoring isoxyids versus Kylinxia as the outgroup
to other deuteropods. Numbers indicate endopod
podomeres.
References:
• FU D J, ZHANG X L, BUDD G E, LIU W, PAN X Y. Ontogeny and dimorphism of Isoxys auritus (Arthropoda) from the early Cambrian Chengjiang biota, South China[J]. Gondwana
Research, 2014, 25(3): 975-982.
• GARCÍA-BELLIDO D C, VANNIER J, COLLINS D. Soft-part preservation in two species of the arthropod Isoxys from the middle Cambrian Burgess Shale of British Columbia,
Canada[J]. Acta Palaeontologica Polonica, 2009b, 54: 699-712.
• VANNIER J, GARCÍA-BELLIDO D C, HU S X, CHEN A L. Arthropod visual predators in the early pelagic ecosystem: evidence from the Burgess Shale and Chengjiang biotas[J].
Proceedings of the Royal Society B: Biological Sciences, 2009, 276(1667): 2567-2574.
• WILLIAMS M, SIVETER D J, PEEL J S. Isoxys (Arthropoda) from the Early Cambrian Sirius Passet Lagerstätte, North Greenland[J]. Journal of Paleontology, 1996, 70: 947-954.
Phylogenetic Analysis
 Arthropodization: Either a single origin for trunk arthrodization in Deuteropoda (evidenced by Kylinxia)
with secondary loss among monophyletic isoxyids, or the repeated evolution of trunk arthrodization in
Kylinxia and crown-group Euarthropoda.
 Inference: Isoxyids are viable candidates as the earliest branching members of Deuteropoda.
More details: http://dx.doi.org/10.21203/rs.3.rs-861892/v1 (preprint).