Evolutionary Biology

Multiple origins of dorsal ecdysial sutures in trilobites and their relatives

Kun-sheng Du
Jin Guo
Sarah R Losso author has email address
Stephen Pates author has email address
Ming Li
Ai-lin Chen author has email address

Research Center of Paleobiology, Yuxi Normal University, Yuxi 653100, China
Key Laboratory for Palaeobiology and MEC International Joint Laboratory for Palaeoenvironment, Institute of Palaeontology, Yunnan University, Kunming 650091, China
Management Committee of the Chengjiang Fossil Site World Heritage, Chengjiang, China
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA
Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Penryn, Cornwall, TR10 9FE, UK
Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, CAS, Nanjing, 210008, China

https://doi.org/10.7554/eLife.93113.3

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Figures and data

Hypotheses of homology in sutures between Acanthomeridion and trilobites.
(a) Cephalic morphology of Acanthomeridion serratum. (b) Cephalic morphology of a proetid trilobite (modified from Daley and Drage, 2016). (c) Cephalic morphology of a redlichid trilobite (modified from Whittington 1989). (d) Ventral plates and librigenae as homologous with dorsal suture. (e) Ventral plates and doublure as homologous with marginal suture. (f) Non-homology between ventral plates of Acanthomeridion, librigenae and doublure.

Sites yielding Acanthomeridion from the Cambrian Stage 3 Chengjiang Biota (red circles).
All the specimens of A. serratum used here are collected from Jiucun town, Chengjiang County.

Acanthomeridion serratum from the Cambrian Stage 3 Chengjiang Biota. (a, b) CJHMD 00052a/b respectively, individual with hypostome, ventral plates, and 11 thoracic tergites. (c, d) CJHMD 00053a, showing ventral plate, dorsal sutures, and 11 thoracic tergites. (e–g) YRCP 0016a, showing the ventral plates, and three post-antennal limbs. (h–k) Showing the post-antennal appendages under head, gnathobases of trunk limbs, stick-like exopodites with bristles (black arrows), and endopodites with long spines (white arrows). (f) Micro-CT image of YRCP 0016a; (h, i, k) Micro-CT images of CJHMD 00053a. Abbreviations: ant, antenna; can, post-antennal appendage n beneath head; dbl, doublure; ds, dorsal suture; en, endopodites; ex, exopodites; ey, eye; hs, head shield; hy, hypostome; L, left; lam, lamellae; no, notch; pn, podomere n; R, right; Tn, tergite n; ts, terminal spine; vp, ventral plate.

Acanthomeridion serratum from the Cambrian Stage 3 Chengjiang Biota (a) CJHMD 00054, complete individual with head, eye, and 11 tergites. (b) Micro-CT image of panel (a) showing the stick-like exopods. Abbreviations: ex, exopod; ey, eye; es, eyestalk; hs, head shield; Tn, tergite n.

Acanthomeridion serratum from the Cambrian Stage 3 Chengjiang Biota. (a–e) CJHMD 00055, showing the antenna, ventral plate, endopodites with long spines (arrows in d), 11 tergites, and paddle-like structure (red arrow). (a) Overview of whole specimen. (b) Detail of long left antenna. (c) Close-up of ventral plate. (d) Details of long spines (arrows) of right endopodites. (e) Close-up of paddle-like structure (red arrow). Abbreviations same as Figs. 3, 4.

Ontogenetic series (a–r) of Acanthomeridion and their ventrally curling pleurae (s, t).
(a–r) Showing the individuals from smallest to largest with same scale bar. (s) Lateral view of (o), note the right curling pleurae and left flat pleurae. (t) Lateral view of (f), showing the left curling pleurae.

Scatterplot of axial length and width of thorax of Acanthomeridion serratum.
(a) scatterplot of raw data. (b) data with 95% forecast shown in pink. Specimens previously assigned to A. anacanthus indicated by blue lettering.

Reconstruction of Acanthomeridion serratum in dorsal and ventral view.
Only the protopodite of the head appendages have been observed. There appear to be three post-antennal limbs with the fourth pair below the cephalic-thoracic boundary.

Four representative artiopodans from early Cambrian with their dorsal sutures, free cheeks and ventral plates.
(a, b) Trilobite Wutingaspis tingi from Chengjiang Biota, note its free cheeks and dorsal or facial sutures. (c, d) Xandarellid artiopodan Xandarella spectaculum from Chengjiang Biota bearing the distinctive dorsal sutures. (e, f) Holotype of the protosuturan artiopodan Zhiwenia coronata from Xiaoshiba Biota developing dorsal sutures. (g) The left dorsal suture of Acanthomeridion serratum from Chengjiang Biota, showing the morphological and positional similarities to that of W. tingi (a, b), X. spectaculum (c, d), and Z. coronata (e, f). (h, i) Right ventral plates of A. serratum from Chengjiang Biota bearing a terminal spine, which is similar to free cheek of trilobite like W. tingi (a, b).

Simplified results of phylogenetic analyses.
Comparison of results from matrices with different coding strategies, with coding of key characters for terminals within the analysis indicated by colored circles. (a) Ventral plates considered homologous to free cheeks. (b) Ventral plates considered homologous to trilobite doublure. (c) Ventral plates not considered homologous to any artiopodan character.

Treespace visualisation of Bayesian Inference phylogenetic analyses.
Multidimensional treespace plotted by bipartition relating to coding strategies (color of point). (a) Analyses were unconstrained. (b) Trilobites were constrained so that Olenellus getzi was resolved as the earliest diverging member of the group. Area included within a 95% confidence interval (CI) indicated by points inside the line of different colors. Area where Acanthomeridion is recovered sister to trilobites (95% CI) also shown by area within black line. Where Acanthomeridion is not recovered sister to trilobites (95% CI), shown by area within grey line. Note the overlap in the areas of the black and purple lines.

number of trees in posterior sample supporting Acanthomeridion as sister to trilobites / number of trees in the posterior sample total. Results tabulated for each coding strategy, with unconstrained and constrained analyses (constrained analyses where a monophyletic group of all trilobites except for Olenellus getzi was forced, in order to recover trilobite relationships compatible with Paterson et al. 2019).

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