Evolutionary Biology

Multiple origins of cephalic 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
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.93113.1

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Figures and data

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 tergites. (c, d) CJHMD 00053a, showing ventral plate, dorsal sutures, and 11 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–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 Fig. 2.

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.

Artistic reconstruction of Acanthomeridion serratum.
Only the protopodite of the head appendages have been observed.

Four representative artiopodans from early Cambrian with their dorsal sutures, free cheeks and ventral plates.
(a, b) The iconic trilobite Wutingaspis tingi from Chengjiang Biota, note its free cheeks and dorsal or facial sutures. (c, d) The symbolic 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.
(a) Maximum parsimony, a simplified version of Fig. S7a. (b) Bayesian inference, simplified result of Fig. S8a and S9a. (c) Maximum parsimony, a simplified version of Fig. S7b. (d) Bayesian inference, simplified result of Fig. S8b and S9b. Circles indicate presence of cephalic ecdysial sutures. Colors of circles indicate hypothesized homology, as coded in the morphological matrix (either multiple origins – 2 colours – or a single origin – 1 color). Dotted lines indicate the results of minimum assumptions strategy.

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