Thompson JR et al. (2018), A new stem group echinoid from the Triassic of ...

ECB-ART-46117

R Soc Open Sci 2018 Jan 01;51:171548. doi: 10.1098/rsos.171548.

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A new stem group echinoid from the Triassic of China leads to a revised macroevolutionary history of echinoids during the end-Permian mass extinction.

Thompson JR , Hu SX , Zhang QY , Petsios E , Cotton LJ , Huang JY , Zhou CY , Wen W , Bottjer DJ .

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The Permian-Triassic bottleneck has long been thought to have drastically altered the course of echinoid evolution, with the extinction of the entire echinoid stem group having taken place during the end-Permian mass extinction. The Early Triassic fossil record of echinoids is, however, sparse, and new fossils are paving the way for a revised interpretation of the evolutionary history of echinoids during the Permian-Triassic crisis and Early Mesozoic. A new species of echinoid, Yunnanechinus luopingensis n. sp. recovered from the Middle Triassic (Anisian) Luoping Biota fossil Lagerstätte of South China, displays morphologies that are not characteristic of the echinoid crown group. We have used phylogenetic analyses to further demonstrate that Yunnanechinus is not a member of the echinoid crown group. Thus a clade of stem group echinoids survived into the Middle Triassic, enduring the global crisis that characterized the end-Permian and Early Triassic. Therefore, stem group echinoids did not go extinct during the Palaeozoic, as previously thought, and appear to have coexisted with the echinoid crown group for at least 23 million years. Stem group echinoids thus exhibited the Lazarus effect during the latest Permian and Early Triassic, while crown group echinoids did not.

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	Figure 1. Specimens and location of Y. luopingensis n. sp. (a) Locality map showing the location of the Luoping Biota marked as star. Adapted from [18] (b) specimen 61701; note the bulge in the centre of the test which probably indicates the Aristotle's lantern inside of the compressed test. (c) Specimen 32321 which shows an apical view of a compressed test with apical disc with genital plates, an ocular plate and the madreporite. (d) Specimen 61163 showing a compressed test with spines. (e) Close-up of spines and ambulacral plate on specimen 32321. Note the absence of a milled ring and the striate nature of the spines. (f) Close-up view of the madreporite, ocular plate and adapical coronal plating of specimen 32321. Note the imbrication of the plates, with more adoral plating imbricating over more adapical plates. (g) Close-up of coronal plating and spines on specimen 32321. Spines and tubercles are arranged in distinct rows with larger spines lying slightly below corresponding imperforate and non-crenulate tubercles. Scale bars in (b,d) are 1 cm, bar in (c) is 2 mm and bars in (e–g) are 500 µm.
	Figure 2. Results of phylogenetic analyses showing phylogenetic position of Y. luopingensis n. sp. (indicated in bold) relative to Late Palaeozoic and Early Mesozoic stem and crown group echinoid genera. (a) Single MPT resulting from equally weighted parsimony analyses using 69 characters and rooted on Echinocystites ponum. Tree length = 121 steps, CI = 0.69, RI = 0.74. Numbers at nodes in bold represent bootstrap proportions resulting from 10 000 ‘fast’ bootstrap replicates while italicized numbers are decay indices for each node. (b) Fifty percent majority rule consensus tree summarizing posterior distribution of trees resulting from Bayesian analyses using a symmetric Dirichlet prior with parameter α = ∞. Numbers at nodes represent PP of each node. Scale bar denotes scale for branch lengths, which are the average branch lengths from the posterior distribution of trees.
	Figure 3. Diversity of echinoids from the Late Palaeozoic to the Early Mesozoic. (a) Stratigraphic ranges of echinoid taxonomic groups included in phylogenetic analyses. The phylogenetic relationship of Yunnanechinus to other stem group echinoids is uncertain, thus its phylogenetic relationships are depicted with a dashed line. The Early Triassic, which experienced significant environmental stress [4,27], lacks any fossil representation of stem group echinoids, which are Lazarus taxa during this time. (b) Diversity curve of stem group and crown group echinoids during the Permian and Triassic. The curve was compiled using the range through method [29]. The dashed line for stem group echinoids in the Induan and Olenekian represents the inferred presence of stem group precursors to Yunnanechinus but which have yet to be sampled from the fossil record. Crown group echinoids are first known from the Roadian, while stem group echinoids go extinct at the oldest in the Carnian. Throughout the entire figure, stem group echinoids are indicated in red and crown group echinoids in blue. Data are compiled from electronic supplementary material, tables S3 and S4, which list Permian and Triassic echinoid taxa.

References [+] :

Hu, The Luoping biota: exceptional preservation, and new evidence on the Triassic recovery from end-Permian mass extinction. 2011, Pubmed, Echinobase