Saturday, March 10, 2018

[Paleontology • 2018] Yunnanechinus luopingensis • A New Stem Group Echinoid from the Triassic of China leads to A Revised Macroevolutionary History of Echinoids during the end-Permian Mass Extinction

Yunnanechinus luopingensis
Thompson, Hu, Zhang, Petsios, Cotton, Huang, Zhou, Wen & Bottjer, 2018

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.

KEYWORDSsea urchin, Triassic, Lazarus effect, echinoderm, Luoping Biota

Figure 1. Specimens and location of Yunnanechinus luopingensis n. sp.
(a) Locality map showing the location of the Luoping Biota marked as star.
(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.

Systematic palaeontology

Echinoidea Leske, 1778
Stem group Echinoidea

Incertae familiae

Genus Yunnanechinus n. gen.

Etymology. Named for Yunnan, China from whence the type species is known.

Type species. Yunnanechinus luopingensis n. sp.

Yunnanechinus luopingensis n. sp.

Etymology. Named for the Luoping Biota, the fossil Lagerstätte from which the species is described.

Diagnosis. Test with imbricate plating, at least adapically and ambitally. Genital plates with one gonopore per plate (figure 1f). Plates of apical system covered with small, imperforate non-crenulate tubercles. Interambulacral plates polygonal to subpentagonal in shape. Interambulacral plates with a single imperforate non-crenulate tubercle, and sparse imperforate non-crenulate secondary tubercles. Spines less than half the diameter of the test in length, finely striate and without a milled ring (figure 1e,g).

Material. The holotype is specimen LPI-32321, paratypes are specimens LPI-2638, LPI-61163, and LPI-61701A,B.

Occurrence. All specimens from the Middle Anisian (Pelsonian) Guanling Formation of the Luoping Biota of Yunnan Province, South China.

Jeffrey R. Thompson, Shi-xue Hu, Qi-Yue Zhang, Elizabeth Petsios, Laura J. Cotton, Jin-Yuan Huang, Chang-yong Zhou, Wen Wen and David J. Bottjer. 2018. A New Stem Group Echinoid from the Triassic of China leads to A Revised Macroevolutionary History of Echinoids during the end-Permian Mass Extinction. ROYAL SOCIETY OPEN SCIENCE. DOI: 10.1098/rsos.171548
For #FossilFriday here's Yunnanechinus luopingensis from our paper that came out earlier this week. The #phylogenetic placement of this species indicates that a lineage of the stem group #seaurchins, which were diverse in the #Palaeozoic Era, actually survived into the #Mesozoic! 

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