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

Jeffrey R Thompson,Qi-Yue Zhang,Shi-Xue Hu,Wen Wen,David J Bottjer,Elizabeth Petsios,Chang-Yong Zhou,Laura J Cotton,Jin-Yuan Huang

doi:10.1098/rsos.171548

Abstract

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.

Highlights

The effect of the end-Permian mass extinction on the macroevolutionary history of echinoids has become a classic example of the extinction event’s devastating influence on the macroevolutionary history of metazoans [1,2,3,4]
Bayesian and parsimony-based phylogenetic analyses are shown in figure 2, while results of sensitivity analyses are in electronic supplementary material, figures S1–S3
We resolved the members of the echinoid crown group with fairly high bootstrap support (73) and the clade of Archaeocidaris plus the crown group with even higher support (89)

Summary

Introduction

The effect of the end-Permian mass extinction on the macroevolutionary history of echinoids has become a classic example of the extinction event’s devastating influence on the macroevolutionary history of metazoans [1,2,3,4]. Palaeozoic echinoids, which make up the majority of the clade’s stem group, have tests composed of multiple columns of interambulacral and ambulacral plates which articulate flexibly and disarticulated rapidly following death [8,9]. This is in stark contrast to the echinoid crown group, which has a test structure consisting of only two columns of ambulacral plates, and two columns of interambulacral plates, and in many taxa displays test plating with interlocking stereom [6,10]. It was accepted that the Permian–Triassic extinction spelled the end for the echinoid stem group, which presumably never survived into the Mesozoic

Methods

Results

Conclusion