THE PERMIAN-TRIASSIC MACROEVOLUTIONARY HISTORY OF ECHINOIDS

The Permian–Triassic bottleneck has long been thought to have drastically altered the course of echinoid (sea urchin) evolution. Previous hypotheses postulated that the entire echinoid stem group, which were abundant and morphologically diverse in the Paleozoic, went extinct as a result of the end-Permian mass extinction. However, recent fossil finds of stem group echinoids from the Middle Triassic of France and China are revising the scenario for the extinction of echinoids during the end-Permian extinction. We herein present results using phylogenetic analyses of stem group and crown group echinoids from the late Palaeozoic and early Mesozoic to document a revised macroevolutionary history of echinoids during the end-Permian mass extinction. By including both stem group and crown group echinoids in our analyses, including new and recently described species from the Early and Middle Triassic, we show that stem group echinoids did survive into the early Triassic, and thus were not limited in their occurrence to the Palaeozoic Era. We propose that stem group echinoids exhibited the Lazarus effect during the latest Permian and Early Triassic, implying mass rarity, restricted distribution, low diversity or all of these conditions in the post-Paleozoic stem group. Crown group echinoids, however, were comparatively diverse in the Early Triassic. The crown group and stem group echinoids thus overlap over 20 million years in their stratigraphic ranges, and the initial post-Paleozoic diversification of the crown group echinoids accompanied the decline of the echinoid stem group.