The Mesozoic return of Paleozoic faunal constituents: A decoupling of taxonomic and ecological dominance during the recovery from the end-Permian mass extinction

Abstract

Phanerozoic diversity curves illustrate that the transition from taxonomic dominance of the Paleozoic Fauna to taxonomic dominance of the Modern Fauna is coincident with the Permo–Triassic boundary. Although a corresponding transition in ecological dominance must have occurred, its timing is unclear. One way to assess ecological dominance during the Early-Middle Triassic recovery interval is through the study of bioclastic accumulations. Bioclastic accumulations from the Early Triassic are largely dominated by constituents of the Modern Fauna. In this study, we examined bioclastic accumulations from the Middle Triassic, the interval after the environmental stress associated with the end-Permian extinction abated. Bioclastic accumulations from Panthalassan deposits (Ladinian portion of the Liard Formation at Williston Lake, Canada) and Tethyan deposits (Anisian–Ladinian Muschelkalk Group, Germany) were examined. Although the most common shell bed-producer differed between the two sites (bivalves, constituents of the Modern Fauna, in Germany and brachiopods, constituents of the Paleozoic Fauna, in Canada), both sites, though geographically disparate, contained significant crinoidal bioclastic accumulations (encrinites). Crinoids, like many constituents of the Paleozoic Fauna, suffered drastic diversity losses at the end-Permian mass extinction and maintained relatively low diversity through the Middle Triassic. Despite reduced diversity, bioclastic accumulations show that crinoids regained some measure of ecologic dominance in the Middle Triassic. In addition, brachiopods, another constituent of the Paleozoic Fauna that suffered severe diversity losses at the end-Permian extinction, regained at minimum local ecological dominance in the Middle Triassic of Panthalassa. These data demonstrate that the transition to Modern faunal taxonomic dominance was decoupled from the transition to Modern faunal ecological dominance.