A GLOBAL RECORD OF ECOLOGICAL RESURGENCE OF THE PALEOZOIC EVOLUTIONARY FAUNA IN THE MIDDLE TRIASSIC

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 Permian-Triassic boundary. Although a corresponding transition in ecological dominance must have occurred, its timing is less clear. One way to assess ecological dominance is through the study of bioclastic accumulations. To test whether the Modern Fauna were ecologically dominant following the end-Permian mass extinction, we compiled a global survey of bioclastic accumulations from the Middle Triassic, the interval after which the environmental stress associated with the end-Permian extinction abated.

Middle Triassic bioclastic accumulations from level-bottom marine deposits in Canada, Nevada, Germany, Hungary, Israel, and China were examined. The most common shell bed-producer differed amongst the sites (bivalves, constituents of the Modern Fauna, in Germany and Nevada and brachiopods, constituents of the Paleozoic Fauna, in Canada, China, Hungary, and Israel). In addition, four out of six sites (Canada, Germany, China, Hungary) contained significant bioclastic accumulations (up to several meters thick) comprised of crinoids, also constituents of the Paleozoic Fauna.

Crinoids suffered drastic diversity losses at the end-Permian mass extinction and maintained at relatively low diversity levels through the Middle Triassic. Despite reduced diversity, this study shows that crinoids temporarily 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, temporarily regained ecological dominance in several widely disparate locations. Thus the transition to ecological dominance of the Modern Fauna was not coincident with the end-Permian mass extinction and was therefore decoupled from the transition to Modern faunal taxonomic dominance.

Since the transition to Modern faunal ecological dominance was not synchronous with the end-Permian mass extinction, its ultimate cause remains unclear. Future studies of ecological dominance, including bioclastic accumulation studies, may elicit the timing and ultimately the cause of this transition.