ECOLOGICAL DOMINANCE OF EARLY TRIASSIC DISASTER BIVALVES IN PANTHALASSA AND TETHYS BENTHIC PALEOCOMMUNITIES

Post end-Permian mass extinction marine assemblages exhibit decreased faunal diversity, abundance, and community eveness as a consequence of environmental stresses. A group historically labeled as the “disaster bivalves” has been qualitatively observed by previous workers to numerically dominate many Early Triassic collections around the world. These four genera, Claraia, Eumorphotis, Promyalina, and Unionites, are hypothesized to fill the role of opportunistic generalists, thriving in the depauperate conditions after the extinction. Presented here is preliminary work quantifying the nature of disaster bivalve dominance in Early Triassic benthic communities in Panthalassa and Tethys carbonate deposits. Paleobiology Database (PBDB) entries for North American collections are compared to database and field collections made in Italy using numerous community ecology metrics (mean rank abundance, breadth of distribution, relative abundance), calculated at the genus level. Database entries were deemed suitable for this study if abundance counts of all bivalves, brachiopods, and gastropods were reported, and more than 5 individuals were counted. Field collection abundance counts were made from bulk samples. Preliminary results show high ecological importance of Unionites and Claraia in Induan North American collections, based on mean rank abundance and distribution, which then diminishes in the Olenekian. Promyalina and Eumorphotis increase in ecological importance over the course of the Early Triassic, with Promyalina becoming the highest ranking disaster bivalve in Olenekian North American communities. A similar ecological dominance pattern is observed in Italian collections, except Promyalina is not as important. Community eveness, based on Simpson’s Dominance Index, remains relatively unchanged throughout the Early Triassic, while Middle Triassic assemblages are significantly more even. Interestingly, the relative abundance of the disaster bivalves as a group decreases over time as other genera become the ecological dominants. This second wave of low diversity, high dominance taxa differ in composition between the North American assemblages, which see a resurgence of brachiopod ecological importance, and Italian assemblages, where the dominant taxa are typically bivalves.