WHEN BIVALVES TOOK OVER THE WORLD

The Permian-Triassic boundary marks the most devastating mass extinction in the history of life and signifies the drastic faunal change that characterizes the end of the Paleozoic and the beginning of the Mesozoic. One particularly striking aspect of this faunal change is the abrupt post-Paleozoic switch from brachiopod-dominated to bivalve-dominated level-bottom marine communities, a phenomenon dubbed the "brachiopod-bivalve question". During the Paleozoic, brachiopods were extremely abundant and diverse in benthic faunas around the world, while bivalves played only a secondary role. Then, as a result of the end-Permian mass extinction, brachiopods suffered their greatest crisis and bivalves escaped relatively unscathed. Since then, bivalves have surpassed brachiopods in number and diversity and have "taken over the world". The "brachiopod-bivalve question" remains largely unanswered and, until this study, no quantitative study of Early Triassic bivalve paleoecology has ever been conducted.

Field work from nearshore to slope environments from three temporally-distinct western Pangea seaways was conducted to determine bivalve paleoecology during the Early Triassic. Bivalves are typically the largest fossils within these generally diminutive faunas and are characteristically ecologically dominant. Only crinoids ever commonly exceed bivalves in abundance, but not until the late Early Triassic. Other groups are occasionally locally more abundant than bivalves (e.g., microgastropods and Lingula), but these abundances represent the "boom" times characteristic of biotic recovery opportunism and bivalves are still ever-present as the second most abundant group in these intervals. Except for opportunistic pulses of Lingula in the Griesbachian, brachiopods are extremely rare. Bivalves clearly constitute a cosmopolitan fauna that dominates Lower Triassic strata in the western United States. It is evident that the end-Permian mass extinction and its aftermath played a pivotal role in the post-Paleozoic shift from brachiopod-dominated to bivalve-dominated marine paleocommunities. The ultimate goal of this research is to delineate the paleoecological and paleoenvironmental context of bivalves during the very beginning of their dominion in the world’s oceans.