PALEOECOLOGY OF BIVALVES DURING THEIR INITIAL RISE TO DOMINANCE

Perhaps the most intriguing and long-lasting legacy of the end-Permian mass extinction 251 million years ago is the numerical domination of bivalves in today’s skeletonized benthic marine fauna. Previous studies using taxonomic data have indicated that the end-Permian mass extinction and the subsequent recovery during the Early Triassic were crucial in the rise of the Class Bivalvia: the Permian/Triassic boundary marks the phyletic change from the rhynchonelliform brachiopod-dominated Paleozoic Fauna to the bivalve-dominated Modern Fauna, and bivalves survived the end-Permian mass extinction better than any other group. This research represents the first quantitative study of the paleoecology of Early Triassic bivalves in an attempt to determine what roles the end-Permian mass extinction and its aftermath played in the bivalve rise to dominance.

Through a detailed, global, field-based study examining strata deposited in a wide range of marine environments during all stages of the Lower Triassic, a broad picture of Early Triassic bivalve paleoecology is emerging. The majority of examined shell beds, ranging from centimeters to meters in thickness, are dominated by bivalves. These include shell beds formed in nearshore to outer shelf environments in Lower Triassic strata deposited in eastern Panthalassa, represented by the Dinwoody, Moenkopi, and Thaynes Formations of the western United States, and in Lower Triassic strata deposited in western Tethys, represented by the Werfen Formation of northern Italy. Bivalves also dominate lags formed in nearshore siliciclastic environments of the Lower Triassic Hiraiso Formation of Japan. In the Lower Triassic Kamura Formation of Japan, bivalves dominate sediments deposited on a seamount in open-ocean Panthalassa; bivalve grainstones reach up to 6 meters in thickness. The same bivalve genera, Eumorphotis, Promyalina, Claraia and Unionites, occur abundantly in all of these global localities. Rhynchonelliform brachiopods are nearly completely excluded from Lower Triassic strata. Around the world, only inarticulate brachiopods, microgastropods, and crinoids intermittently compete with bivalves as the dominant skeletonized biota. This global domination by bivalves during the Early Triassic does not appear to be a taphonomic effect.