BATTLE OF THE BENTHOS: DOCUMENTING THE TRANSITION FROM BRACHIOPOD TO BIVALVE DOMINATED FAUNAS

Diversity studies indicate that a major reconstructing of marine benthos took place at the Permian-Triassic boundary. This boundary marked the transition between the Paleozoic fauna, dominated in part by brachiopods and crinoids, and the Modern fauna, dominated by bivalves and gastropods. Using abundance data, our study examines the switch between brachiopods and bivalves. It is well documented that Late Permian mass extinctions affected these two clades but a detailed history of this faunal switch has yet to be resolved. To help in understanding this transition, we documented late Paleozoic and early Mesozoic benthic associations and tested the possible driving mechanisms that could control faunal distributions. Subsequently, we compared the late Paleozoic and early Mesozoic results to better understand the causes of faunal patterns exhibited. Using a total of over 387,000 specimens (i.e., 366,321 from the late Paleozoic, 21,671 from the early Mesozoic) from the Paleobiology Database and other literature sources, the similarities and differences between late Paleozoic and early Mesozoic faunal patterns were analyzed using detrended correspondence analysis. Multi-response permutation procedure was utilized to confirm resulting faunal patterns. Among factors hypothesized to control faunal patterns, results indicate that ecology is more important than paleogeography, depositional environment and time. Specifically, a strong segregation between samples dominated by sessile benthos (epifaunal brachiopods and bivalves) and samples dominated by mobile benthos (infaunal bivalves and grazing gastropods) exists. Our results indicate that the relative abundance of sessile benthos versus mobile benthos dictates faunal patterns in both late Paleozoic and early Mesozoic associations. We hypothesize that the end-Permian mass extinction effectively dislodged the incumbent brachiopods allowing stress tolerant epifaunal bivalves to proliferate in the Early Triassic. As environmental conditions improved in the Middle Triassic, sessile benthos such as epifaunal brachiopods and bivalves proliferated and as environmental conditions improved further, infaunal, high-energy, mobile benthos slowly began to replace sessile benthos as the dominant marine group beginning in the Late Triassic.