| This study documents brachiopod and bivalve relative abundance patterns from the Middle and Late Triassic. Brachiopod- and bivalve-dominated fossil assemblages from before and after the end-Permian mass extinction were analyzed to gain perspective on the brachiopod radiation within the early Mesozoic and to more completely understand the causal mechanisms behind the switch between Paleozoic brachiopod-dominated faunas and Modern bivalve-dominated faunas. Using data from bulk samples, this study examines quantitative patterns recorded by marine benthos. In total 387,992 specimens were pooled from primary and summary literature resources, including 21,671 Middle and Late Triassic specimens from Italy, Poland, Slovakia, Hungary, China, Austria, and Nevada. In addition, a total of 336,321 Late Carboniferous and Early Permian specimens were analyzed from Nevada, Kansas, Oklahoma, Texas, Utah, New Mexico, Venezuela, Thailand, and Australia. For each time interval (i.e., late Paleozoic and early Mesozoic), multivariate statistics determined whether paleogeography, depositional environment, age, substrate preference, or taxonomic membership influenced faunal patterns and if so, to what extent. Both time intervals reveal that ecological preferences largely control faunal patterns. That is, the abundance of sessile benthos (epifaunal brachiopods and bivalves) versus mobile benthos (infaunal bivalves and grazing gastropods) controlled faunal distributions. Comparison of late Paleozoic and early Mesozoic faunal patterns indicates that taxonomic structure differed between time intervals. The late Paleozoic faunas consisted of numerous brachiopod orders varying in their attachment modes, compared to the one order of shallow burrowing deposit-feeder bivalves. In contrast, the Middle Triassic records a decrease in brachiopod diversity compared to bivalves and the fauna consisted primarily of epifaunal brachiopods and bivalves. As the Late Triassic approached, infaunal suspension feeding bivalves slowly replaced epifaunal brachiopods and bivalves. Faunal patterns reveal that Middle Triassic faunas represents a stress-tolerant, sessile fauna suggesting that environmental disruption leading to the end-Permian mass extinction affected oceanic condition such that stress-tolerant faunas continued into the Middle Triassic. Slowly, from the Middle to Late Triassic, high energy, mobile fauna replaced this stress-tolerant fauna. As a result, the brachiopod to bivalve switch was due to a combination of brachiopod incumbency, mass extinction, and key bivalve adaptations. |
