TIMING OF ECOLOGICAL DOMINANCE SHIFTS IN MARINE BENTHIC GROUPS DURING THE MESOZOIC OF NORTH AMERICA

Sepkoski’s work on diversity patterns suggest that the Mesozoic was a time of great ecosystem restructuring in the world’s oceans as the bivalve-dominated Modern fauna replaced the brachiopod-dominated Paleozoic fauna following the end-Permian mass extinction. Additionally, research suggests increased predatory stress induced an increase in epifaunal cementation and an increase in infaunalization in a series of events known as the marine Mesozoic revolution. Previous studies disagree on the timing of these events; Sepkoski’s diversity patterns suggest these events began shortly after the Permian-Triassic boundary, McRoberts (2001) suggests Late Triassic, and Vermeij (1977, 1987) suggests Late Cretaceous. To help resolve the timing of this major marine transition we track clade diversity combined with ecological dominance patterns via occurrence patterns of the Mesozoic marine bivalves and brachiopods of North America. Generic and ecological data were downloaded from the Paleobiology Database (www.paleodb.org) and focused only on genera occurrences within ~11 million year bins. To determine ecological dominance, this study used occurrences data due to inadequate abundance data. A total of 18,884 bivalve occurrences comprise 471 genera and 458 brachiopod occurrences comprise 53 genera. Diversity and occurrence plots show that epifaunal organisms were the dominant group throughout the Triassic and most of the Jurassic both in terms of diversity and occurrences. However, during the Late Jurassic (Jurassic bins 5 and 6) infauna was more diverse while epifauna had more occurrences. Early Cretaceous patterns depict a return to epifaunal dominance. During the early Late Cretaceous (Cretaceous bin 5) infauna diversity surpasses epifaunal diversity and remains dominant into the Cenozoic. However, infaunal occurrences begin their dominance during the late Late Cretaceous (Cretaceous bin 7). These results suggest that the take over of infaunal organisms occurred during the Late Cretaceous, consistent with Vermeij (1977, 1987), but was asynchronous in North America in terms of diversity and ecological dominance.