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A TEST OF BIOGEOGRAPHICAL, ENVIRONMENTAL, AND ECOLOGICAL EFFECT ON MIDDLE AND LATE TRIASSIC BRACHIOPOD AND BIVALVE ABUNDANCE PATTERNS

¹ California State University, Fullerton, Department of Geological Sciences, Fullerton, California 92834-6850, USA
² University of Southern California, Department of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, California 90089-0740, USA nbonuso{at}fullerton.edu

Analysis of Middle Triassic data indicates that biogeography influences sample distributions, whereas depositional environment and stratigraphic position play secondary roles in governing sample patterns. During this time, taxa differed among biogeographic realms, while the general ecology remained the same: epifaunal benthos— pedunculate and epibyssate suspension feeders—dominate Middle Triassic samples much as they did in the Early Triassic. In contrast, Late Triassic data prove to be more complex in terms of ecology compared to Middle Triassic. Here, guild structure dictates the faunal patterns in addition to biogeographic realm and stratigraphic position, and an overall increase of infaunal life habits occurs—burrowing suspension and deposit feeders increase. Although diversity after mass extinction began to recover at the Early-Middle Triassic boundary, our results indicate that ecology remained stable through the Middle Triassic until the more modern life habits (e.g., infaunalization) increased in the Late Triassic. We conclude that the taxonomic and ecological differences among Late Triassic geographic regions recorded the initiation of a more mobile and infaunal life habit indicative of a modern lifestyle. Our results also indicate that this modernization did not necessarily unfold simultaneously and in coordinated fashion within regions and throughout time. Instead, details of guild expansion or stability may be region specific.

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