Mesozoic Return of Brachiopods: Brachiopod-Dominated Shell Beds in the Middle Triassic

Sarah E. Greene, David J. Bottjer, and John-Paul Zonneveld
University of Southern California, Department of Earth Sciences Los Angeles, CA, USA [email protected]

The benthic marine diversity and paleocommunity structure of the Early Triassic indicates that deleterious environmental conditions associated with the end-Permian mass extinction persisted for ~5 million years. It is known that benthic marine faunal diversity increased in the Middle Triassic, but it has yet to be established how these communities reorganized once environmental stress abated.

To further understanding of Middle Triassic benthic marine paleoecology, shell beds from the Ladinian Liard Formation, exposed at Williston Lake, British Columbia (Canada), were examined. The Liard Formation, a mixed siliciclastic-bioclastic succession, represents a shallow shelf setting with a prograding, storm-dominated, barrier island shoreline. Shell beds, in addition to providing information about biological diversity, are important indicators of ecological structure, particularly dominance. 58 shell beds were surveyed to establish thicknesses and fossil constituents. 33 shell beds were dominated by terebratulid brachiopods, 23 were encrinites (composed primarily of crinoid material), and 2 were brachiopod-encrinite composites. Average bed thickness was 0.35 meters, ranging from thin terebratulid pavements to a 3.48 meter-thick encrinite. Although bivalves were often present, they were not numerically dominant in any bed.

Though global brachiopod diversity was severely reduced by the end-Permian extinction, these data demonstrate that brachiopods regained their status as important components of the benthic fauna in the Middle Triassic. Thus, at least in parts of the Panthalassic ocean, the transition from the Paleozoic to the Modern Evolutionary Fauna included a resurgence of Paleozoic faunal components during the Middle Triassic recovery from the deleterious environmental stress associated with the end-Permian mass extinction.

 

AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid