MESOZOIC RETURN OF BRACHIOPODS: BRACHIOPOD-DOMINATED SHELL BEDS IN THE MIDDLE TRIASSIC

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 beyond that determined from analyses of trends in diversity, 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 also important indicators of ecological structure, particularly dominance. A total of 58 shell beds were surveyed to establish thicknesses and fossil constituents. Of these 58 shell beds 33 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 present in many of these units, 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 as part of the Middle Triassic recovery from the deleterious environmental stress associated with the end-Permian mass extinction.