LATE TRIASSIC BIVALVE-BRACHIOPOD INTERACTIONS

The switch from the brachiopod-dominated to bivalve-dominated benthic communities has long fascinated paleobiologists. This change has often been cited as the classic example of ecological competition, but more recent analyses have portrayed it as primarily due to the differential effects upon each group at the end-Permian mass extinction. Most recent studies have used global patterns of taxonomic diversity through geologic time to analyze this pattern. These broad based investigations are the first step to understanding interactions between these two clades, but they may provide too simplistic an approach for resolving why this change in dominance occurred. In particular, the nature of brachiopod and bivalve occurrence on a local ecological scale during the Triassic may provide important insights towards fully understand the dynamics of this transition. As vanquished incumbents, brachiopods did not just retreat from the bivalve onslaught after the end-Permian mass extinction. Global taxonomic diversity analyses show an increase in brachiopod diversity during the Late Triassic. An analysis of this early Mesozoic brachiopod resurgence will allow a better understanding of the ultimate post-Paleozoic brachiopod decline. To begin this analysis we present a localized, ecological investigation of the Upper Triassic, Luning Formation of Nevada, which contains the most diverse Mesozoic brachiopod fauna know in North America. Hogler (1992) completed general paleoecological studies that reveal an atypical biota including bivalve-dominated and brachiopod-dominated biofacies. Bulk rock samples were collected from these biofacies and lithological and faunal characteristics were compared. Paleoenvironmental analysis indicates that the two biofacies derive from separate paleoenvironments. The brachiopod-dominated biofacies characterizes a moderately agitated, carbonate shelf environment while the bivalve-dominated biofacies formed in a more distal, low energy environment with a less stable substrate. These results suggest that the Late Triassic brachiopod global diversity increase was not caused by an across-the-board resurgence of brachiopods in benthic communities, but that they were able to again dominate some benthic environments on a local scale.