Abstract

The end-Permian mass extinction was the largest Phanerozoic biotic crisis that resulted in significant and permanent ecological change. In order to examine ecological aspects of the recovery, shell beds deposited in environments ranging from nearshore to storm wave base were examined from three stratigraphic intervals in the Lower Triassic of the western U.S. Shell beds of the first interval, the Griesbachian Dinwoody Formation, are low-diversity, monospecific beds of Claraia and Promyalina, commonly with the inarticulate brachiopod Lingula. Data from the Nammalian Sinbad Limestone (Moenkopi Formation) provide a small window into the second time interval, in which common low-diversity (bivalves and microgastropods) shell beds occur. Within the third interval, represented by the Spathian Virgin Limestone (Moenkopi Formation) and Thaynes Formation, the bivalves Promyalina and Permophorus are found in both monospecific and polytaxic beds. Crinoids are also commonly found as encrinites and as significant contributors to the matrix of these beds. Shell beds range in thickness from pavements to 10s of centimeters and show variable internal complexity. The persistence of monospecific shell beds throughout these three intervals is significant as support for long-term stress during the recovery interval that is not apparent from sedimentological data alone. Although these Early Triassic beds primarily are comprised of members of the Modern Evolutionary Fauna, they are more similar to beds from the Paleozoic in thickness and taphonomic characteristics.

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