LOWER TRIASSIC SHELL BEDS OF THE WESTERN UNITED STATES

Shell beds are an important and conspicuous part of the stratigraphic record. Fossil concentrations have been examined over major diversification events and the results have revealed that shell beds can accurately record changes in dominance and abundance, and other changes in community dynamics through periods of turnover. Little work, however, has been done utilizing shell beds over major extinction events.

The end-Permian mass extinction resulted in the extinction of up to 96% of all marine species. Ecologically this is an event of great magnitude that is examined here through fossil concentrations in the western U. S. Shell beds from three time periods represented in the Lower Triassic are examined.

The first time slice examined is recorded in the Dinwoody Formation (Griesbachian). The shell beds in this formation are of low diversity, and monospecific beds of Claraia and Promyalina as well as the inarticulate Lingula are common. Data from time two are incomplete, but the Sinbad Formation (Nammalian) provides a small window into this time period, in which common low diversity (bivalves and microgastropods) shell beds occur. Within the third period, represented by the Virgin and Thaynes Formations (Spathian) the bivalves Promyalina and Permaphorus 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.

The shell beds in these formations were deposited in environments ranging from nearshore to storm wave base. These shell beds range in thickness from pavements to 10's of cm and show variable internal complexity. The shell beds occurring in these Lower Triassic sections are of low diversity, and are relatively thin throughout the entire interval. Although the beds of the Early Triassic are primarily comprised of Modern taxa, they are more similar to beds of the Paleozoic in thickness and taphonomic characteristics.