Environmentally Controlled Variation in Shell Size of Ambonychia Hall (Mollusca: Bivalvia) in the Type Cincinnatian (Upper Ordovician)

Abstract

The common Ordovician bivalve genus Ambonychia Hall of the type Cincinnatian Series displays great variation in shell size that is strongly correlated with paleoenvironment. In older Cincinnatian sequences the largest mean and maximum shell size are found in the highest energy paleo-environments with decreasing size found in lower energy paleoenvironments. Further, the smallest size class of Ambonychia frequently is excluded from the higher energy paleoenvironments, possibly associated with differential taphonomic destruction of smaller shells. The deep-to-shallow water trend in mean and maximum shell size is not present in younger Cincinnatian sequences, perhaps because of a change in the morphology of the younger Ambonychia species, such as the development of a keeled umbo.