Environmentally controlled variation in shell size of Ambonychia (Mollusca: Bivalvia) from the type Cincinnatian (Upper Orodvician)

Daley, G.M. 1999.
Palaios 14: 520-529

Abstract: The common Ordovician bivalve genus Ambonychia 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 paleoenvironments, both of which decrease in lower energy paleoenvironments. Further, the smallest size class of Ambonychia is frequently 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 to the morphology of the younger Ambonychia species, such as the development of a keeled umbo.