Body size evolution of the Late Cretaceous bivalves from Ariyalur, southern India

Abstract

An organism's body size is one of its most fundamental and observable traits for extant and fossil taxa. In the present study, we document long-term evolutionary trends in body size of Late Cretaceous bivalves from the Ariyalur Sub-basin, southern India. Our data suggest a significant increase in bivalve body size from the Cenomanian to the Maastrichtian. However, this increase is not gradual and does not strictly follow Cope's Rule, the tendency for size to increase over time. Rather, a non-directional trajectory of body size increase is evident across various taxonomic ranks. The similarity between the long-term trends observed at higher (class and order) and lower taxonomic levels (family and genus) suggests congruence of the underlying macroevolutionary and microevolutionary processes. We evaluate these body size trends with respect to decreasing paleolatitudes and cyclic changes in the relative sea-level of the basin. Both of these factors are recognized to have influenced body size variation, but isolating the impacts of individual factors is challenging.