Body-size evolution in cretaceous molluscs and the status of Cope's rule

Abstract

Cope's rule, the tendency for lineages to evolve to larger body size, is widely seen as a pervasive evolutionary pattern. However, only a few studies have gone beyond enumerating isolated examples to assess its overall frequency relative to body-size decrease or stasis. Thus, although size is clearly an important parameter for microevolution and ecology, including conservation biology, its impact on large-scale patterns remains poorly understood. The prevalence of Cope's rule is even more uncertain, as some reported cases of evolutionary size increase may actually represent an expansion of a clade's size range (a pattern generally termed an 'increase in variance', although not necessarily in the formal statistical sense) rather than a phyletic, directional trend. I have performed a comprehensive census of body- size changes in a large fauna of Cretaceous bivalve and gastropod genera. A directional net increase in body size (including the loss of small- sized species and thus representing Cope's rule in the strict sense) is no more frequent than an increase in size range among species or a net evolutionary size decrease. Thus the undisputed ecological importance of body size does not translate into a preferred macroevolutionary pattern.