TESTING THE ‘PLUS ÇA CHANGE’ MODEL: AN EVALUATION OF NUCULID BIVALVE EVOLUTION FROM THE LATE CRETACEOUS GREENHOUSE AND NEOGENE-QUATERNAY ICEHOUSE

Evolutionary patterns are known to vary through time and space within lineages and clades. However, the environmental context for these different evolutionary patterns remains unsettled. Thus, the goal of this study is to examine evolutionary patterns framed within an environmental context by testing Sheldon’s (1996) ‘Plus ça change’ model, which predicts that stasis and phyletic gradualism are linked with increased and decreased environmental variation, respectively. To test this model, we examine the role that broad-scale climatic regimes have on evolutionary patterns by documenting evolutionary change among nuculid bivalves from the stable greenhouse climate of the Late Cretaceous and the more climatically dynamic icehouse climate of the Neogene-Quaternary in the U.S. Gulf Coast. Change in morphology through time is evaluated using both size and outline shape data. Comparison among changes in size and outline shape patterns for Nucula indicates that phyletic gradualism dominated the Late Cretaceous greenhouse and unbiased random walk to stasis dominated the Neogene-Quaternary. These results provide support for the ‘Plus ça change’ model and point to an important role for the nature of environmental variability in controlling evolutionary pattern.