Testing the “Plus ça change” model: a comparison of nuculid bivalve evolution across contrasting broadscale climatic regimes

Abstract Documenting patterns of evolution and stasis has been a major focus of paleobiology. However, despite substantial knowledge gleaned on this topic, many questions related to the underlying environmental processes that determine the dynamics of evolution and stasis remain unresolved. Therefore, this study focuses on examining these evolutionary patterns framed within an environmental context. Specifically, we test Sheldon’s “Plus ça change” model, which predicts that morphological change is associated with more stable environments, such as in tropical latitudes or greenhouse climates, whereas stasis is linked to less stable environments, like those found in temperate latitudes or during icehouse climates. We examine the role that broadscale climatic variation exerts on evolutionary dynamics by documenting morphological change among nuculid bivalves in shallow-shelf settings from three different climate regimes: (1) the stable Late Cretaceous greenhouse climate; (2) the moderately stable Neogene transitional climate; and (3) the less stable Quaternary icehouse climate. Morphological changes over time were assessed using both bivalve size and outline shape. Comparison among changes in size and outline-shape patterns for Late Cretaceous and Neogene–Quaternary Nucula indicates that morphological change over time and stasis, respectively, dominated these different time intervals. In all cases, morphological change over time coincided with the more stable and less climatically variable greenhouse conditions, whereas stasis was associated with the more variable regimes characteristic of icehouse climates. These data provide strong support for the need to consider broad environmental factors—in this case climate—when assessing evolutionary modes. Furthermore, they point to the relevance of the Plus ça change model to explain patterns of evolution and stasis.