PUTTING EVOLUTIONARY PATTERNS IN CONTEXT: A COMPARISON OF THE BIVALVE NUCULA DURING GREEN- AND ICEHOUSE CLIMATES

Documenting speciation patterns (i.e., punctuated equilibrium, phyletic gradualism, stasis) has been a major focus of evolutionary paleobiology over the past several decades. Despite substantial knowledge gleaned concerning this topic, a number of questions related to the processes that underlie different evolutionary patterns remain unresolved. Most studies have concentrated on testing the legitimacy and frequency of different evolutionary patterns, while ignoring their environmental context. Therefore, this study is part of a broader project investigating evolutionary patterns within an environmental framework. To test for potential environmental controls on evolutionary patterns, we examined the morphological variability among four species of the bivalve Nucula from contrasting climatic regimes: Late Cretaceous greenhouse and Neogene-Quaternary icehouse of the Mississippi Embayment and Florida Platform, respectively. This group was chosen because they are abundant fossils in both regions and time intervals under study. To determine their evolutionary trajectories, outline shape data was utilized as a measure of morphologic change through time and space. Shape parameters were analyzed using both Principle Component Analysis (PCA) and Canonical Variate Analysis (CVA) in the statistical package R. This analysis reveals high variability among the Late Cretaceous intervals, suggesting gradual change, whereas the two Neogene-Quaternary species are almost morphologically indistinguishable through time, pointing to evolutionary stasis. These data support Sheldon’s (1996) ‘Plus ça Change’ model, which hypothesizes that increased environmental variability, such as during icehouse climates, will result in stasis, whereas reduced environmental variability, characteristic of greenhouse climates, will drive gradualistic morphologic responses.