EXTINCTION AND INFAUNALITY: ECOMORPHOLOGICAL PATTERNS IN LATE CRETACEOUS AND EARLY CENOZOIC VENEROID BIVALVES

Shell morphology and burrowing behavior are closely correlated in veneroid bivalves, making it possible to examine ecological and functional traits in this clade over long timescales in the fossil record. The purpose of this study is to document morphological and ecological trends in Late Cretaceous to early Oligocene veneroids, focusing specifically on the influence of two extinctions: (1) the end-Cretaceous (K/T) and (2) end-Eocene events. Questions addressed include: (1) What are the major trends in morphological and ecological diversity in veneroids during this time? (2) Are patterns of extinction selectivity or preferential rebound evident across either of these events? (3) What effect do these differences have on the short- and long-term evolutionary history of this clade? I explore these questions by establishing simple morphological measures and constructing an empirical morphospace based on shell outline and pallial line data for approximately 1200 specimens representing 101 subgenera of veneroids, arcticoids, and glossoids from North America and Europe. Patterns are summarized via Elliptical Fourier Analysis, using points equally distributed around the shell circumference and the pallial line. The resulting morphological measures and morphospace are interpreted in terms of ecological and functional traits such as burrowing rate and depth. Extinction selectivity and preferential recovery are assessed by comparing the distributions of taxa in morphospace among time intervals. Patterns of morphological and ecological diversity are compared to data on extinction selectivity and preferential rebound, in order to assess the short and long-term effects of these extinction events on the evolutionary history of veneroids. Given the relationship between shell morphology and burrowing behavior in veneroids, it appears that deeper, more rapidly burrowing taxa were more likely to originate during the K/T rebound. Results indicate that the K/T rebound may have exerted greater long-term influence on overall evolutionary trends than the extinction itself.