SEXUAL DIMORPHISM AND SPECIES SELECTION IN LATE CRETACEOUS FOSSIL OSTRACODES FROM THE U.S. GULF COASTAL PLAIN

Species level selection, a variant of natural selection in which entire species, as opposed to individual organisms, are selected for or against, is a relatively under-studied process that may play an important role in evolution. Recent studies have used sexual dimorphism to examine potential instances of species selection because it is a trait that is determined by heritable physical characteristics and occurs solely at the species level. These studies have primarily been conducted on extant bird species, using extinction threat levels as a proxy for extinction rates. The results of such studies have varied; in some instances the data have supported a correlation between dimorphism and extinction. The fossil record offers an opportunity to examine the relationship between sexual dimorphism and extinction in the context of actual extinction rates, as opposed to extinction risk. In this study, we will determine if sexual size dimorphism is correlated with extinction or origination in Late Cretaceous ostracodes from the U.S. Gulf Coastal Plain, and whether the magnitude of dimorphism relates to the rates of extinction and origination in these species.

We examined 30 species of ostracodes collected from samples of Late Cretaceous sediment in Alabama and Mississippi. Male ostracodes, because of their very large sperm and bulky copulatory apparatus, tend to be more elongate than females. We quantified this dimorphism by digitizing the outline of adult ostracodes, fitting an ellipse to this outline, and calculating the major and minor ellipse axes as measures of valve length and height. We modeled these data as a mixture of two bivariate normal distributions, one for each sex, and used the Mahalanobis distance between males and females to quantify the magnitude of sexual dimorphism for each species. Preliminary analyses indicate that the magnitudes of dimorphism differ among clades. We also compiled stratigraphic ranges for each species from previously published literature. We compared the magnitude of dimorphism for each clade to its stratigraphic range in the Gulf Coastal Plain in order to determine if a correlation exists between sexual size dimorphism and extinction or origination.