89 Paleontology: Morphological Evolution & Morphology and Systematics (Posters)

Monday, 5 November 2012
9:00 AM - 6:00 PM
Hall B

SEXUAL DIMORPHISM AND EXTINCTION AND ORIGINATION IN LATE CRETACEOUS OSTRACODES FROM THE U.S. GULF COASTAL PLAIN

Paper number: 89-12
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Species selection, a process through which evolution acts on entire species as opposed to individuals, is considered plausible in principle but few empirical studies have documented its effect. Because sexual dimorphism is a trait that occurs at the species level, recent studies, focusing primarily on modern birds, have used it to test the validity of species selection. Although results have varied, some have found a correlation between dimorphism and extinction risk or diversification rate. The fossil record makes it possible to explore this relationship in species for which we can determine actual, rather than projected, extinction rates. In this study, we documented patterns of sexual dimorphism in Late Cretaceous ostracodes from the U.S. Gulf Coastal Plain, and tested whether the magnitude of sexual dimorphism relates to extinction or origination rates in these species.

We obtained morphometric data from 33 species of ostracodes collected from sediment samples from the Late Cretaceous of Alabama and Mississippi. In order to accommodate their large sperm, male ostracodes are more elongate than females. We approximated the shape of ostracode valves as an ellipse and used the major and minor axes of this ellipse as measures of length and width. We modeled body size within each species as a mixture of two bivariate normal distributions, one each for males and females. We then used the Mahalanobis distance between the male and female distributions as a measure of sexual dimorphism. We determined that related species tend to have similar magnitudes of dimorphism, suggesting that this trait is phylogenetically conserved across these clades. Comparing stratigraphic ranges of highly dimorphic species to those with limited dimorphism indicated no systematic difference in extinction or origination rates with respect to dimorphism. In addition, we found no evidence of selectivity in origination or extinction with respect to body size or taxonomic family.