ASSESSING CHANGES IN SEXUAL DIMORPHISM IN OSTRACODE LINEAGES DURING THE LATE CRETACEOUS

Traits such as body size, geographic range, and abundance have been linked with lineage origination and extinction. However, the historical influence of sexual selection—a form of evolutionary selection concerning an individual’s ability to locate, attract, and compete for mates—on lineage success is less well understood.

The link between lineage origination and extinction and sexual selection may help to better understand the diversity of life in the past as well as modern biodiversity. Studies of extant taxa (e.g. birds) have used indirect proxies to test the influence of sexual selection on macroevolution, but the fossil record may permit direct investigations into the correlation between sexual selection and extinction and origination rates.

Sexual dimorphism, as a proxy for sexual selection is well documented in modern Ostracoda. Among members of the superfamily Cytheroidea, the carapaces of males are visibly elongate, accommodating large reproductive organs and large sperm. Here we conducted a time-series exploration of sexual dimorphism in Late Cretaceous ostracode samples from the U.S. Gulf Coastal Plain in order to (1) quantify the amount of evolutionary change in sexual dimorphism within species-level lineages, and (2) to test for directional trends in sexual dimorphism in these lineages.

In this study we digitized the outlines of adult fossil ostracodes and measured the minor (height) and major (length) axis of a fitted ellipse. We calculated shape dimorphism (the difference in log length/height ratio) and size dimorphism (difference in log area) in a number of ostracode genera and species to track the differential trajectory of changes in dimorphism throughout 15 million years during the Late Cretaceous (Santonian to Maastrichtian). Recent work has begun to elucidate links between the magnitude and style of dimorphism and its relationship to lineage success and the rate of macroevolution.

In most samples, we find evidence for two statistically significantly clusters, representing two sexes. We were able to estimate dimorphism patterns in over 100 species of our focal fauna, with replicated estimates for about 35 species. With the exception of one species, Haplocytheridea renfroensis, evolutionary changes within species were generally modest compared to differences between species.