DID SEXUAL SELECTION HINDER SURVIVAL OF OSTRACODE SPECIES THROUGH THE K/PG MASS EXTINCTION?

Sexual selection is a special case of natural selection that relates to investment in traits that help organisms compete for mates. Sexual selection is suggested to affect the probability of species extinction: Investment in expensive traits that do not directly increase survival may elevate extinction risk when conditions change, in particular under extreme ecological shifts. However, empirical testing has been restricted to the living biota, where indirect proxies are used to calculate extinction. Thus the role of sexual selection in dictating extinction selectivity will remain largely untested without the inclusion of fossil data.

We developed a study system using ostracodes that allows us to examine the role of sexual selection in fossil lineages. In cytheroid ostracodes the male shell is systematically more elongate than that of females, and the sexes can be assigned even in fossils. Moreover, as verified in the living fauna, the strength of dimorphism can be taken as an indicator of the intensity of sexual selection. By leveraging the rich fossil record of these organisms, we can test sexual selection as a driver of extinction selectivity across the Cretaceous/Paleocene mass extinction.

Here we report the preliminary results of an investigation using the fossil record of early Paleocene (Danian) cytheroid ostracodes from the US Gulf Coastal Plain. We digitized the valve outlines of adult specimens, and used mixture modeling to identify sex clusters in samples. We successfully estimated sexual dimorphism in body size (difference in log area between the sexes) and shape (difference in log length/height ratio) for 35 populations assigned to 24 species and 14 genera.

Comparing dimorphism data between the Late Cretaceous and Paleocene, we observe a systematic reduction in size and shape dimorphism in the Paleocene. This is most recognizable by the absence in the Paleocene of species with males that are much bigger and much more elongate then females, a style of dimorphism that reflects higher investment in mating traits. Our results suggest that aspects of resource allocation (survival vs reproductive investment) may be an important factor for species survival at the end Cretaceous mass extinction.