Temporal shifts in ostracode sexual dimorphism from the Late Cretaceous to the late Eocene of the US Coastal Plain

Ostracodes of the superfamily Cytheroidea exhibit sexual dimorphism in the carapace such that males are more elongate than females. This sex difference is attributed to the need of the carapace to accommodate the large male copulatory apparatus, and the degree of dimorphism is an indication of male investment in reproduction. In this study, we examine trends in sexual dimorphism, as a proxy for sexual selection, from the Late Cretaceous to the late Eocene to better understand the long-term effects of the Cretaceous/Paleogene mass extinction. We used mixture models to identify sex clusters from digitized outlines of photographed specimens and estimated size and shape dimorphism as the difference in the mean log area and the mean log length-to-height ratio for male and female clusters. We found dimorphism exhibits a phylogenetic signal; families and genera tend to occupy various restricted subsets of dimorphism space. Previous work documented that the mean and variance in size and shape dimorphism decreased sharply at the Cretaceous/Paleogene boundary, and here we show that this fauna only partially returns to Cretaceous dimorphism patterns by the late Eocene. Most surprisingly, species with both high size and shape dimorphism, which occurred in a diverse set of taxa before the extinction, remain rare into the late Eocene. These trends suggest sexual selection may respond to several possible demographic and environmental factors, which warrant further investigation.

Automated summary

This study examines trends in sexual dimorphism in ostracodes from the Late Cretaceous to the late Eocene to understand the long-term effects of the Cretaceous/Paleogene mass extinction. The results show that sexual dimorphism has a phylogenetic signal and only partially returns to pre-extinction patterns by the late Eocene. Surprisingly, species with high size and shape dimorphism remain rare into the late Eocene. These findings suggest that sexual selection may respond to demographic and environmental factors.