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- Temporal shifts in ostracode sexual dimorphism from the Late Cretaceous to the late Eocene of the U.S. Coastal PlainPublication . Samuels-Fair, Maya; Martins, Maria Joao Fernandes; Lockwood, Rowan; Swaddle, John P.; Hunt, GeneOstracodes 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.
- Shifts in sexual dimorphism across a mass extinction in ostracods: implications for sexual selection as a factor in extinction riskPublication . Martins, Maria Joao Fernandes; Hunt, Gene; Thompson, Carmi Milagros; Lockwood, Rowan; Swaddle, John P.; Puckett, T. MarkhamSexual selection often favours investment in expensive sexual traits that help individuals compete for mates. In a rapidly changing environment, however, allocation of resources to traits related to reproduction at the expense of those related to survival may elevate extinction risk. Empirical testing of this hypothesis in the fossil record, where extinction can be directly documented, is largely lacking. The rich fossil record of cytheroid ostracods offers a unique study system in this context: the male shell is systematically more elongate than that of females, and thus the sexes can be distinguished, even in fossils. Using mixture models to identify sex clusters from size and shape variables derived from the digitized valve outlines of adult ostracods, we estimated sexual dimorphism in ostracod species before and after the Cretaceous/Palaeogene mass extinction in the United States Coastal Plain. Across this boundary, we document a substantial shift in sexual dimorphism, driven largely by a pronounced decline in the taxa with dimorphism indicating both very high and very low male investment. The shift away from high male investment, which arises largely from evolutionary changes within genera that persist through the extinction, parallels extinction selectivity previously documented during the Late Cretaceous under a background extinction regime. Our results suggest that sexual selection and the allocation of resources towards survival versus reproduction may be an important factor for species extinction during both background and mass extinctions.