326-9 Sexual dimorphism and reproductive mode in Middle Eocene Ostracoda from the U.S. Coastal Plain

Abstract:

The means by which organisms reproduce is fundamental to their biology, but it is seldom inferable from their fossilized remains.  Cytheroid ostracodes offer a rare exception to this limitation.  Sexual dimorphism of the mineralized carapace presents as differences in shape and size between males and females; usually, males have a more-elongate carapace. By digitizing the outlines of ostracode carapaces, we not only can differentiate males from females but also can determine sex ratios, which can reveal the existence of asexual populations if no males are present.  Here we present the results of a broad-scale examination of sexual dimorphism and sex ratios for Middle Eocene (Claibornian regional stage) cytheroid ostracodes from the U.S. Coastal Plain. From this fauna, we collected dimorphism and sex ratio data from over 100 populations representing more than 75 species and 30 genera.  Sexual populations were compared to similar published data from earlier intervals (Late Cretaceous, early Paleocene) and younger Late Eocene faunas.  Overall, Paleogene dimorphism is generally lower than that of the Cretaceous, with notably fewer species with dimorphism indicative of very high male investment in reproduction.  In addition, we document four species that have female-only, presumably asexual populations.  This incidence of asexuality is similar to that of other time periods in this region, and to the living cytheroid fauna found around the United Kingdom today.  These results contribute to a greater understanding of the evolutionary history of ostracodes as well as the ways reproductive mode and sexual dimorphism evolve across marine taxa.   

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