SEXUAL DIMORPHISM IS GENERALLY STABLE OVER THE LIFETIME OF FOSSIL OSTRACODE SPECIES

Sexual dimorphism can vary among closely related species, but there is little data about how sexual dimorphism varies through the lifetime of a species. Here we document sexual dimorphism in the carapaces of ostracode crustaceans from the superfamily Cytheroidea collected from the Late Cretaceous (~85 to 66 million years ago) of the US Coastal Plain. We measured size and shape from carapace outlines of over 100 species and used mixture models to detect male and female clusters, with males assigned to be the more elongate cluster as in living cytheroids. We quantified dimorphism as the difference between male and female means in size (log area) and shape (log of the length to height ratio). Across the fauna, a variety of dimorphism patterns were observed. Males are usually larger than females, sometimes by as much as 30%, but in some species they are up to 20% smaller. Similarly, males range from slightly (2%) to substantially (15%) more elongate than females across species. Despite this variation in dimorphism, repeated measures within species almost always find little to no difference in dimorphism, even among samples spanning hundred of kilometers and millions of years. We found evidence for substantial change in dimorphism within only one species, Haplocytheridea renfroensis, which starts out highly dimorphic in size and shape but becomes much less strongly dimorphic over about 15 million years. These findings suggest that sexual dimorphism, despite its likely links to sexual selection and organismal fitness, can often behave as a stable, species-level trait.