SEXUAL SELECTION AND SEXUAL DIMORPHISM TRENDS IN CYTHEROID OSTRACODES FROM THE U.S. COASTAL PLAIN

Sexual selection maximizes individual reproductive success, but it has been suggested that allocating resources to reproduction at the expense of survival may compromise a lineage’s longevity. Despite sexual selection’s ubiquity, testing how sexual selection affects extinction is difficult because males and females are rarely distinguishable in the fossil record. Cytheroid ostracodes have been abundant in the fossil record since the Mesozoic, and their males are distinctly more elongate than females. In addition, the magnitude of sexual dimorphism in shell size and shape can be a proxy for the investment by males in reproduction, probably reflecting sperm competition. Recent work found that ostracode species with stronger sexual dimorphism were more likely to go extinct during the Late Cretaceous of the U.S. Coastal Plain. If species with higher dimorphism continued to experience higher extinction, we expect size dimorphism to decrease over time, holding all else equal. However, high size dimorphism may persist if sexual selection for sperm competition remains strong, or if dimorphism was uniquely disadvantageous during the Late Cretaceous. To test this, we photographed diverse populations of cytheroid ostracodes from the late Eocene of the Coastal Plain, and measured size as valve area and shape as length:height ratio, where length and height were estimated as the major and minor axes of an ellipse approximating the valve outline. We fit mixture models to infer sex clusters, and estimated size and shape differences between them. Results thus far indicate extreme dimorphism, with males much larger and more elongate than females, although common before the end-Cretaceous extinction, has not evolved again in the 30 million years since. However, the variance in shape dimorphism in the Eocene is higher than in the Cretaceous (F = 2.47, P = 0.024, df = 9, 142) or Paleocene (F = 4.95, P < 0.0001, df = 9, 64), as genera newly originating in the Eocene evolved stronger shape dimorphism than previously observed. Although the end-Cretaceous extinction seems to have eliminated extreme size dimorphism, the increase in shape dimorphism may indicate sexual selection is still a strong driver of morphological evolution in Eocene cytheroid ostracodes.