Por Maria João Martins (Department of Paleobiology, Smithsonian Institution, National Museum of Natural History, Washington, DC).
Abstract: Sexual selection relates to investment in traits that help organisms compete for mates. Sexual selection is suggested to affect the probability of species extinction: Investment in expensive traits that do not directly increase survival may elevate extinction risk when conditions change. However, empirical testing of this hypothesis has been mainly restricted to the living biota, where indirect proxies are used to calculate extinction. An alternative approach is to explore the topic in the fossil record. Cytheroid ostracods offer an unique insight into the role of sexual selection in extinction selectivity in the deep-time; in this group the male shell is systematically more elongate than that of females, and the sexes can be assigned even in fossils. By leveraging the rich fossil record of cytheroids from the US Coastal Plain, we tested sexual selection as a driver of extinction selectivity across two extinction regimes: a background regime (Late Cretaceous) and a mass extinction regime (Cretaceous-Paleocene). Using data from a composite stratigraphic section from Mississippi, we verified that species with increased male investment had higher probability of extinction in the Late Cretaceous. Specifically, species with strong sexual dimorphism, where males are much bigger and elongate than females, were generally shorter lived. Next, comparing sexual dimorphism data between the Late Cretaceous and the Paleocene (K-Pg), we verified that genera survivorship was linked to the reduction of dimorphism in Pg species compared to their K relatives. Moreover, species with dimorphism style reflecting highest investment in male reproductive traits were absent in the Pg, in agreement with data from the Late Cretaceous. We hypothesize that survivorship at the end of the Cretaceous mass extinction was probably dependent on a taxa’s ability to reallocate resources from reproduction to survival.