TESTING WHETHER THE STRENGTH OF SEXUAL SELECTION INFLUENCES SPECIES EXTINCTION AND ORIGINATION IN LATE CRETACEOUS CYTHEROID OSTRACODES

To explain the diversity of life researchers have sought to understand how and why species originate and go extinct. To this end, paleontologists have linked lineage origination and extinction with traits such as body size and geographic range. One evolutionary driver not yet considered is the extent of sexual selection—selection that specifically influences mating outcomes—operating within species. In extant species, sexual selection may cause reproductive isolation and, hence, promote speciation. Theoretical models and experimental studies indicate that strong sexual selection can increase extinction risk from the demographic drag caused by investment in the costly structures that commonly evolve under sexual section (e.g., weapons or large body size). However, it is often impossible to study actual extinction and origination in living fauna and therefore this issue will remain largely untested without the inclusion of fossil data.

Here we present an analysis of species-level origination and extinction of cytheroid ostracodes from a composite reference section spanning 15 million years of the Late Cretaceous (Santonian to early Maastrichtian). Cytheroids possess sexually dimorphic shells, with males relatively more elongate and often absolutely larger than females. The degree of sexual dimorphism is thought to reflect the intensity of sexual selection. We used mixture modeling to identify sex clusters in biometric samples. From these, we computed sexual dimorphism in body size (difference in log area between the sexes) and shape (difference in log length/height ratio). Sexual dimorphism was directly estimated for 75 of the 101 species documented in this fauna; dimorphism in 20 more species was estimated as the mean dimorphism of their genus. We applied capture-mark-recapture analysis to species occurrences in 30 analytical samples (resolution of ~0.5 myr). We tested a variety of models for origination, extinction and preservation, including those that specify origination and extinction as dependent on the strength of sexual dimorphism. Initial results suggest that the probability of extinction increases with increasing size (but not shape) sexual dimorphism. Such a finding has implications for understanding the drivers of biodiversity over time and for managing current conservation efforts.