Adult sex ratio, sexual dimorphism and sexual selection in a Mesozoic reptile

Abstract

The evolutionary history of sexual selection in the geologic past is poorlydocumented based on quantification, largely because of difficulty in sexingfossil specimens. Even such essential ecological parameters as adult sex ratio(ASR) and sexual size dimorphism (SSD) are rarely quantified, despite theirimplications for sexual selection. To enable their estimation, we propose amethod for unbiased sex identification based on sexual shape dimorphism,using size-independent principal components of phenotypic data.We appliedthe method to test sexual selection in Keichousaurus hui, a Middle Triassic(about 237 Ma) sauropterygian with an unusually large sample size for afossil reptile. Keichousaurus hui exhibited SSD biased towards males, as inthe majority of extant reptiles, to a minor degree (sexual dimorphism index20.087). The ASR is about 60% females, suggesting higher mortality ofmales over females. Both values support sexual selection of males in thisspecies. The method may be applied to other fossil species. We also used theGompertz allometric equation to study the sexual shape dimorphism ofK. hui and found that two sexes had largely homogeneous phenotypes atbirth except in the humeral width, contrary to previous suggestions derivedfrom the standard allometric equation.