title: Adult sex ratio, sexual dimorphism and sexual selection in a Mesozoic reptile.
abstract: The evolutionary history of sexual selection in the geologic past is poorly documented based on quantification, largely because of difficulty in sexing fossil specimens. Even such essential ecological parameters as adult sex ratio (ASR) and sexual size dimorphism (SSD) are rarely quantified, despite their implications for sexual selection. To enable their estimation, we propose a method for unbiased sex identification based on sexual shape dimorphism, using size-independent principal components of phenotypic data. We applied the method to test sexual selection in Keichousaurus hui, a Middle Triassic (about 237 Ma) sauropterygian with an unusually large sample size for a fossil reptile. Keichousaurus hui exhibited SSD biased towards males, as in the majority of extant reptiles, to a minor degree (sexual dimorphism index -0.087). The ASR is about 60% females, suggesting higher mortality of males over females. Both values support sexual selection of males in this species. The method may be applied to other fossil species. We also used the Gompertz allometric equation to study the sexual shape dimorphism of K. hui and found that two sexes had largely homogeneous phenotypes at birth except in the humeral width, contrary to previous suggestions derived from the standard allometric equation.
authors.affiliation: Department of Earth and Planetary Sciences, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA rmotani@ucdavis.edu.
authors.forename: Ryosuke
authors.identifier: http://orcid.org/0000-0001-5022-1053
authors.initials: R
authors.lastname: Motani
authors.affiliation: Laboratory of Orogenic Belt and Crustal Evolution, Ministry of Education; Department of Geology and Geological Museum, Peking University, Yiheyuan Street 5, Beijing 100871, People's Republic of China djiang@pku.edu.cn.
authors.forename: Da-yong
authors.identifier: http://orcid.org/0000-0002-4668-2081
authors.initials: DY
authors.lastname: Jiang
authors.affiliation: Center of Integrative Research, The Field Museum, Chicago, IL 60605-2496, USA.
authors.forename: Olivier
authors.initials: O
authors.lastname: Rieppel
authors.affiliation: Laboratory of Orogenic Belt and Crustal Evolution, Ministry of Education; Department of Geology and Geological Museum, Peking University, Yiheyuan Street 5, Beijing 100871, People's Republic of China.
authors.forename: Yi-fan
authors.initials: YF
authors.lastname: Xue
authors.affiliation: Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Mangiagalli, Milan 34-20133, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Tintori
mesh_terms: D000818:Animals; D049628:Body Size; D025061:Dinosaurs / Q000033:anatomy & histology*; D005260:Female; D005580:Fossils; D006811:Humerus / Q000033:anatomy & histology; D008297:Male; D012727:Sex Characteristics; D012744:Sex Ratio
keywords: adult sex ratio; gompertz allometry; sauropterygia; sexual selection; sexual size dimorphism