[N] 2006 Ontogenetic changes in Psittacosaurus
Makovicky, P., Gao, K-Q., Zhao, C-F. & Erickson, G. (2006) Ontogenetic changes in Psittacosaurus: Implications for taxonomyand phylogeny. JVP 26(3) Abstracts pp.94
The ceratopsian genus Psittacosaurus comprises 8-11 relatively uniform species from the Lower Cretaceous of Asia. The taxonomy and phylogeny of Psittacosaurus is complicated by a conservative body plan combined with a poor understanding of the distribution of taxonomically significant cranial characters, some of which vary ontogenetically. In order to better understand the relationship between ontogeny and the distribution of potentially diagnostic characters, we studied their development throughout the ontogeny of two species, P. mongoliensis from Mongolia, and a second taxon from the Yixian Formation of Liaoning, China. Specimens were aged using skeletal histology and growth stage determination was established using the Developmental Mass Extrapolation technique. Presence or absence of 17 potentially informative characters such as head shape, dentary flange, jugal horn shape, and denticle counts were noted for individual specimens along the curves.
Growth curves for the two species differ substantially with P. mongoliensis showing greater maximum growth rates and larger maximum size than the Yixian Fm. taxon, indicating that life history patterns can be used to distinguish between Psittacosaurus species. Both ontogenetic and individual variation was observed in some of the characters examined. For example, presence of a well developed flange along the lower edge of the dentary, a trait diagnostic for some species, is correlated with ontogeny and a fully developed flange only occurs in older, subadult individuals. Individual variation in the expression of this character during ontogeny is also observed for the Yixian Fm. species. Conversely, characters such as skull and orbit shape and denticle counts appear to show relatively little ontogenetic variation, which does not transcend character state boundaries.
Novel understanding of character evolution gained through this study has important implications for the validity of some species of Psittacosaurus and also for the phylogeny of the group. This study demonstrates the potential of life history studies for understanding evolutionary patterns and processes in extinct clades.