Morphology and Evolution of the Ceratopsian Skull

Abstract

Socio-sexual selection has long been recognised as an important driver of evolution and is known to be an important influence on the morphology and behaviour of extant organisms. The long term macroevolutionary effects of socio-sexual selection are difficult to observe because they operate over long timescales. Theoretical work has suggested that socio-sexual selection can possibly influence speciation, adaptation and extinction rates, but these predictions remain largely untested. The fossil record provides a potential solution for testing macroevolutionary hypotheses regarding socio-sexual selection over millions of years. Recent studies have suggested that the growth patterns of cranial ornamentation in Ceratopsian dinosaurs resemble the growth patterns predicted for a trait under socio-sexual selection. Identifying the presence of socio-sexual selection in extinct organisms is problematic, but evidence from extant taxa provide important evidence in detecting its effects on morphology. In this project, I use a combination of traditionally defined character states and three-dimensional geometric morphometrics, on a large scale for the first time in a dinosaur clade, to test competing macroevolutionary hypotheses of socio-sexual selection. I first test the hypothesis that morphological diversity in ceratopsian dinosaurs is driven by species recognition by comparing differences in character states between ceratopsian clades. I evaluate morphological evidence for socio-sexual selection in fossil specimens, employing novel techniques to assess growth and morphological variation in the skull of the ceratopsian that is best-represented by fossil specimens, Protoceratops andrewsi. Lastly, I extend the morphological dataset to encompass other ceratopsian taxa and examine modularity, morphological disparity and evolutionary rates across the clade. My results refute the hypothesis that exaggerated ceratopsian traits evolved for species recognition and provide support for predictions that ceratopsian traits associated with socio-sexual selection, namely the frill and horns, formed distinct units that both developed and evolved at comparatively rapid rates.