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Foth C, Hedrick BP, Ezcurra MD. (2015) Cranial ontogenetic variation in early saurischians and the role of heterochrony in the diversification of predatory dinosaurs. PeerJ PrePrints3:e1185v1https://doi.org/10.7287/peerj.preprints.1185v1
Non-avian saurischian skulls underwent at least 165 million years of evolution and shapes varied from elongated skulls, such as in Coelophysis, to short and box-shaped skulls, such as in Camarasaurus. A number of factors have long been considered to drive skull shape, including phylogeny, dietary preferences and functional constraints. However, heterochrony is increasingly being recognized as a major factor in dinosaur evolution. In order to quantitatively analyse the impact of heterochrony on saurischian skull shape, we have analysed five ontogenetic trajectories using two-dimensional geometric morphometrics in a phylogenetic framework. This allowed for the evaluation of how heterochrony affected overall skull shape through both ontogenetic and phylogenetic trajectories and how it impacted modular changes within the skull. Using principal component analyses and multivariate regressions, it was possible to quantify different ontogenetic trajectories in light of heterochrony. The results recovered here indicate that taxa underwent a combination of local paedomorphosis and peramorphosis within the skull along individual ontogenies and phylogenies, but that either peramorphosis or paedomorphosis dominated when the skull was considered as a whole. We found that the hypothetical ancestor of Saurischia led to basal Sauropodomorpha mainly through paedomorphosis, and to Neotheropoda mainly through peramorphosis. Paedomorphosis then led from Orionides to Avetheropoda, indicating that the paedomorphic trend previously found in advanced coelurosaurs may extend back into the early evolution of Avetheropoda. Not only are changes in saurischian skull shape complex due to the large number of factors that affect shape, but heterochrony itself is complex, with a number of reversals throughout non-avian saurischian evolution. The sampling of ontogenetic trajectories is considerably lower than the sampling of adult species and the current study represents a first exploratory analysis. To better understand the impact of heterochrony on cranial evolution in saurischians, the data set we present must be expanded and complemented with further sampling from future fossil discoveries, especially of juvenile taxa.
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