Cranial ontogenetic variation in early saurischians and the role of heterochrony in the diversification of predatory dinosaurs

FOTH, C.; HENDRICK, B. P.; EZCURRA, M. D.

PeerJ

PeerJ

Año: 2016

Non-avian saurischian skulls underwent at least 165 million years of evolution andshapes varied from elongated skulls, such as in the theropod Coelophysis, to short andbox-shaped skulls, such as in the sauropod Camarasaurus. A number of factors havelong been considered to drive skull shape, including phylogeny, dietary preferences andfunctional constraints. However, heterochrony is increasingly being recognized as animportant factor in dinosaur evolution. In order to quantitatively analyse the impactof heterochrony on saurischian skull shape, we analysed five ontogenetic trajectoriesusing two-dimensional geometric morphometrics in a phylogenetic framework. Thisallowed for the comparative investigation of main ontogenetic shape changes andthe evaluation of how heterochrony affected skull shape through both ontogeneticand phylogenetic trajectories. Using principal component analyses and multivariateregressions, it was possible to quantify different ontogenetic trajectories and evaluatethem for evidence of heterochronic events allowing testing of previous hypotheses oncranial heterochrony in saurischians. We found that the skull shape of the hypotheticalancestor of Saurischia likely led to basal Sauropodomorpha through paedomorphosis,and to basal Theropoda mainly through peramorphosis. Paedomorphosis then ledfrom Orionides to Avetheropoda, indicating that the paedomorphic trend found byprevious authors in advanced coelurosaurs may extend back into the early evolution ofAvetheropoda. Not only are changes in saurischian skull shape complex due to the largenumber of factors that affected it, but heterochrony itself is complex, with a number ofpossible reversals throughout non-avian saurischian evolution. In general, the samplingof complete ontogenetic trajectories including early juveniles is considerably lower thanthe sampling of single adult or subadult individuals, which is a major impediment tothe study of heterochrony on non-avian dinosaurs. Thus, the current work representsan exploratory analysis. To better understand the cranial ontogeny and the impact ofheterochrony on skull evolution in saurischians, the data set that we present here mustbe expanded and complemented with further sampling from future fossil discoveries,especially of juvenile individuals.