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Cranial ontogenetic variation in early saurischians (free pdf)

Ben Creisler

A new paper in open access:

Christian Foth, Brandon P. Hedrick & Martin D. Ezcurra (2016)
Cranial ontogenetic variation in early saurischians and the role of
heterochrony in the diversification of predatory dinosaurs.
PeerJ 4:e1589
doi: https://doi.org/10.7717/peerj.1589

Non-avian saurischian skulls underwent at least 165 million years of
evolution and shapes varied from elongated skulls, such as in the
theropod Coelophysis, to short and box-shaped skulls, such as in the
sauropod 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 an important factor in dinosaur evolution. In order to
quantitatively analyse the impact of heterochrony on saurischian skull
shape, we analysed five ontogenetic trajectories using two-dimensional
geometric morphometrics in a phylogenetic framework. This allowed for
the comparative investigation of main ontogenetic shape changes and
the evaluation of how heterochrony affected skull shape through both
ontogenetic and phylogenetic trajectories. Using principal component
analyses and multivariate regressions, it was possible to quantify
different ontogenetic trajectories and evaluate them for evidence of
heterochronic events allowing testing of previous hypotheses on
cranial heterochrony in saurischians. We found that the skull shape of
the hypothetical ancestor of Saurischia likely led to basal
Sauropodomorpha through paedomorphosis, and to basal Theropoda mainly
through peramorphosis. Paedomorphosis then led from Orionides to
Avetheropoda, indicating that the paedomorphic trend found by previous
authors 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 affected it, but
heterochrony itself is complex, with a number of possible reversals
throughout non-avian saurischian evolution. In general, the sampling
of complete ontogenetic trajectories including early juveniles is
considerably lower than the sampling of single adult or subadult
individuals, which is a major impediment to the study of heterochrony
on non-avian dinosaurs. Thus, the current work represents an
exploratory analysis. To better understand the cranial ontogeny and
the impact of heterochrony on skull evolution in saurischians, the
data set that we present here must be expanded and complemented with
further sampling from future fossil discoveries, especially of
juvenile individuals