Exploring the cranial morphology and possible convergences of Triassic non-crocodylomorph pseudosuchians (Archosauria: Pseudosuchia) with other sauropsids through linear morphometrics

Abstract

Triassic pseudosuchians had highly diversified cranial morphologies. These archosaurs occupied diverse ecological roles, ranging from terrestrial predators and herbivores to semiaquatic ambush predators and possible waders. Here, we apply linear cranial morphometrics to assess possible convergences with other sauropsids from the clades Theropoda, Squamata, and Plesiosauria and discuss their paleoecology, based upon what was known for the aforementioned clades. Thus, a total of 34 specimens of Triassic pseudosuchians and theropod dinosaurs were added to a morphometrics dataset originally used to infer the ecology of the enigmatic theropod Spinosaurus aegyptiacus. Principal component analysis and permutational multivariate analyses of variance were used to evaluate morphospace occupation and group differentiation. Our results indicate that linear variables are capable of distinguishing semiaquatic and aquatic taxa from terrestrial taxa, but fail to distinguish between semiaquatic and fully aquatic taxa, and could not distinguish between the different niches of the sampled terrestrial taxa. Phytosaurs cluster close to marine squamates, Qianosuchus mixtus occupies an intermediate position between the phytosaurs and the crocodylomorphs, erpetosuchids plotted near varanids. Beaked pseudosuchians did not cluster close to beaked theropods, with Effigia okeeffeae overlapping with Baryonyx walkeri. The loricatans, and possible synonymous, Decuriasuchus quartacolonia and Prestosuchus chiniquensis overlapped. Some loricatans plot near ornithomimids, and Saurosuchus galilei cluster close to Acrocanthosaurus atokensis. We conclude that convergent morphologies do not necessarily reflect convergent function in Triassic pseudosuchians. The research of their paleoecology would benefit from the integration of morphology and function rather than morphology and convergence, especially with obscure taxa such as the poposauroids.