129-74 The endocast of the early-diverging phytosaur Parasuchus bransoni (Archosauriformes) and preliminary incorporation of endocranial characters into phytosaur phylogenetics

Abstract:

Phytosaurs are superficially crocodile-like archosauriform reptiles that are relatively common macrofaunal components of the non-marine Upper Triassic. Most phytosaur taxa are described almost exclusively from their highly diagnostic cranial material which, in turn, forms the basis for our understanding of their phylogenetic relationships. Most anatomical studies of phytosaurs have focused on the external features of the cranium, with few instances of internal cranial descriptions. This is clear in character matrices used in phylogenetic inference of the internal relationships of phytosaurs. Advances in computed tomography have facilitated a new wave of investigation into the endocranial anatomy of phytosaurs without the need for destructive sampling techniques. Despite this, most phytosaur species lack an endocranial description, and those descriptions that do exist have not incorporated these new observations into a phylogenetic framework. 

Here we present the first description of the cranial endocast of a referred specimen of Parasuchus bransoni, an early diverging phytosaur, from the ochre unit of the Popo Agie Formation in Fremont County, Wyoming (UWGM 1978). The endosseous labyrinth (inner ear) of UWGM 1978 exhibits similar semicircular canal thickness and anteroposterior lobe asymmetry to the more basal phytosaur Wannia scurriensis than to the putative congener Parasuchus angustifrons. In addition, the morphology of a sinus within the roofing bones of the skull, described in other phytosaurs as either the pineal body or dural venous sinus, is clarified. This structure may represent part of the paratympanic sinus system, which is considered plesiomorphic to Archosauria, but its presence in phytosaurs may indicate 1) an earlier appearance of the paratympanic sinus system in Archosauriformes or 2) rather than a sister relationship of Phytosauria+Archosauria, a more derived position of phytosaurs within crown clade archosaurs. Incorporation of novel morphological characters of the endocranium into the phylogenetic study of phytosaurs may help clarify the taxonomy of Parasuchus and further refine our understanding of phytosaur phylogenetics.

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