Novel cranial endocast of the South American “rauisuchian” Saurosuchus galilei (Pseudosuchia: Loricata) from the Ischigualasto Formation of Argentina

VON BACZKO, M. B.; CARDILLO, A. F.; ULLOA-GUAIQUIN, K.; DESOJO, J.B.; PAULINA CARABAJAL, A.

Salta

Congreso; Reunión de Comunicaciones de la Asociación Paleontologica Argentina; 2022

Asociación Paleontologica Argentina

Saurosuchus gallilei is a well-known prestosuchid loricatan from the Upper Triassic of Ischigualasto Formation in Argentina. Basal loricatans are terrestrial quadrupedal hypercarnivores with recurved serrated teeth, sizes reaching up to 7 meters, and numerous convergent characteristics with theropod dinosaurs. This group is considered a key component of the Triassic continental faunas, being an important part of the early diversification of archosaurs. They filled the top predator niches during the stabilization period following the Permo-Triassic mass extinction, before the emergence of the well-known carnivorous dinosaur clades. This study presents the first reconstruction of the cranial endocast of a South American rauisuchian including the encephalon, cranial nerves, and inner ear. In the endocast of the referred specimen PVSJ 32 (Museo de Ciencias Naturales de la Universidad Nacional de San Juan), the encephalon is tall and sigmoid and has a remarkably large hypophysis. This endocast lacks the anterior portion of the forebrain, however, the mould of elliptic olfactory bulbs can be identified in the holotype PVL 2062 (Instituto Miguel Lillo, Tucumán). The optic lobes cannot be distinguished and the flocculi are poorly developed as small lumps anterior to the labyrinths. Cranial nerves V to XII follow a pattern similar to other basal pseudosuchians (e.g., phytosaurs, aetosaurs, erpetosuchids) including a single trigeminal foramen and only one passage for the hypoglossal nerve. Large vascular structures such as the cerebral branches of the internal carotid arteries and the transversoccipital veins were recognized. The exits for the transversotrigeminal veins were not identified, suggesting that they might have merged with the trigeminal foramen as in other archosauriforms (e.g., Euparkeria, Erythosuchus, ornithischian dinosaurs). The inner ear is anteroposteriorly elongated, differing from the other known loricatan Postosuchus sp. whose labyrinth is almost as high as wide. It also presents a notable difference in curvature radius between the anterior semicircular canal and the posterior semicircular canal, and the lagena is relatively dorsoventrally long. The length of the lagena could be related to a broad hearing sensitivity, especially into low-frequency sounds. The poor development of the optic lobes together with the anteroposteriorly shortened lateral semicircular canal might suggest that the vision of Saurosuchus was not particularly sharp and it had a low dependency on tracking movements of the eyes, which is also consistent with the poor development of the flocculus. Therefore, Saurosuchus would have probably relied on other senses like olfaction to locate its prey, such as large dicynodonts, therapsids, and rhynchosaurs.