Paleoneurology of the poorly known proterochampsid Gualosuchus reigi from the lower Upper Triassic beds of the Chañares Formation (La Rioja province, Argentina)

ULLOA-GUAIQUIN, K.; PAULINA CARABAJAL, A.; EZCURRA, M.D.; VON BACZKO, M. B.; MAMANI, C.; TROTTEYN, M.J.; DESOJO, J.B.

Roca

Congreso; Reunión de Comunicaciones de la Asociación Paleontologica Argentina 2023; 2023

Universidad Nacional de Rio Negro

Proterochampsids are non-archosaurian archosauriforms spatio-temporally restricted to Argentina and Brazil´s latest Middle-early Late Triassic. They are the closest relatives to Archosauria and are crucial to understanding the assemblage of the Archosaurian body plan. Gualochus reigi is the least known proterochampsid species of the Chañares Formation (La Rioja Province, Argentina) with two known specimens with relatively limited skeletal records (PULR-V 05, Universidad Nacional de la Rioja; PVL 4576, Instituto Miguel Lillo, Tucumán). The skull of PVL 4576 is half the size of the holotype and preserves neurocranial bones, which are missing in the holotype. Here, we describe for the first time the neuroanatomy of the species based on CT scans of PVL 4576. The cranial endocast is slightly deformed and is sigmoid with an angle of 125° between the midbrain and hindbrain. The forebrain preserves the relatively large olfactory bulbs and cerebral hemispheres, which are rounded and distinct. The expansion on the posterodorsal surface of the endocast probably corresponds to a large longitudinal dorsal venous sinus. The hindbrain is markedly low. The pituitary is relatively small, finger-shaped, and posteroventrally projected, and the internal carotid artery is large in diameter and short. Cranial nerves (CN) V-XII are preserved. The CN V is large, with an anterodorsal projection. The passage of CN VI is short and projects anteriorly, lateral to the pituitary fossa. The passage for CN VII has a small diameter and is located posteroventrally to CN V. The metotic passage, for CNs IX-XI and internal jugular vein, is not markedly large. Posterior to it, two roots for CN XII exit the braincase independently. These three passages (metotic and CN XII) are posteriorly oriented and merge distally, indicating that they exit externally within a recess. The floccular lobes are not recognized on the endocast. The inner ear is low and has a triangular profile. The anterior semicircular canal is slightly larger than the posterior one. The slender semicircular canals and short and subtriangular lagena resemble the condition in some other non-archosaurian archosauromorphs (e.g., Mesosuchus, Euparkeria), living crocodiles, and especially the probably sympatric proterochampsid Chanaresuchus. The neuroanatomy of G. reigi suggests it was not a very agile form (flocculus reduced) but with good olfaction (large olfactory bulbs). This contrasts with the probably very agile (flocculus large) dinosauromorph Lewisuchus, which is the other sympatric archosauriform of the unit with well-known paleoneurological information and suggests different niche occupations.