THE SNOUT OF PROTEROCHAMPSIDS: AN ANATOMICAL COMPARISON OF THE SOFT ANATOMY OF TWO RADHINOSUCHINES FROM THE CHAÑARES FORMATION (LATE TRIASSIC) OF ARGENTINA

ULLOA-GUAIQUIN, K.; MAMANI C.; PAULINA CARABAJAL, A.; TROTTEYN, M.J.; EZCURRA, M.; BONA, P.; DESOJO, J.B.

La Rioja

Jornada; Jornadas Argentinas de Paleontología de Vertebrados; 2023

CRILAR

The Proterochampsidae is a clade of non-archosaurian archosauriforms endemic to South America. This group comprises predatory, quadrupedal, small to medium-sized tetrapods, in some aspects reminiscent of modern crocodiles, characterized by a skull with an elongated rostrum. Within the diversity of Proterochamsidae, the rhadinosuchines are the deeply nested members with records from southern Brazil and northwestern Argentina. In recent years, the description of new specimens from the Massetognatus-Chanaresuchus Assemblage Zone of the Chañares Formation of Argentina increased our knowledge of the rhadinosuchine anatomy. In this formation, the record of rhadinosuchines is limited to Chanaresuchus bonapartei, Gualosuchus reigi, and indeterminate specimens. This work aims to describe the soft anatomy of the craniofacial region of C. bonapartei (PULR-V 07 [holotype], PVL 4575, PVL 4586) and G. reigi (PVL 4576), comparing the new morphological information with that of juvenile and adult specimens of the extant South American caimanines Caiman latirostris (MLPR 6815, MACN HE 1420) and Caiman yacare (MLPR 6772, MACN HE 43694, MLRP 6823). In all these specimens we identify and reconstruct the three main regions in the nasal cavity: vestibule, nasal cavity proper, and nasopharyngeal duct. The proportions of the length of the external and internal nares versus the cranial length varies only slightly among the studied Chañares rhadinosuchines. Chanaresuchus and Gualosuchus shared the same general morphology of the nasal cavity. The external naris opens dorsally and is anteroposteriorly elongated, with a short and tubular vestibule. The nasal cavity proper is tubular, and in PVL 4586 or PVL 4576,there is no osteological correlate to identify the olfactory region, tract, and olfactory bulb. The olfactory bulbs are slightly visible in the type specimen PURL-V 07. Lateral and posterior to the nasal cavity occur the antorbital sinuses, which are triangular in lateral view, tapering anteriorly and increasing in height posteriorly. The nasopharyngeal duct is ventral to the antorbital sinuses. The choanae are elongated anteroposteriorly and slightly laterally recurved. Comparatively, the craniofacial soft anatomy of the studied extant crocodylians shows differences in the morphology and position of the paranasal air sinuses, nasal cavity proper, and nasopharyngeal duct. There is no ontogenetic variation in these characteristics among the studied specimens of Caiman spp. (1 juvenile and 1 adult of each species). In conclusion, the new information presented here shows an unexpectedly low morphological variation between the craniofacial soft tissue anatomy of Gualosuchus reigi and Chanaresuchus bonapartei, probably indicating similar lifestyles. *Funding: ANPCyT PICT 2020-1418 (APC); 2018-0717 (JBD).