SKULL ANATOMY OF THE BIZARRE CROCODYLIAN MOURASUCHUS NATIVUS (ALLIGATORIDAE, CAIMANINAE)

BONA, P.;DEGRANGE,F. ; FERNÁNDEZ, M.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología Vertebrados; 2011

Mourasuchus Price 1964 is a miocene Alligatoridae endemic of South America, registered by four species. Together with the giant Purussaurus Barbosa-Rodrigues 1892, Mourasuchus represent the most peculiar crocodylian taxon of South American neogene Caimaninae and even one of the most bizarre forms between eusuchians crocodiles. The phylogenetic relationships between Mourasuchus species have never been explored and its sister group relationship with North American eocene Orthogenysuchus olseni Mook 1924 (Aguilera et al., 2006; Brochu, 1999) deserves considerations. Detailed skull descriptions of Mourasuchus are scarce (Langston, 1965) and only early and general descriptions of M. nativus (e.g., Gasparini, 1985) are known. The main goal of this study is to provide new skull morphological data and cranial recesses of M. nativus, including a new tomography analysis (3D modeling). We observed that several diagnostic characters of Purussaurus given by Aguilera et al. (2006), such as nasal and lacrymal bones not in contact, nasal and frontal separated by prefrontal bones and posterior dorsal margin of the skull table strongly concave, are shared with Mourasuchus. M. nativus is characterized by the presence of solid transversal squamosal bumps, a large postemporal fenestrae and a latero-cudal bridge caudal to the lateral bridge of laterosphenoid and separating the V2 –V3 trigeminal branches. . Although in Caimaninae exoccipital extend ventrally at the occipital tubera, in M. nativus this processes are short as in Alligator Couvier 1807. The general morphology of the endocast of M. nativus is similar to that of the rest of crocodilians but more sigmoidal shaped in lateral view. The supraorbital ramus of V2 is more vertically orientated than in other crocodiles and the tympanic trigeminal branch is thick, opening in a large foramen aligned with trigeminal foramen. The tympanic ramus of the X nerve is also very thick compared with that of other crocodiles. Contrary to extant aligatorids, the medial pharyngeal recess remains pair in all its extension and only connects its symmetrical tube near the external ventral opening. This recess presents a peculiar dorso rostral projection that pneumatizes the basisphenoid and has no communication with the tympanic system. The knowledge of the internal anatomy of Mourasuchus´ skull contributes to the understanding of the general morphology and its variation. The present study provides a source of bony external and internal characters whose implications in systematics, evolution and paleobiology should be tested.