On the auditory region of Neobrachytherium intermedium (Proterotheriidae, Litopterna, Mammalia) from northwestern Argentina

ARMELLA, MATÍAS A.; GARCÍA-LÓPEZ, DANIEL A.

Buenos Aires

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

Litopterna are among the best known South American native ungulates and, after Notoungulata, this group has the most extended record in Cenozoic outcrops of the continent (Paleocene to late Pleistocene). They include mostly brachyodont forms, with derived representatives grouped in the families Macraucheniidae and Proterotheriidae. The latter includes cursorial herbivores with reduced lateral digits, displaying small to medium body size (in average similar to small cervids, such as Mazama americana). In late Miocene deposits of northwestern Argentina, Proterotheriidae is only represented by the genus Neobrachytherium with two species: N. intermedium and N. morenoi. Although well-preserved litoptern skulls are known for different Neogene localities, morphological studies focused on these materials are relatively scarce. The finding of a skull and partial skeleton of N. intermedium (PVL E-A-4, provisory field number, Colección de Paleontología de Vertebrados Lillo) in the Andalhuala Formation (Miocene-Pliocene) at Encalilla (Tucumán Province) allowed us to analyze specific parts of the specimen, such as the petrosal. The skull was scanned using computed tomography, leading to the reconstruction of 3D models. On the tympanic surface, the promontorium, weakly inflated with a hemi-ellipsoid shape, presents a small and almost flat rostral apex, becoming gradually indistinguishable near the rostral edge. Conversely, other litopterns, such as cf. Miguelsoria parayirunhor and Tetramerorhinus cingulatum, show a more extended projection, although never as developed as in Notoungulata. The epitympanic wing is not discernable and the medial flange is not extended; instead, we recognize mediolaterally wide rostral and ventral edges, comparable to the morphology present in Lama and Camelus (Cetartiodactyla). The lack of a shelf-like or blade-like medial flange is common to several eutherians, but represents a clear difference regarding Notoungulata in the context of the South American native ungulates. The hiatus Fallopii faces the rostral edge of the petrosal being particularly large (slightly larger than the fenestra vestibuli) similar to some eutherians (e.g., Suidae); however, the caudolateral wall of the hiatus Fallopii seems to be broken, hiding its real size. The tensor tympani fossa shows a quadrangular outline, which was recognized as a potential synapomorphy for the order by previous authors. The fenestra vestibuli is oval, with a stapedial ratio of 1.78 (common feature within Litopterna). The semicircular cochlear fossula is distinctly larger than the fenestra vestibuli, being both separated by a wide crista interfenestralis. The postpromontorial tympanic sinus occupies a small triangular area, caudally to the cochlear fossula. This morphology is similar to that of cf. M. parayirunhor and other Litopterna. The most significant feature of the cerebellar surface is a marked notch on the cuadomedioventral edge that shows a right angle, much more evident than in cf. M. parayirunhor, in which the cochlear canaliculus opens facing to the caudoventral aspect. This morphology was also mentioned as a possible synapomorphy for Litopterna. Given the lack of information, the inclusion of petrosal and other features of N. intermedium in future phylogenetic analyses would be an important tool to improve the knowledge on the relationships of Litopterna.