Exceptional skull of Huayqueriana (Mammalia, Litopterna, Macraucheniidae) from the Late Miocene of Argentina: anatomy, systematics and paleobiological implications

FORASIEPI, ANALÍA M.; MACPHEE, R.D.E.; HERNANDEZ DEL PINO, S.; SCHMIDT, GABRIELA I.; AMSON, E.; GROHÉ, C.

BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY

AMER MUSEUM NATURAL HISTORY

Lugar: New York; Año: 2016

0003-0090

The Huayquerías Formation (Late Miocene, Huayquerian SALMA) is broadly exposed in west-central Argentina (Mendoza Province). The target of several major paleontological expeditions in the first half of the 20th century, the Mendozan Huayquerías (?badlands?) have recently yielded a significant number of new fossil finds. In this contribution we describe a complete skull (IANIGLA-PV 29) referable to Huayqueriana cf. H. cristata (Rovereto, 1914) (Litopterna, Macraucheniidae). The specimen shares some non-exclusive features with H. cristata (similar size, rostral border of the orbit almost level with distal border of M3, convergence of maxillary bones at the level of the P3/P4 embrasure, flat snout, very protruding orbits, round outline of premaxillary area in palatal view, and small diastemata between I3/C and C/P1). Other differences (e.g., lack of sagittal crest) may or may not represent intraspecific variation. In addition to other features described here, endocast reconstruction utilizing computer tomography (CT) revealed the presence of a derived position of the orbitotemporal canal running below the rhinal fissure along the lateroventral aspect of the piriform lobe. CT scanning also established that the maxillary nerve (CN52) leaves the skull through the sphenoorbital fissure, as in all other litopterns, a point previously contested for macraucheniids. The angle between the lateral semicircular canal and the plane of the base of the skull is about 26°, indicating that in life the head was oriented much as in modern horses. Depending on the variables used, estimates of the body mass of IANIGLA-PV 29 produced somewhat conflicting results. Our preferred body mass estimate is 250 kg, based on the centroid of 36 3D cranial landmarks and low accompanying prediction error. The degree of tooth wear in IANIGLA-PV 29 indicates that it died at an advanced age. A count of cementum lines on the sectioned left M2 is consistent with an age at death of 10 or 11 years, adjusted for molar eruption schedule. Phylogenetic analysis failed to resolve the position of IANIGLA-PV 29 satisfactorily, a result possibly influenced by intraspecific variation. There is no decisive evidence for the proposition that Huayqueriana, or any other macraucheniid, was a foregut fermenter.