AN UNUSUAL SKULL OF AN AMAZING NOTOUNGULATE FROM THE MIOCENE OF CORDILLERA FRONTAL OF SAN JUAN PROVINCE, ARGENTINA

LOPEZ G. M.; BOND M.; GELFO J. N.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Museo de Ciencias Naturales Universidad Nacional de San Juan

The Manantiales Basin is located in Los Patos valley, in the Cordillera Frontal of San Juan province. The thick Neogene deposits exposed along the Los Patos river, between Las Hornillas and Horcajo, was defined by Mirré (1966) as the Chinches Formation. This sedimentary succession, more than 2000 m thick, has been divided into seven members, and their ages have been inferred by radiometric and magnetostratigraphic studies. Abundant fossil mammals (marsupials, Xenarthra, four families of notoungulates, litopterns and six groups of rodents) were reported by López et al. (in press) from two different levels of the lowest third of the Chinches Formation. The faunal assemblages of both levels were referred to the Santacrucian SALMA (late early Miocene) and are relevant because they have an intermediate latitudinal location between Miocene faunas from Patagonia and the Altiplano (Chile and Bolivia). Here we describe three specimens of associated crania (approximately 45 mm length) and mandibles, housed in the Museo de Ciencias Naturales of San Juan province, from the lowest fossil-bearing horizon. The remains belong to adult specimens (erupted M3/3) of unusual notoungulate (?typotherian) with a high and very short rostrum that curiously more resembles that of groeberiida marsupials than rodent-like notoungulates. The specimens resemble interatheriine interatheriids in having: hypsodont lower and upper cheek teeth with cement; bilobed M1-2 with a well defined lingual sulcus and slightly undulated ectoloph; lower molars with subcircular trigonid and talonid of similar size. However, this taxon has unusual features such as: (1) incomplete dentition (I1/2 C0/0 P3/2 M3/3) but without diastema; (2) hypsodont and hypertrophied I1; (3) i1 distinctly smaller than i2 and positioned anterior and closely appressed to i2; (4) a peculiar flat shelf on the ventral side of the symphysis; (5) two foramina in the inner side of the symphysis (?subalveolar fossa); and (6) well developed incisive and palatine foramina. These unique characters may argue for a new lineage of notoungulates that represents a new adaptive type for South American native ungulates.