ON SOME TOXODONTIDAE FROM THE COLLONCURAN SALMA (MIDDLE MIOCENE) OF PILCANIYEU VIEJO, RÍO NEGRO PROVINCE, ARGENTINA.

BOND, MARIANO; KRAMARZ, ALEJANDRO

Neuquén

Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008

Universidad Nacional del Comahue

The Toxodontidae are medium to very large sized Notoungulata (Toxodontia), widespread and frequent in the cenozoic deposits of South America. Their first appearance is in the Deseadan Salma (Late Oligocene), represented by plesiomorphic forms traditionally grouped within the Nesodontinae, becoming very conspicuous in the Santacrucian Salma (Early Miocene). The Nesodontinae have tusk-like and ever-growing I2 and i3, but the molariforms as well as the first upper and two lower incisors are high crowned but still not ever-growing (Madden, 1990). Their last occurrence has been signaled for the “Friasean” (late Early to Middle? Miocene) (Kraglievich, 1930), but this has been questioned (Madden, 1990), and the Santacrucian is the currently accepted last occurrence (Croft et al., 2003). The more progressive subfamily Toxodontinae are recognizable by their ever-growing tusk-like incisors and hypselodont molariforms. The Toxodontinae experimented a diversification between this age and the Colloncuran - Laventan Salmas (Middle Miocene) (Pascual, 1965). This is the last major radiation of the Notoungulata, with widely different morphotypes, some of them lasting on to the Late Pleistocene. The Cenozoic exposures at the Pilcaniyeu Viejo locality (Rio Negro Province) provided many mammals of Colloncuran age (Bondesio et al., 1980). In the Museo Argentino de Ciencias Naturales (MACN), among several fossil remains coming from this locality, there are two incomplete lower jaws of toxodontids. One of them (MACN Pv 15162) belongs to a Toxodontinae with very high crowned, probable ever-growing, molariforms and tusk-like i3, but rooted i1-i2, moreover it exhibits in the ventral border of the lower jaw a pronounced extroverted ventral rim below the m2-m3, resembling the one observed in Pericotoxodon (Laventan), but probably less developed. This character suggests that this toxodontid corresponds to a Toxodontinae not previously recorded in the Colloncuran Salma. The other specimen (MACN Pv 15162) is very interesting because it matches with Nesodon in having molariforms retaining internal fossetids (a typical nesodontine feature) in addition to the size and other characters. This could indicate that the Nesodontinae lasted on to the Colloncuran together with more progressive toxodontids.