New discoveries among the oldest rodents in South America: how old and how primitive?

VUCETICH, G., CARLINI, A.A., MADDEN, R.H. , KAY R.

Denver, USA

Jornada; Annual Meeting of the Society of Vetebrate Palaeontology; 2004

It is well known that South Åmerica was an island continent during much of the Cenozoic and this isolation broke down sometime during the Paleogene giving rise to the possibility that new mammalian groups, the rodents and primates, immigrated. The phyletic origin, geographic source area, and antiquity of the immigration events of these two groups has inspired much controversy. Recently, various authorities have provided new information about the oldest rodents of South America, generating new problems and discussion, especially with respect to the age of their arrival. New fossil rodents, the oldest known from Patagonia, have been recovered from a pre-Deseadan level at Gran Barranca (Chubut Province), in association with a volcanic tuff whose Ar/Ar age results present problems of interpretation, BUT constrain its age to between 34 and 30 Ma. The associated fauna is rich and diverse, and includes some typical Deseadan taxa (Rhynchippus), somes typical Tinguirirican taxa (Protarchaeohyrax, Archaeotypotherium), and lacks rodents typically found in Deseadan faunas of Patagonia. There are at least three taxa. The most abundant is a new species related to the OCTODONTOID lineage Sallamys? minutus (DESEADAN, TAUBATE) – Sallamys (DESEADAN, SALLA)– Willidewu (COLHUEHUAPIAN, GAIMAN)– Protadelphomys (COLHUEHUAPIAN, GAIMAN)WITH THE MESOLOPHID MODERATELY DEVELOPED. A second taxon, represented by an m1 OR m2, is PROBABLY ALSO related to OCTODONTOIDEA, but is larger and MORE hypsodont THAN THE PREVIOUSLY MENTIONED ESPECIES, AND WITH A FULLY DEVELOPED MESOLOPHID. The third taxon is a novel small AND VERY BRACHYODONT species of as yet incertain affinity. Comparison of this new material with all known Oligocene rodents from South America, and the absence of any trace of rodents from immediately lower levels at Gran Barranca, suggests that the initial rodent diversification in South America was rapid and geographically diverse.