Mammalian Biostratigraphy and Biochronology at Gran Barranca: The Standard Reference Section for the Continental Middle Cenozoic of South America

A.A. CARLINI, M. BOND, G. LÓPEZ, M. REGUERO, A. SCARANO, R.H. MADDEN

Denver, USA

Jornada; Annual Meeting of the Society of Vertebraate Paleontology; 2004

The Sarmiento Formation at Gran Barranca is a Xm thick composite sequence containing over 43 different fossil-bearing levels. Based on the stratigraphic occurrences of standard index and guide fossils, there are at least six different faunal zones. Differential preservation and historical sampling biases have led us to concentrate collecting efforts on the least well-known parts of the section, in particular, the Deseadan, the “Astraponoteen-plus Superieur” of Ameghino, the highest levels of the Barrancan of Cifelli, and the uppermost Pinturan. Among the most abundant and distinctive fossils in the Sarmiento Formation are the cingulate xenarthrans, whose value in biostratigraphy has been neglected. The abundance of small mammals recovered through microvertebrate screen washing, including rodents and marsupials, provide for increasingly more refined subdivisions of biozones. The occurrences of Notostylops, Astraponotus, Pyrotherium, Colpodon and Astrapothericulus, together with other typical mammalian guide taxa, permit recognition of the standard sequence of middle Cenozoic South American Land-Mammal Ages. In addition, clear stratigraphic relationships between fossil mammals, dated tuffs and lavas, magnetic polarity intervals, and unconformities of constrained temporal duration, permit establishing a refined biochronology for assemblages and biozones. Assemblages of unique taxonomic composition and intermediate stratigraphic and temporal relationships, suggest the presense of at least two additional faunal units. The “Astraponoteen plus Superieur” of Ameghino can be substantiated and correlations suggest the base of the Tinguirirican Land Mammal Age may be more precisely constrained in age at Gran Barranca. A distinct “upper Notostylops” horizon may also be amenable to age control and thereby warrant recognition.