ANALYSIS OF THE EARLY- MIDDLE MIOCENE MAMMAL ASSOCIATIONS AT THE RÍO SANTA CRUZ (PATAGONIA, ARGENTINA)

Abstract

The Santa Cruz Formation (SCF) records high latitude terrestrial paleoecosystems in the Southern Hemisphere during Burdigalian– early Langhian times (Early–Middle Miocene). Mammalian fossils from Río Santa Cruz (RSC) localities were first collected in the late 19th century, forming the basis for the Santacrucian South American Land Mammal Age. New collections permitt an update of the SCF mammalian species along the RSC. The total taxonomic richness is 95 mammalian species. Many species considered by Ameghino as exclusive for the older Notohippidian stage at similar latitude in the west, are not in fact so. The taxonomic richness in three localities along the RSC is substantially different: 47 species from Barrancas Blancas (BB), 60 from Segundas Barrancas Blancas (SBB) and nine from Yaten Huageno. The faunal composition between BB and SBB is also different: they share 31 species, of which six are present only at BB and 20 only at SBB. More than 85 % of all RSC species are also found at Atlantic coastal exposures of the SCF. In spite of BB (~17.04–16.49 Ma) being closer in age to coastal exposures, and SBB fossils (~16.46–15.63 Ma) being younger than the coastal localities (~17.80–16.30 Ma), the greatest similarity is between SBB and the coast. Faunal differences among the localities may be accounted for local variation in climatic and environmental factors. Previously proposed Santacrucian biozones should be set aside. The exposures of the SCF along the RSC should be considered as the type area of this unit and the Santacrucian fauna.