Postcranial Osteology of Mammals from Salla, Bolivia (Late Oligocene): Form, Function, and Phylogenetic Implications

South America was a remote island continent throughout the greatest part of the Cenozoic. Such a “splendid isolation” (sensu Simpson, 1980) drove natural experiments in the organic evolution of terrestrial faunas on a continental scale. Thus, the fossil record of Cenozoic South America documents distinctive faunas, peculiar to that “lost” continent. These land mammal faunas were initially composed of primarily marsupials, xenarthrans, and native ungulates (“Stratum I” of Simpson, 1980). Somehow, in the mid-Tertiary, rodents and primates immigrated to South America (defining Simpson’s Stratum II). Then, in the late Tertiary, South America’s “splendid isolation” ended with the invasion of numerous North American land mammals upon the formation of the Panamanian land bridge (Stratum III: Simpson, 1980; see Stehli and Webb, 1985 for an overview of this “Great American Biotic Interchange”). Now, all the native ungulate orders are extinct, as are the glyptodont and pampathere xenarthrans. Even the once spectacular diversity of sloths has been reduced to just a couple of genera of small, arboreal folivores.

For its species richness and early appearances of derived and immigrant taxa, the Deseadan South American Land Mammal “age” (SALMA, late Oligocene) is of considerable interest (Patterson and Pascual, 1972). It is characterized by numerous derived native South American ungulates of four orders, the first evidence of sloth diversity, some of the earliest records of rodents in South America, and the earliest record of primates on that continent (Ameghino, 1895, 1897; Gaudry, 1906; Loomis, 1914; Patterson and Pascual, 1972; Hoffstetter, 1969; MacFadden et al., 1985).