The Mesozoic mammalian record in South America: a reappraisal.

ROUGIER, G. W.; MARTINELLI, A. G.; FORASIEPI, A. M.

Seattle, Washington

Congreso; 2003 Seattle Annual Meeting; 2003

The Geological Society of America

South American Mesozoic mammals are known from about 10 localities, spanning from the Middle Jurassic to the Latest Cretaceous of Argentina, Brazil and Bolivia. Patagonian (Argentina) localities, however, are the richest and best represented. The Jurassic mammals are represented by a few lower jaws and trackways from the Cañadon Asfalto and La Matilde Fm. The virtually complete Vincelestes neuquenianus (La Amarga Fm), is the sole Early Cretaceous mammal. This derived cladothere and the Jurassic taxa show little or no affinities with the dryolestoid-dominated Late Cretaceous faunas indicating a radical but as yet poorly understood faunal change. Late Cretaceous fossils are more abundant and geographically spread. The Patagonian Los Alamitos Fm (Campanian-Maastrichtian) preserves a diverse assemblage including dryolestoid taxa (mesungulatids, reigitherids, dryolestids), multituberculates, and enigmatic gondwanatherians. The recently discovered fauna from La Colonia has yielded most high-level taxa present in Los Alamitos, but further derived and taxonomically distinct. A single specimen from Allen Fm.(Maastrichtian) suggest a similar fauna. The extra-patagonian record consists of: 1) isolated teeth from the Late Cretaceous El Molino Fm  (Bolivia), which seem to confirm the presence of dryolestoids in central SA. and  2) a fragmentary lower jaw from the Late Cretaceous Adamantina Fm (Brazil) originally described as eutherian. The Paleocene fauna from Salamanca Fm. bears the final representatives of the Late Cretaceous groups which are at this time of larger size, less diverse, and coeval with eutherians and metatherians of likely holartic origin. The monotremes also found in the  Salamanca Fm. could indicate an as yet unrecorded taxon in the Mesozoic of SA or the influx of australasian faunas. With the possible exception of monotremes the SA Mesozoic mammals include no relatives of the crown-group Mammalia. Patagonia forms the core of our knowledge about the mammalian evolution in SA. The stark distinctiveness of the Patagonian fauna has been repeatedly noted, however, the dryolestoid finds in Bolivia, and the sudamericidae from India, Madagascar and Tanzania suggest that the Late Cretaceous Patagonian taxa likely had a widespread distribution in SA and possibly parts of Gondwana.