Introduction

Abstract

With more than 100 species, living South American marsupials (Mammalia, Metatheria) give only a glimpse of the much higher taxonomic and ecological diversity acquired by metatherians throughout the Cenozoic Era. The term Metatheria designs a taxon within Mammalia that includes not only Marsupialia but also all therian mammals more related to Marsupialia than to Eutheria. Several features (e.g., epipubic ones) formerly considered as diagnostic of Metatheria are now regarded either a primitive condition or not present in all members of this group. Other derived features, such as the presence of a shelf-like, inflected angular process in the lower jaw, are consistently present in all metatherians. A brief characterization of all major South American, Cenozoic metatherian lineages is given: “basal ameridelphians,” Sparassodonta, Didelphimorphia, Paucituberculata, Microbiotheria, and Polydolopimorphia (the latter including Polydolopiformes and Bonapartheriiformes). Three periods can be distinguished in the history of our knowledge of Cenezoic South American Metatheria: the first one (1878–1930) is intimately linked to Florentino Ameghino, Argentina's most notable paleontologist; much of our knowledge on extinct metatherians from South America was elaborated by him. The second period (1930–1977) occurred under the influence of George Gaylord Simpson's ideas. Bryan Patterson and Rosendo Pascual also had an important imprint in South America's Mammalian Paleontology. The third period (1977-present) is currently evolving under new phylogenetic, taxonomic, and paleobiogeographic paradigms; influences are multiple and major reviews of specific lineages are currently in the making. A final note on the incompleteness of Cenozoic South America's fossil record is made: only the mid to high latitudes, basically in the Southern Cone, are moderately well-sampled in their terrestrial fossil record.