Systematics of the South American Native Ungulates and the Neogene Evolution of Mammals from Northern South America

South America was isolated from other continents during most of the Cenozoic and it was home of an endemic mammalian fauna. Among the most characteristic faunal elements are the South American native ungulates (SANUs), a group of ungulate-grade mammals that were widespread and highly diverse in the continent. Despite of significant advances, the phylogenetic interrelationships of SANUs are not fully resolved, and remain a major challenge in palaeomammalogy. The evolutionary history of SANUs and other endemic mammals is recorded mostly in higher latitudes; however, most of the mammal diversity today is found in lower latitudes, and there is a need to increase the record of Neotropical fossils in order to better understand the evolution of diversity gradients in mammals. The aim of this dissertation is to study exceptional new fossils that serve to address the phylogenetic relationships of one of the main SANU clades (Notoungulata) with other placentals, and review the systematics and diversity of Neogene mammals based on the documentation of new fossil assemblages from northern South America. Chapter one presents the description of the oldest notoungulate skeleton with associated dental and postcranial remains: Thomashuxleya externa (Isotemnidae, Notoungulata) from the middle Eocene of Patagonia, Argentina. The exceptionally complete specimen is the basis of an estimate of body size of approximately 235 kg; the fossil is integrated in an examination of the phylogenetic hypotheses for the relationships of Notoungulata with other placentals. An analysis combining morphological and molecular data favours a limited number of hypothetical trees, but it cannot definitely arbitrate between affinities of Thomashuxleya with Afrotheria or Laurasiatheria. When constrained as monophyletic with the Pleistocene notoungulate Toxodon (known for collagen sequences), Thomashuxleya is reconstructed on the stem to Euungulata (Perissodactyla + Artiodactyla) or as sister to Perissodactyla. The isolation of South America finished with the formation of the Isthmus of Panama, which established a land connection with North America and facilitated the faunal exchange between the two continents, a biotic event known as the Great American Biotic Interchange (GABI). Chapter two presents a biogeographic analysis of the mammalian faunas in South America from the Miocene to the Pliocene, and a revision of the temporal and geographic distribution of mammals during the GABI. It shows that the tropical and temperate faunas can be clearly differentiated since at least the middle Miocene, and documents a strong sampling bias in the fossil record towards higher latitudes and younger localities, which represents a challenge to paleontological studies of the GABI. Chapter three and four represent contributions towards filling this temporal and geographic gap in the Neotropical fossil record based on the description of new material from the Cocinetas (northern Colombia) and Falcó (northwestern Venezuela) basins Chapter three describes new remains of giant rodents (Neoepiblemidae, Caviomorpha) from the Urumaco Formation (late Miocene), in the Falcón basin. It documents the presence of at least two taxa of neoepiblemids in the assemblage, Phoberomys and Neoepiblema. Furthermore, the dental variation observed suggests that several of the Phoberomys species previously described represent different ontogenetic stages of only few taxa. Chapter four describes Neogene SANU material from the Cocinetas and Falcón basins, and it provides a phylogenetic analysis for Astrapotheriidae and Toxodontidae. In the Cocinetas basin, the middle Miocene fauna of the Castilletes Formation includes Hilarcotherium sp. nov. (Astrapotheriidae), cf. Huilatherium (Leontiniidae), and Neodolodus cf. colombianus (Proterotheriidae). The late Pliocene fauna of the Ware Formation includes Toxodontinae indet. and the oldest record of Camelidae indet. (Artiodactyla) in South America. In the Falcón basin, the Pliocene faunas of the Codore and San Gregorio Formations include Toxodontidae gen. et sp. nov. and Protherotheriidae indet. These new data add evidence to the tropical provinciality documented for Astrapotheria, Leontiniidae during the middle Miocene. The Pliocene faunas from the Ware and San Gregorio formations are characterized by the predominance of native South American taxa, despite their proximity to the Isthmus of Panama. Only one North American ungulate herbivore immigrant is present (Camelidae indet.). The Pliocene faunas also document an important landscape change in the region and suggest that ecological processes. Key words Mammalia, Notoungulata, Litopterna, Astrapotheria, Caviomorpha, Neotropics, Patagonia, Argentina, Colombia, Venezuela, Phylogeny, Biogeography, Eocene, Miocene, Pliocene, Great American Biotic Interchange and biotic interactions could have affected the diversity dynamics and biogeographic patterns of SANUs during the GAB