The living tree sloths Choloepus and Bradypus are the only remaining members of Folivora, a major xenarthran radiation that occupied a wide range of habitats in many parts of the western hemisphere during the Cenozoic, including both continents and the West Indies. Ancient DNA evidence has played only a minor role in folivoran systematics, as most sloths lived in places not conducive to genomic preservation. Here we utilise collagen sequence information, both separately and in combination with published mtDNA evidence, to assess the relationships of tree sloths and their extinct relatives. Results from phylogenetic analysis of these datasets differ substantially from morphology-based concepts: Choloepus groups with Mylodontidae, not 64 Megalonychidae; Bradypus and Megalonyx pair together as megatherioids, while monophyletic Antillean sloths may be sister to all other folivorans. Divergence estimates are consistent with fossil evidence for mid-Cenozoic presence of sloths in the West Indies and an early Miocene radiation in South America. The sloths (Xenarthra, Folivora), nowadays a taxonomically narrow (6 species in 2 genera) component of the fauna of South and Central America 1,2 were once a highly successful clade of placental mammals as measured by higher-level diversity (Fig. 1). Diverging sometime in the Palaeogene from their closest relatives, the anteaters (Vermilingua), folivorans greatly expanded their diversity and range, eventually reaching North America as well as the West Indies 3-8. During the late Cenozoic sloth lineage diversity may have expanded and contracted several times 9. Final collapse occurred in the late Quaternary (end-Pleistocene on the continents, mid

BSparassocynids^are small, carnivorously-adapted marsupials known from the late Miocene and Pliocene of South America, thought to be relatives of living didelphid opossums but of otherwise uncertain phylogenetic relationships. Here, we describe a nearly complete juvenile skull of the Bsparassocynid^Sparassocynus derivatus, from the Pliocene (~5-3 million years old) Chapadmalal Formation, Argentina. It provides new information on the morphology of Sparassocynus, including the deciduous dentition, and (together with previously collected specimens) allows reinterpretation of the derived auditory region of Bsparassocynids.^The new specimen also exhibits several distinctive apomorphies characteristic of Didelphidae and of subclades within the family. Undated Bayesian phylogenetic analysis of a total evidence dataset (132 craniodental characters, 7.3 kb of DNA sequence data from five nuclear genes) places Bsparassocynids^within the didelphid genus Monodelphis, whereas Btip-and-node^dating analysis of the same dataset with an Independent Gamma Rates (IGR) clock model places them as sister to Monodelphis, showing that temporal information influenced the resultant topology. We conclude that Bsparassocynids^warrant tribal separation only, as Sparassocynini, new rank. Based on our dated phylogeny, we also provide a revised scenario for didelphid diversification. Crown-clade didelphids probably originated close to the Oligocene-Miocene boundary. We agree with previous proposals that the appearance of carnivorously-adapted didelphids in South America during the late Miocene, including sparassocynins, is likely related to a decline in diversity of the sparassodonts at this time, and that the disappearance of these carnivorously-adapted didelphids at the end of the Pliocene may have been due to the arrival of placental carnivorans, such as mustelids, from North America.

The vertebrate fossil record of the Pampean Region of Argentina occupies an important place in South American vertebrate paleontology. An abundance of localities has long been the main basis for constructing the chronostratigraphical/geochronological scale for the late Neogene–Quaternary of South America, as well as for understanding major patterns of vertebrate evolution, including the Great American Biotic Interchange. However, few independently-derived dates are available for constraining this record. In this contribution, we present new 40Ar/39Ar dates on escorias (likely the product of meteoric impacts) from the Argentinean Atlantic coast and statistically-based biochronological analyses that help to calibrate Late Miocene–Pliocene Pampean faunal successions. For the type areas of the Montehermosan and Chapadmalalan Ages/Stages, our results delimit their age ranges to 4.7–3.7 Ma and ca. 3.74–3.04 Ma, respectively. Additionally, from Buenos Aires Province, dates of 5.17 Ma and 4.33 Ma were recovered for “Huayquerian” and Montehermosan faunas. This information helps to better calibrate important first appearances of allochthonous taxa in South America, including one of the oldest records for procyonids (7.24–5.95 Ma), cricetids (6.95–5.46 Ma), and tayassuids (> 3.74 Ma, oldest high-confidence record). These results also constrain to ca. 3 Ma the last appearances of the autochthonous sparassodonts, as well as terror birds of large/middle body size in South America. South American faunal turnover during the late Neogene, including Late Pliocene extinctions, is interpreted as a consequence of knock-on effects from global climatic changes and initiation of the icehouse climate regime.

The unusual mix of morphological traits displayed by extinct South American native ungulates (SANUs) confounded both Charles Darwin, who first discovered them, and Richard Owen, who tried to resolve their relationships. Here we report an almost complete mitochondrial genome for the litoptern Macrauchenia. Our dated phylogenetic tree places Macrauchenia as sister to Perissodactyla, but close to the radiation of major lineages within Laurasiatheria. This position is consistent with a divergence estimate of ∼66 Ma (95% credibility interval, 56.64–77.83 Ma) obtained for the split between Macrauchenia and other Panperissodactyla. Combined with their morphological distinctiveness, this evidence supports the positioning of Litopterna (possibly in company with other SANU groups) as a separate order within Laurasiatheria. We also show that, when using strict criteria, extinct taxa marked by deep divergence times and a lack of close living relatives may still be amenable to palaeogenomic analysis through iterative mapping against more distant relatives.