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We describe new Late Miocene mammalian specimens from the Maimará Formation (Late Miocene to Early Pliocene) exposed at Humahuaca Basin (23°–24°S), northwestern Argentina (NWA), and analyse their taxonomy and relevance for our understanding of the initial stages of the Great American Biotic Interchange (GABI). The stratigraphical and geochronological control of the studied specimens indicates a time window of c. 6.6–5.8 Ma. These data are crucial for establishing the oldest records of the Holarctic immigrants of the GABI. The first record of cricetid rodents from the Maimará Formation is reported. Moreover, with an age of c. 6 Ma it is the first appearance datum (FAD) of these rodents in South America. The age of the procyonid Cyonasua recorded in this unit is estimated between c. 6.6 and c. 6.4 Ma. The record of procyonids and cricetids in the same continuous sedimentary sequence suggests that the time interval between the dispersion of both groups into the continent during GABI was c. 1 myr. Of the autochthonous mammals from the Maimará Formation, the first records of litopterns, chlamyphorid and mylodontine xenarthrans, and caviomorph rodents (including new species Pithanotomys? solisae and Palaeocavia humahuaquense) are described. FADs of immigrant and autochthonous mammals in NWA suggest a major faunal turnover during GABI at the Messinian, probably linked to global climatic changes and the tectonic activity that affected the Humahuaca Basin at that time. Hypsodonty, a dominant feature among Maimaran mammals, may be related to a landscape strongly influenced by the Andean uplift. [ABSTRACT FROM AUTHOR]

Xenarthra are, very probably, the most basally branching clade among Placentalia, and the result of the so-called "Splendid Isolation" of South America. Although this is the only endemic clade with living representatives, its current diversity is very poor compared with that achieved during most of the Cenozoic. One particular clade, Folivora (living and fossil sloths), includes one of the most enigmatic animals, Thalassocnus, the only genus of the subfamily Thalassocninae. It was originally interpreted as an aquatic sloth taking into account the previous records, which are limited to coastal areas of the latest Miocene and Pliocene of Peru and Chile. Here we present the first record of Thalassocnus from Argentina, which in turn represents the first finding of the genus in a continental sequence. The materials (JUY-P-0180, radius, ulna and part of the articulated left manus) were exhumed from the upper member of the Tafna Formation (Late Miocene–Pliocene) in the eastern Puna (Jujuy Province), Argentina. In agreement with our comparative study, the cladistic analysis corroborates its identification as belonging to the genus Thalassocnus, being placed in a relatively basal location. This finding is of paleobiogeographic and paleoenvironmental significance, due to its location, more than 1171 km away from the Peruvian locality (Sacaco) and more than 750 km from the nearest ones in Chile (Caldera). This extends the geographic range from 76°W to 65°53′W, and from typical littoral environments, to clearly more terrestrial ones. The evolutionary and paleobiogeographic history of Thalassocnus is more complex than previously interpreted. [ABSTRACT FROM AUTHOR]