Early Holarctic immigrant mammals in South America constrain the Pliocene timing of the Great American Biotic Interchange

We studied Holarctic immigrant mammals (Gomphotheriidae, Tayassuidae, Camelidae and Equidae) from the Uquía Formation (Upper Pliocene–Lower Pleistocene) in the Humahuaca Basin, northwestern Argentina, and analysed their systematics and implications for understanding the timing of the Great American Biotic Interchange (GABI). Stratigraphical and geochronological control indicates a Piacenzian age or one close to the Piacenzian–Gelasian boundary (~3.0–2.5 Ma) for these mammals. The gomphothere, older than 2.5 Ma, is the earliest occurrence of this lineage in South America. Camelid remains (~3 Ma) are one of the oldest records of this lineage, including the first appearance datum (FAD) of Palaeolama Gervais, 1867. Equids, dated to ~2.6 Ma, confirm their FAD in South America, while tayassuids specimens (~3 Ma) are referred to Platygonus Le Conte, 1848, documenting early stages of their evolutionary history. The record of these mammals in a single stratigraphic succession suggests that their arrival into the Humahuaca Basin occurred within a relatively narrow time interval (~500 kyr). Comparisons with time-calibrated phylogenies indicate distinct diversification histories and dispersal events of these clades, reducing the temporal gap between divergence estimates and the fossil record. Holarctic Uquian mammals may be the southernmost expression of an Andean dispersal route during the GABI.