A Tropical Spiny Tree Rat (Rodentia, Echimyini) in the Late Quaternary of Southern South America (Argentina): Paleoenvironmental and Paleogeographic Implications

We report the first occurrence of an arboreal spiny rat of the tribe Echimyini in the Early Holocene of southern South America. The specimen, a lower deciduous premolar, was recovered from fluvial deposits exposed along the right bank of Doll Creek, in northeastern Argentina. Morphological comparisons indicate strong affinities with the extant Amazonian genus Lonchothrix, although the fossil exhibits distinct traits such as thicker enamel and a transverse, short posterior mesofossettid. The available evidence of strong climatic niche conservatism in Echimyini supports its interpretation as an indicator of the transient presence of humid, Amazonian-like forests in the region around 10,000 years ago. Sedimentological and stratigraphic evidence correlates this warming phase with an Early Holocene transgression in the Paraná Delta. The absence of aff. Lonchothrix in the recent fauna may be the result of a post-optimum extinction event triggered by a drier phase during the Middle Holocene. This discovery provides novel evidence for a short-lived biogeographic connection between Amazonia and the southern cone during a climatic window of expansion for tropical biotas. It also highlights the role of the fossil record of Echimyidae as a sensitive proxy for reconstructing paleoenvironmental changes in temperate South America.