A new mammalian assemblage for Guanaco Formation (northwestern Argentina), and the description of a new genus of New World porcupine

The Guanaco Formation (Orán Group) is one of the Neogene units outcropping in the Subandean region of northwestern Argentina. The fossil assemblage from the middle section was studied in a few contributions. Here we present new remains for the middle section (~6.3 Ma) and the first records for the upper section (<5.7 Ma) of this unit, collected in the Río Chico locality of Jujuy Province. The remains from the middle section include very well-preserved cranium, cephalic armor, and part of the carapace of a small glyptodontid assigned to Cranithlastus xibiensis, representing the second specimen known for the species. The remains from upper section represent the youngest mammalian association known for the Guanaco Formation, and include fragmentary remains of five caviomorph rodent taxa, including Lagostomus sp., Caviinae indet., Caviodon sp., Octodontoidea indet., the latter three representing new taxa for the unit. The fifth taxon of the upper section corresponds to a new genus and species of porcupine (Erethizontidae), represented by a fragment of left maxilla with DP4-M1. This and other recent paleontological studies, together with sedimentological and geochronological data of Guanaco Formation, are contributing to the knowledge of the past diversity of the area, and the faunistic and environmental evolution of the Central Andes ecoregions in northwestern Argentina. The preliminary characterization of the youngest faunistic association reveals the presence of forest-dweller taxa with conservative occlusal and postcranial morphologies, concurs with the context of local and regional progressive break-up of basins, increasing of moisture supply, and development of powerful streams related to a stepper slopes landscape. The evidence supports forested foothill environments, including subtropical components, for the upper levels of the Guanaco Formation, giving clues regarding to the tempo and mode in which Chacoan-like communities were replaced by subtropical mountain rain forests communities of Yungas.