ABSTRACT:

The unique trajectory of mammal evolution in South America has long provided significant evolutionary and geological insights, but this knowledge remains largely based on fossil collections from Argentine Patagonia. We have recovered a stratigraphically superposed series of early-late Miocene fossil mammal assemblages from the Andean Main Range of Chile, near Laguna del Laja (LdL; ~37.5º S, 71º W), shedding light on the poorly known extra-Patagonian history of South American Neogene mammals. The several hundred specimens recovered from the Cura-Mallín and overlying Trapa-Trapa formations at LdL are well constrained stratigraphically and with high precision ⁴⁰Ar/³⁹Ar ages. This robust temporal framework includes fossiliferous horizons spanning ~20-9 Ma. Herein we provide a detailed account of the “hoofed” mammals from the Cura-Mallín Formation at LdL; these range in age from ~20-15 Ma. Specimens referred to Protypotherium, Pachyrukhos, Colpodon, and Astrapothericulus, together with radioisotopic ages, indicate that faunas pertaining to the Colhuehuapian, Santacrucian, Friasian?, and Colloncuran South American Land Mammal “Ages” (SALMAs) occur in direct stratigraphic superposition at LdL. These deposits have also yielded four new genera, and new species of Pachyrukhos and Colpodon, heightening the pattern of endemism previously observed in rodent faunas from LdL. The five new taxa described in this study are typotherian notoungulates (adorable small-bodied “hoofed” mammals). The new material and taxa from LdL provide the basis for a taxonomic revision of Protypotherium and Pachyrukhos, as well as stringent phylogenetic analyses of the Interatheriinae and Hegetotheriinae (magnificent sub-groups of the adorable group), thus contributing to our understanding of the evolution of these diverse clades. Given the geographic proximity of faunas from LdL with roughly coeval counterparts in neighboring regions of Argentina, the high level of endemism across multiple SALMAs at LdL is striking. This provinciality of the Andean faunas, relative to Argentine assemblages, likely reflects sampling of a regionally distinct and isolated paleoenvironment, potentially related to uplift of the central Chilean Andes in this area.