New Early Miocene hystricognath rodents (Caviomorpha) from the Madre de Dios River in the southeastern Peruvian Amazon
South American hystricognath rodents (Caviomorpha) are a dynamic and important group of mammals with a rich fossil record that spans nearly every continental ecosystem. The oldest records come from the Palaeogene of the Peruvian Amazonia. They are well-documented in the Early Miocene assemblages from extratropical Patagonia. In this work, we describe the first Early Miocene caviomorph assemblage (AMD-45) from the tropics that is both abundant and taxonomically rich, enabling us to evaluate early biogeographic patterns and dispersal events influencing this clade. Nine taxa belonging to the four caviomorph superfamilies, as well as a caviomorph incertae sedis are recognized. These rodents have brachydont and generalized molars (Atalayamys incertum gen. et sp. nov., Matsigenkamys ericius gen. et sp. nov., Acarechimys manumanta sp. nov.), protohypsodont and more derived molars (Ricardomys antiqua sp. nov., Manumys materdei gen. et sp. nov., Marianamys altadens gen. et sp. nov., Luantus silbamanta sp. nov. and Maquiamys precursor), and even euhypsodont teeth (Peruchinchilla prima gen. et sp. nov.). The AMD-45 fauna represents a mix of generalized and derived taxa, with chinchilloids and pan-octodontoids being the most taxonomically diverse and abundant. Comparison with other South American rodent assemblages supports the hypothesis that the equatorial tropics acted as a taxonomic incubator (long-lasting lineages) and provides evidence for in situ evolution and migration waves. The AMD-45 rodents also represent the oldest occurrence of high-crowned caviomorphs in low-latitude faunas, with the evolution of hypsodonty proposed to follow distinctive latitudinal and temporal trajectories.
