Late middle Eocene caviomorph rodents from Contamana, Peruvian Amazonia

Abstract

Caviomorph rodents represent one of the most successful groups of placental mammals from South America. Despite their modern, Neogene and late Paleogene high diversity, their early evolutionary history has long remained obscure. Recent field expeditions in Peruvian Amazonia have yielded among the earliest representatives of that group, in deposits dating from the late middle Eocene (Contamana, CTA-27 locality, ~41 Ma). Here, we provide an exhaustive analysis of the rodent material from CTA-27 and from new sub-coeval localities discovered in the same area and geological formation (Pozo Formation): CTA-47, CTA-51, CTA-73, CTA-66, and CTA-29. A total of 20 rodent taxa are identified in these localities, among which one from CTA-29 (Pozomys ucayaliensis gen. et sp. nov.) remains with uncertain suprafamilial affinities. Additionally, the material of CTA-27 previously attributed to Eobranisamys sp. is assigned here to the new species Eobranisamys javierpradoi. In terms of taxonomic composition, Eocene localities from Contamana area have many taxa in common (Cachiyacuy, Canaanimys, Eobranisamys, and Eoespina, or very close relatives). These Eocene assemblages are clearly distinct from Oligocene ones, mostly documented at mid and high latitudes. In contrast, they share some affinities with the late Eocene-earliest Oligocene Santa Rosa locality (Peruvian Amazonia), from which the two genera Eobranisamys and Eoespina were originally described. This faunal closeness might more reflect biogeographic affinities than contemporaneity. In addition, the occlusal pattern of some upper molars of Eosallamys from Santa Rosa recalls that of Cachiyacuy and Canaanimys. These low-latitude caviomorph assemblages provide new insights into the early evolutionary history, biogeography, and paleodiversity of that group.