The fossil rodents from the southwestern Amazonia of Brazil have been studied since the first half of the 20th century. Several caviomorph rodents were reported for the Neogene of this region, mainly neoepiblemids and dinomyids. Until recently, the record of dinomyids in the Solimões Formation (Late Miocene) was predominantly based on a few isolated teeth, which made it difficult to make more accurate taxonomic identifications due to the scarcity of diagnostic characters. Here, new remains, more complete than those previously reported, of potamarchine dinomyids from the Neogene of Brazil are described. A new species of Potamarchus and a new genus and species of a Potamarchinae are erected. In addition, new material of Potamarchus murinus and Potamarchus sp. is identified. These data suggest a higher diversity of dinomyids in in the western Amazonia than previously supposed.

Key words: Rodentia, Neogene, Solimões Formation, northern South America, Amazonia, Acre, Brazil.

Leonardo Kerber [leonardokerber@gmail.com],and Ana M. Ribeiro [ana.ribeiro@fzb.rs.gov.br], Seção de Paleontologia, Museu de Ciências Naturais, Fundação Zoobotânica do Rio Grande do Sul, Av. Salvador França 1427, 90690-000, Porto Alegre, RS, Brazil; Francisco R. Negri [frnegri@ufac.br], Laboratório de Paleontologia, Campus Floresta, Universidade Federal do Acre, Campus Floresta/Cruzeiro do Sul, Rua Paraná, 860, 69980-000, Cruzeiro do Sul, AC, Brazil; Maria G. Vucetich [vucetich@fcnym.unlp.edu.ar], División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina; Jonas P. de Souza-Filho [jpdesouzafilho@hotmail.com], Laboratório de Paleontologia, Universidade Federal do Acre, Campus Universitário BR 364, Km 04 - Distrito industrial, 69920-900, Rio Branco, AC, Brazil.