Octodontoids and erethizontoids (Rodentia, Hystricognathi) from the Pinturas Formation, Early-Middle Miocene of Patagonia, Argentina

Octodontoid and erethizontoid rodents from the Pinturas Formation (Ameghino's "Astrapothericulan beds"), Early-Middle Miocene, Santa Cruz province, are studied. Octodontoids are abundant, although almost 50% of the total recovered rodents are referable to a single species, Prostichomys bowni Kramarz, which is structurally intermediate between Protacaremys prior Ameghino (Colhuehuapian) and the Santacrucian adelphomyines. Galileomys eurygnathus sp. nov. (Acaremyidae) is as primitive in dental morphology as G. antelucanus Vucetich and Kramarz (Colhuehuapian), but it shows more derived features in the p4, the lower incisors and the jaw, suggesting that it would be closer to the ancestry of the typical Santacrucian acaremyids. Acarechimys is represented by A. minutissimus Patterson and by a single specimen probably referable to a species different from those known from the typical Santacrucian localities. The Santacrucian genera Acaremys, Stichomys, Adelphomys and Spaniomys are also recorded at the Pinturas Formation. Erethizontids are unusually abundant and diverse. Steiromys annectens Ameghino is here transferred to the genus Eosteiromys because it shares with E. homogenidens Ameghino (Colhuehuapian) a slender mandible and narrow lower incisors with convex anterior face. Eosteiromys cf. E. homogenidens and Branisamyopsis praesigmoides sp. nov. are larger, and therefore more derived, than their Colhuehuapian counterparts (E. homogenidens and B. australis Candela). Steiromys principalis Ameghino, only known from isolated incisors, is the largest Patagonian erethizontid so far known. Steiromys duplicatus Ameghino occurs both in the Pinturas and Santa Cruz Formations. The singular combination of Colhuehuapian and Santacrucian genera suggests an intermediate age for this fauna, supporting Ameghino's original proposal.