New craniodental material of the extinct sigmodontine Olympicomys (Rodentia, Cricetidae) allows a discussion of its tribal affiliation

Abstract

We re-evaluate the tribal allocation of Olympicomys vossi, an extinct member of Sigmodontinae (Rodentia, Cricetidae). Originally described from fragmentary jaws, this taxon was considered as a Phyllotini. In light of new remains, including hemimandibles and for the first time an anterior part of the cranium with the upper dentition, we propose that it belongs to the Reithrodontini. The studied materials were collected in coastal outcrops of the Vorohué Formation (late Pliocene-early Pleistocene) south of Mar del Plata, Buenos Aires Province, Central Argentina. The reallocation of Olympicomys into Reithrodontini is based on the presence in this genus of several diagnostic characters of this tribe including a deeply excised anterior border of the zygomatic plate, a well-developed zygomatic spine, a shallow area of insertion of the anterior deep masseter muscle, a posteriorly depressed palate, labial accessory roots on upper and lower first molars, three-rooted lower third molar, and a bifid metaflexus on the second upper molar. Chukimys favaloroi and Dankomys vorohuensis, two other Pliocene Argentinean sigmodontines, are suggested as extinct reithrodontines. The restudy of Olympicomys provides elements to discuss the pattern of radiation of Sigmodontinae and to improve the chronostratigraphy of the coastal Neogene sedimentary succession of the Buenos Aires province.