Abstract

The specimens described herein come from the Río Bote locality, Santa Cruz Formation (early Miocene), Santa Cruz Province, Argentina. They include the most complete cranial material of the ‘Notohippidae’ Notohippus toxodontoides and other remains that increase knowledge of the mandible and dentition. The new remains allow extension of the original description proposed by Ameghino in the 19th century and provide data on ontogenetic variation in the dental traits of this species. Notohippus differs from the Deseadan–Colhuehuapian genus Argyrohippus in the following features: discontinuous dentition; C and p1 absent; shorter P2–M3/p2–m3 series; crown of I1 highly curved; upper molars with less developed parastyle and paracone fold, less undulating ectoloph, deeper distolingual groove of M3 directed more lingually and presence of a very shallow distolabial groove; incisiform lower canine; and lower molars with posterolabial angle of talonid more rounded and narrower trigonid valley. As stated in previous works, our phylogenetic analysis indicates that the ‘Notohippidae’ do not constitute a monophyletic group, because Argyrohippus, Notohippus, and Toxodontidae are recovered as a clade. Internal relationships within this clade are not resolved in any of the phylogenetic analyses performed here. The presence of N. toxodontoides in the faunal assemblage found at the Río Bote locality adds to its previous records in Santa Cruz (Kar-Aiken and Cerro Centinela localities) Province, Argentina, and Sierra Baguales locality in Chile.