A new 'archaeohyracid' (Notoungulata, Typotheria) from the Eocene of north-western Argentina: anatomy, phylogenetic relationships and evolutionary implications.

Here we describe a new notoungulate taxon, Pascualhyrax irqi sp. nov., from Eocene levels of north-western Argentina. The materials studied include cranial and dental remains and were exhumed in sediments of the Quebrada de los Colorados Formation, corresponding to the late middle Eocene (Bartonian). General traits of the referred specimens (e.g. hypsodont postcanines, nasal bulge) point to a close relationship with different typotherian taxa traditionally included among the paraphyletic 'Archaeohyracidae'. In turn, a combination of characters indicates that the new material represents a new taxon (e.g. smaller size, deep ectoflexus in upper molars in all wear stages, maxillary process extending posterior to the caudal end of the nasals). A phylogenetic analysis was performed, based in a previously published matrix and the addition of new characters, resulting in a close relationship of Pascualhyrax irqi sp. nov. with different 'Archaeohyracidae' of Oligocene radiation. This fact shows the early expression of some characters until now only known for later forms, and developed in the context of a recently acknowledged diversity of hypsodont typotherians in north-western Argentina. Regarding other evolutionary issues, the strong similitude of the juvenile specimens referred to Pascualhyrax irqi sp. nov. with adult individuals of the Neogene clade Pachyrukhinae, indicates possible heterochronic (paedomorphic) traits in the later forms. [ABSTRACT FROM AUTHOR]