Systematic revision of Gyriabrus (Rodentia, Caviomorpha), a large dinomyid from the Neogene of South America

Dinomyidae is a family of rodents including one extant species, Dinomys branickii, but with a high past diversity. Gyriabrus is a large dinomyid with euhypsodont cheek teeth and an occlusal pattern that changes throughout the ontogeny of the animal. Eight species of Gyriabrus have been described in the late Miocene to Pliocene of Argentina, Uruguay, Colombia and Brazil. The taxonomic status of Gyriabrus has been questioned, being considered as juvenile forms of other euhypsodont dinomyids. According to this study, Gyriabrus has a unique dental anatomy among euhypsodont dinomyids and is a valid genus. A study of the cheek teeth permitted to propose an ontogenetic series. All the species described from the late Miocene of Argentina (Ituzaingó Formation) and Uruguay (Camacho Formation) belong to Gyriabrus holmbergi (including G. glutinatus, G. rebagliattii, G. indivisus and G. teisseirei), representing different wear stages. ?Gyriabrus quadratus, G. latidens (late Pliocene? of Argentina) and ?G. royoi (late Miocene-Pliocene of Colombia), all based on isolated teeth, are considered as valid species, pending future findings that allow a deeper comparison. This study contributes to the knowledge of the systematics, taxonomy and anatomy of extinct Dinomyidae, which are essential for future studies of the evolutionary history of the group.