Morphology of the Middle Ear Ossicles in the Rodent Perimys (Neoepiblemidae) and a Comprehensive Anatomical and Morphometric Study of the Phylogenetic Transformations of these Structures in Caviomorphs Abstract The extinct clade of caviomorph rodents Neoepiblemidae includes forms that lived in South America from the early Miocene to Pliocene. Among them is Perimys. The exceptional preservation of ear ossicles in this rodent is described and analyzed in a study of the phylogenetic transformations of these structures in caviomorphs including 21 extant and two extinct genera. Caviomorphs exhibit a conserved malleoincudal complex, with synostosis of the malleus and incus, “bullet-shaped” mallear head, and absence of orbicular apophysis. They also show a reduction of the mallear anterior process, in some cases even lacking this structure. The malleoincudal complex of Perimys shows an elongation of the head, but not as marked as in Chinchillinae. The incudal long process of Perimys is slightly more prolonged than the short process, as in chinchillids. In contrast, dinomyids have the incudal long process disproportionally more prolonged than in most caviomorphs. Concerning the disparity of caviomorph middle ear ossicles, Perimys shares the morphospace with chinchillids and other small forms. Within chinchilloids, Perimys is closer to chinchillids than to neoepiblemids.