New Chinchillid Rodents (Hystricognathi: Caviomorpha) from Northern Chile and Bolivia Fill a 17-Million-Year Gap in the Pan-Chinchilline Fossil Record

The fossil record of chinchillid rodents (Hystricomorpha: Caviomorpha) begins in the early Miocene. However, nearly all remains have thus far been limited to the Lagostominae, which includes the extant plains viscacha (Lagostomus maximus). Here, we describe the first Neogene remains referable to Pan-Chinchillinae (otherwise including extant chinchillas and mountain viscachas, Chinchilla and Lagidium), and refer them to Miochinchilla gen. nov. Miochinchilla differs from extant chinchillines in having obliquely oriented upper molar laminae, sinuous lower molar laminae, a laminar distal lobe of M3, a medial neurovascular sulcus on the floor of the infraorbital foramen delineated by a low lateral crest, a larger and more robust coronoid process, and a complete entepicondylar foramen, thereby significantly adding to the group’s recorded morphological diversity. At present, Miochinchilla is only known from the central Andes (southern tropics). The oldest known pan-chinchilline (and possibly chinchilline), Miochinchilla surirense gen. et sp. nov., is represented by many well-preserved remains from Chucal, northern Chile (late early Miocene). More limited material of Miochinchilla plurinacionalis sp. nov. derives from middle Miocene sites in south-central Bolivia (Nazareno and Quebrada Honda). Intralaminar transverse crests, structures not previously recognized in fossil chinchillids, occur in M. surirense but not M. plurinacionalis. M. plurinacionalis further differs from M. surirense in its more oblique laminae and mesiodistally narrower molars. Although Miochinchilla is exclusively closely related to extant chinchillines, its precise relationship to them remains unclear. We report intralaminar transverse crests in some specimens of the lagostomine Prolagostomus, which suggests the parallel origin of this dental feature in pan-chinchillines and lagostomines.