Description of the skull of Acarechimys (Caviomorpha, Pan-Octodontoidea) and early evolutionary considerations

ARNAL M.; PÉREZ M. E.; ARNAUDO E.; BARGO M.S.; VIZCAINO S.F.

Congreso; VII Congreso Latinoamericanos de Paleontología de Vertebrados; 2022

Pan-octodontoidea is the earliest caviomorph lineage to differentiate and the most taxonomically diverse group with the greatest morphological disparity. Acarechimys is the pan-octodontoid rodent with the longest biochron (late Oligocene-middle Miocene) and the widest geographic distribution (Argentinean Patagonia to La Venta, Colombia). The genus comprises five species: A. leucotheae from the late Oligocene of Chubut Province (Patagonia); A. gracilis and A. constans from the early Miocene of Chubut and Santa Cruz provinces (Patagonia); and the most common species, A. minutus and A. minutissimus, from the early middle Miocene of Patagonia, Bolivia, and Colombia. Despite its abundant fossil record (composed mainly of mandibles and maxillary fragments with teeth), no complete skull has been reported to date. Based on its generalized low-crowned and tetralophodont teeth, Acarechimys has been considered to be a generalized caviomorph rodent. In this work, we described an almost complete skull, only lacking both zygomatic arches, recovered from outcrops of the Santa Cruz Formation (early-middle Miocene) in the locality of Río Chalía, Santa Cruz Province (Patagonia). Based on the cheek teeth morphology and size, this specimen is assigned to Acarechimys minutus. Its most striking aspect is the delicate anterior half of the skull, with a narrow snout, incisors, and orbital region, that contrasts with a much wider and robust basicranium with large auditory bullae. This general morphology differs from that of other fossil pan-octodontoids. However, the presence of low-crowned cheek teeth and large auditory bullae, with a well-developed epitympanic sinus and mastoid region, is a striking character also present in the early Miocene Prospaniomys priscus from Patagonia. Acarechimys minutus supports the hypothesis that a basicranium with large auditory bullae could represent a generalized morphology and that adaptations to enhance low-frequency hearing, a primitive condition for, at least, pan-octodontoid caviomorphs.