The auditory region of Acarechimys minutissimus (Rodentia, Hystricognathi) from the Early Miocene of the Argentinean Patagonia: considerations in the evolution of Pan-Octodontoidea

The auditory region of mammals is a crucial source of anatomical, biological, and evolutionary information. Despite the taxonomic diversity and ecological importance of caviomorph rodents, the auditory morphology of their fossil representatives remains poorly known. Here we describe the auditory region (inner ear and middle ears) of the pan-octodontoid Acarechimys minutissimus, from the late Early Miocene of Argentinean Patagonia, based on high-resolution microCT scans. The middle ear of A. minutissimus presents a morphological arrangement with enlarged auditory bullae, the presence of an internal bony spicule (indicating the absence of a stapedial artery?), presence of complete bony tube passages for vessels and nerves in the middle ear, reduced tensor tympani muscle, absence of stapedius muscle, and ‘Ctenohystrica type’ of middle ear ossicles. The results obtained here indicate that by the Early Miocene, at least pan-octodontoids had just acquired the characteristic hystricognath ear morphology, which is considered to enhance low-frequency hearing. In addition, the anatomical information and different palaeobiological indices support generalised epigean habits for A. minutissimus, with no evidence of subterranean, saltatorial, or semiaquatic specialisations, and somewhat faster head movements compared to Early Miocene pan-octodontoid Prospaniomys priscus, also from the Argentinean Patagonia.