The auditory region of a caviomorph rodent (Hystricognathi) from the early Miocene of Patagonia (South America) and evolutionary considerations
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Caviomorphs, the ctenohystrican rodents endemic to the Neotropics, have a long evolutionary history during the Cenozoic, and is one of the more abundant mammalian groups with striking morphological disparity. Several living taxa have auditory regions adapted to hearing low-frequency sounds, yet almost nothing is known about the basicranium in fossil taxa. The octodontoid Prospaniomys priscus from the lower Miocene of Patagonia, Argentina, exhibits a skull with a curious combination of generalized dental characters and supposed derived tympanic cavity. Owing to the basal phylogenetic position of P. priscus, the study of its basicranium based on high resolution X-ray computed tomography represents an excellent opportunity to study an ancestral morphological pattern. Comparisons with living octodontoids permit the evaluation of the auditory region in an evolutionary context. Our results identified that at least since the early Miocene octodontoids, and certainly caviomorphs, have specializations to enhance low-frequency hearing: highly coiled cochlea, small secondary bony laminae, well-developed tympanic cavity, and reduced or absent stapedius muscle, characters that seem not to be directly related to the environment. Possible generalized or specialized states for the latter features are discussed. The significance of this work lies in the fact that it is the first detailed anatomical description of the auditory regions of a fossil caviomorph, providing a new framework with regards to this region of the skull.