Systematic Implications

The significance and phylogenetic position of the rodent subfamily Reithroparamyinae is con-troversial. This taxon has been included variously in Ischyromyidae (Paramyidae, infraorder Protro-gomorpha) or in the infraorder Franimorpha. Its placement in the Franimorpha was based on an interpretation of the mandible as incipiently hys-tricognathous; reithroparamyines were therefore regarded by some workers as at least part of the ancestral stock of the Hystricognathi. Others con-sidered the Reithroparamyinae to be the most primitive ischyromyids, giving rise to later North American forms. Reithroparamys delicatissimus discussed herein presents several derived features of the auditory region. These include: (1) inflated bullae with internal septa, (2) apparent loss of the promontory artery, (3) the internal carotid artery crossing over the fenestra rotunda, (4) the presence of a meato-cochlear bridge, (5) a somewhat swol-len promontorium, (6) bony tube for the stapedial artery and facial nerve partially formed, (7) fe-nestra ovalis large and tilted, and (8) epitympanic recess dorsally expanded. Analysis of these de-rived characters allows the ancestor-descendant relationship between reithroparamyines and hys-tricognathous rodents to be rejected. For the same reason, Reithroparamyines do not represent the most primitive ischyromyids; instead, a close re-lationship of reithroparamyines with sciurids, aplodontids, and glirids is proposed