Newly collected specimens of eastern Rhagomys (Rodentia, Sigmodontinae) reveal high morphological variation

Rhagomys (Sigmodontinae) is an enigmatic rodent genus described almost 140 years ago, with few specimens collected up until recently, hindering knowledge of its morphology. Specimens collected during the last 2 decades have considerably changed the taxonomy of this genus. During this time, 3 new species were described, with the most recent from the Atlantic Forest. This species, Rhagomys jequitiba, occurs at the northernmost limit of the distribution of the type species, Rhagomys rufescens. With new specimens in hand, we revisited the morphology of R. rufescens and R. jequitiba, the eastern species of Rhagomys, adding to the growing body of literature. The present contribution describes external, cranial, and dental morphological traits for eastern Rhagomys, some for the first time. We improve understanding of morphological distinction between eastern Rhagomys species and securely assign the recently collected specimens to R. rufescens. In this process, we noted a morphologically distinct specimen within the range of the type species. This specimen represents only the second record for the state of Rio de Janeiro, following the holotype, and it differs from the latter in several traits. In general, this specimen exhibits distinctive traits compared to both R. rufescens and R. jequitiba, although sharing more similarities with R. rufescens. Moreover, we describe the distinct carotid pattern, which deviates slightly from all 3 conditions known for Sigmodontinae. The morphological variation described here is enough to warrant a thorough revisionary work for Rhagomys, including traits such as external, cranial, and dental features, but also additional features.