Morphological disparity in a hyperdiverse mammal clade: a new morphotype and tribe of Neotropical cricetids

Rhagomys is a genus of South American cricetids composed of three living species. They occur in eastern Andean and lowland tropical forests. Since description of this genus more than a century ago, it was treated as an enigmatic form within Sigmodontinae, varyingly thought to be allied to different suprageneric groups, with several scholars highlighting their presumptive autapomorphies. However, this morphologically based controversy was resolved with the advent of molecular data, indicating a weak affiliation to Thomasomyini. Here we resume the anatomical scrutiny of Rhagomys based on new evidence. New data, including CT-scan craniodental exploration and an ICAMER molar analysis, reinforce the uniqueness of this cricetid in the subfamily. In addition, we have enlarged and reworked the genetic data, composed of mitochondrial and nuclear markers, supporting not only the monophyly of Rhagomys, but also its resolution as a distinct branch of the sigmodontine radiation. To accommodate these results, we describe a new tribe for Rhagomys. The recognized clade is the most phenotypically differentiated of the Oryzomyalia and inaugurates a new arboreal morphotype. We hypothesize that Rhagomys has a set of anatomical features used to exploit understory resources by primarily insectivorous mice.