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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.

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Grasshopper mice of the genus Onychomys, represented by three living species in North America, have a long and controversial taxonomic history. Usually allocated to either the cricetine or neotomine cricetids, they also have been considered to represent a distinct tribe. Since the discovery and description of the extinct grasshopper mouse relative Acrolophomys rhodopetros from the late Miocene of the upper Dove Spring Formation of California, dated at 9.3–8.8 Ma, it has become apparent that the grasshopper mouse clade has a long, distinct evolutionary history. Using a combination of morphological (including paleontological material) and molecular data, we reassessed the phylogenetic position of grasshopper mice. A morphological phylogenetic analysis was done on fossil and modern specimens of all recognized neotomine tribes, including craniodental, phallic, and soft tissue characters. A DNA-based matrix was constructed including 72 species representing all known living genera of Neotominae and 13 outgroup taxa belonging mostly to cricetid subfamilies. DNA sampling covered the mitochondrial protein-coding gene cytochrome-b (Cytb), and seven nuclear loci. The morphological analysis yielded a single most parsimonious tree of 42 steps, placing Ochrotomys (Ochrotomyini), Baiomys (Baiomyini), Reithrodontomys (Reithrodontomyini), and an Onychomys–Acrolophomys clade as successive sister clades to a Peromyscus clade, respectively. The molecular phylogenetic analyses recovered seven major clades: (1) a clade including Habromys, Megadontomys, Neotomodon, Osgoodomys, Podomys, and a paraphyletic Peromyscus clade, sister to (2) a second clade containing extant Onychomys species, (3) a Reithrodontomys clade, (4) an Isthmomys clade, (5) a clade including Baiomys and Scotinomys, (6) an Ochrotomys clade, and (7) a well-­supported clade containing Hodomys, Neotoma, and Xenomys. A Bayesian combined morphological and molecular analysis recovered the same major phylogenetic associations as the molecular analyses. The sum of molecular markers and morphological traits expressed by Acrolophomys and Onychomys leads to a phylogenetic position supporting their recognition as a distinct tribe.

We studied the enlargement of the upper third molar (M3), with respect to the upper second molar in sigmodontine rodents, the largest subfamily of living cricetids. M3 is enlarged in extant and extinct members of at least six tribes (Andinomyini, Euneomyini, Oryzomyini, Phyllotini, Reithrodontini and Sigmodontini), all of them also sharing hypsodonty, planate crowns and overall dental simplification in the context of Sigmodontinae. Enlargement is expressed in four ways, including simplification or modest complication of occlusal design on a single plane. M3 enlargement in sigmodontines is primarily associated with increasing herbivory rather than strictly with phylogeny, and thus presents a classic example of evolutionary convergence.

We present a topological analysis of the third upper molars (M3) using the recently developed ICAMER nomenclatural system as a way to understand the dental morphological similarity in sigmodontine rodents, the most speciose subfamily of cricetids. The method is explored in Scapteromys aquaticus and Abrothrix olivacea, taxa belonging to two diverse tribes, Akodontini and Abrotrichini, respectively, which exhibit high similarity regarding several craniodental traits as well as external anatomy. Both species show morphologically similar M3 in adults characterized by cylindrification and the isolation of a large central fossette arising from the marginal fusion of the anterior and posterior lobes. The results indicate that, before the wear, these rodents have a strongly different topological pattern at the cuspal level, mostly involving production of the connection between the main cusps. The central fossette derives from the isolation of part of the metaflexus in Scapteromys, while in Abrothrix it originates from the hypoflexus. The topological analysis provides a new approach to sigmodontine systematics, including the ability to detect diagnostic characters of both tribes. More important, it constitutes a new step toward an integrative phylogeny of fossil and living cricetids.