Genus Carpomys Thomas, 1895

Emended diagnosis.— The genus Carpomys comprises two extant, relatively small cloud rats and an extinct giant cloud rat, Ca. dakal n. sp. Carpomys is defined phylogenetically as including the most recent common ancestor of Ca. melanurus, Ca. phaeurus, Ca. dakal n. sp., and all of its descendants. Living members of Carpomys possess cranial traits and features of external appearance that were summarized by Musser and Heaney (1992) and Heaney et al. (2014).

The dentition of Carpomys is distinguished from all others by the combination of the following morphological features: upper and lower first molars have four rows of lophs; second and third upper molars have three rows of lophs; a welldeveloped anterior labial cusp is present in m2 and m3; and posterior cingula in all upper molars are laminar and elongate (Figs. 2A, 2B, 4A, and 4B). Carpomys has moderately hypsodont molars and chevronate molar lophs. In the M1, a well-developed and elongate posterior cingulum forms the fourth lamina, while in the m1 a bicuspid anterocentral forms the first loph (Thomas 1898; Musser and Heaney 1992). The posterior loph of the m3 of Carpomys usually has an irregular “W” outline formed by an additional cusplet between the hypoconid and entoconid (Figs. 5A and 5B).

Description and comparisons.— In Carpomys, the height of crowns is medium andtooth morphology is intermediate between the hypsodont (high-crowned) and more chevronate Crateromys and the lophodont (elongated and transverse) Phloeomys; Carpomys cuspsaretypicallyseparatedby deepclefts (Figs. 2–5). As described by Heaney et al. (2009, 2014), the largely fused and lamellate cusp patterns of Carpomys and Musseromys aresimilar, but certain dental traits differentiate the two. Musseromys teeth and body sizes are substantially smaller than those of Carpomys (Heaney et al. 2014). In M3 (Figs. 2A, 4B, and 4C), Carpomys has a well-developed and elongate posterior cingulum that either is reduced or absent altogether in Musseromys. Carpomys has more chevronate lophs (Figs. 5A and 5B) compared to the more transversely oriented laminae of m1 (second and third lophs) and m3 (anterior loph) in Musseromys (Fig. 5C). The m3 cusplet found in Carpomys is absent in Musseromys and the m3 posterior lamina is transverse in Musseromys (Fig. 5C).

The Carpomys M1 anterocone is slightly arcuate and intermediate in curvature between the transverse anterocone of Phloeomys and the arcuate to chevronate anterocones of Batomys and Crateromys. In the M2 and M3, the third lamina is represented by a well-developed posterior cingulum, which is absent in Batomys and Crateromys, and absent in M3 of all Musseromys.The Carpomys m1 anteroconidiscomposedof two laminae, in contrast to the more typical triangular anteroconid in other phloeomyines. In Carpomys, the anteroconid of the m1 has a midline cusp that forms a ridge connecting the anterocentral and second lamina (anterolabial–anterolingual loph) when it is worn (Figs. 2B and 5A; Supplementary Data SD1G and I); in Crateromys, Batomys, and Musseromys, this ridge is not observed, even when the anteroconid is worn down. Carpomys has more arcuate laminae in m1 and m2 compared to Musseromys, but less strongly chevronate compared to Crateromys and Batomys (Fig. 5). The posterior cingulum is absent in the m3 of Carpomys, but present in about 25% of Cr. schadenbergi included in this study (listed in Table 3). Carpomys upper and lower second and third molars typically have greater occlusal widths than occlusal lengths, but the opposite is seen in Crateromys and Batomys (Tables 2 and 3).