RHAGOMYINI, Pardiñas & Tinoco & Barbière & Ronez & Cañón & Lessa & Koch & Brito, 2022
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publication ID |
https://doi.org/ 10.1093/zoolinnean/zlac016 |
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publication LSID |
lsid:zoobank.org:pub:FAC815A5-0D0B-4F5B-A012-DC59DD4E6AD5 |
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DOI |
https://doi.org/10.5281/zenodo.7318423 |
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persistent identifier |
https://treatment.plazi.org/id/836387C5-FFCC-9573-8161-FA22FACAFA1E |
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treatment provided by |
Plazi |
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scientific name |
RHAGOMYINI |
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trib. nov. |
RHAGOMYINI , TRIB. NOV.
( FIGS 2–9 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 View Figure 9 , 11 View Figure 11 )
Z o o b a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:act: BFA3F94-5A18-4AAF-AD22-3A466DDB5005.
Type genus: Rhagomys Thomas, 1917 View in CoL .
Diagnosis: A tribe of the subfamily Sigmodontinae , clade Oryzomyalia, grouping small-sized cricetids (head and body length ~ 90 to 100 mm; body mass ~ 20 to 35 g) uniquely diagnosed by the following traits: all digits with deep, transverse grooves; tips of manual digits callused and having a crescent-shaped depression appearing heart-shaped in dorsal view with embedded minute claws; nail (as opposed to a claw) on the hallux; elongated pedal digit V, which reaches the middle of the second phalanx of digit IV; cranium with marked basicranial flexure involving a low emplacement of the mastoid bulla; true fossa occupying the retromolar region developed laterally to m2-m3; completely planed ventral surface to the dentary; pro-odont, smoothly grooved lower incisor showing faintly textured enamel; molars with conspicuous persistently enamelled and conical main cusps connected by thin ridges; anteroconid of m1 composed of a single conulid (as opposed to two conulids) connected to the protoconid and with direct connection between protoconid and hypoconid; tongue unusually long, narrow and cylindrical, lacking both the torus linguae and the sulcus semilunaris (the latter two features presumed in R. rufescens ); filiform caecum with the external diameter equal to that of the adjacent portion of the colon (after Thomas, 1917; Luna & Patterson, 2003; Percequillo et al., 2004; Luna, 2015; this paper).
Content: A single genus, Rhagomys Thomas, 1917: 192 .
Geographic distribution: Rhagomyini rodents are distributed extensively in tropical South America in two main cores divided by the Arid Diagonal [see map provided by Moreno Cárdenas et al. (2021: fig. 1) for details]: eastern Andean montane forests, typically above 500 m a.s.l., from southern Ecuador, through Peru to northern Bolivia and in the Atlantic Interior Forest in Brazil. Two isolated records in the Amazonian basin south of the Amazon River in Brazil are also known ( Fig. 10 View Figure 10 ).
Biochron: Recent in Bolivia, Brazil, Ecuador and Peru.
Other morphological traits: Taxonomic uniqueness promoted a detailed morphological description in Rhagomys , paradoxically surpassing in this respect most of the other members of the subfamily. Although hampered by the scarcity of available specimens, which produces an inevitable poor control on trait variability, most of the anatomical fields traditionally scrutinized have been reviewed for this rodent (e.g. Luna & Patterson, 2003; Percequillo et al., 2004, 2017; Pinheiro et al., 2004; Abreu-Júnior & Percequillo, 2019).
From external morphology, almost nothing can be added to the general knowledge of the genus. According to published measurements, the ratio of head and body length to tail length is nearly 1 in adult specimens; rarely does the caudal length exceed the body length by more than 10 mm and in some specimens the latter is slightly longer than the former. Not unexpectedly, some juveniles show markedly longer tails ( Medina et al., 2017: table 1), although we suspect that the values reported by Passamani et al. (2011: 829; head and body length = 65 mm, tail length = 105 mm) are incorrect, based on our examination of the published image ( Passamani et al., 2011: fig. 6; ratio head and body length to tail length ~0.8). Photographs of live Rhagomys (e.g. Luna & Patterson, 2003: fig. 2; Medina et al., 2017: fig. 1; Supporting Information, Appendix S3) allow the size of the eyeballs to be judged as conspicuous. No direct data of the eyeball axial length are available for Rhagomys , hampering any confident quantitative approach. However, based on the detailed picture provided by Medina et al. (2017: fig. 1), we estimated an eyeball axial length of 7.5 mm for the illustrated adult specimen of R. longilingua (with a body mass of 24 g; Medina et al., 2017: table 1). According to the equation of Howland et al. (2004), the expected eyeball axial length for a mammal of 24 g is 3.72 mm; employing the equation restricted for rodents ( Panyutina et al., 2017: 174), this value is 3.75 mm. Both are well below our crude value of 7.5 mm, suggesting that the subjective perception that Rhagomys has a large eye can be supported quantitatively. To connect this eye size with an improved nocturnal vision seems plausible, but also the possibility of an enhanced ability to visually detect small food items, such as insects, invites further experimental research. The rhinarium in Rhagomys remains undescribed. Based on two specimens of R. rufescens and one of R. septentrionalis it seems similar in western and eastern species ( Fig. 9E View Figure 9 ): furred suprarhinarial region, moderate in size and embellished with two finely sculptured and rounded narial pads (or tubercles; Hill, 1948) positioned below the nostril line. While the dorsal integumental fold appears scarcely developed, the ventral integumental fold has a dominant role in the lateral expression of the rhinarium. Nasal openings, internally flanked by folds (parts of dorsal integumental fold?), are dorsally divided by tegumental promontories that produce rounded internal atrioturbinates. The entire rhinarium is medially dissected by the sulcus medianus, a continuation of the philtrum, which divides well-haired, upper lips ( Fig. 9E View Figure 9 ). Almost nothing was described about the ears in Rhagomys beyond pilosity and size (e.g. Luna & Patterson, 2003: 5; Percequillo et al., 2004: 242). The specimen MECN 6172 ( R. septentrionalis ) has a rounded auricular pinna, which is concealed basally by head fur and the internal surface of which is covered with delicate hairs. The striking feature of this apparently simple ear is the deep, well recessed concha (Supporting Information, Appendix S12). To our best understanding, this condition could be linked to the internal flexure of the basicranial region and its effect on the several related structures.
Minor cranial details worthy of comment are the reported absence of an alisphenoid strut and the narrow condition indicated for the zygomatic plate of R. rufescens ( Abreu-Júnior & Percequillo, 2019) . All specimens of this species studied here have a thick alisphenoid strut on both sides of the cranium ( Fig.11A View Figure 11 ), and the same has been reported previously (e.g. Percequillo et al., 2004: 244; Pinheiro et al., 2004: 5); we suspect that the absence highlighted by AbreuJúnior & Percequillo (2019: 77) is an error. In addition, the zygomatic plate in Rhagomys can hardly be judged as narrow; on the contrary, it is a broad structure ( Fig. 11B View Figure 11 ). However, when examined superficially, it seems narrow because the external shaft is arcuate, giving its posterior border a transverse orientation; in addition, the upper free border shows a marked twist, and the plate is basally broadened ( Fig. 11C View Figure 11 ). As mirrored by several bony structures of the cranium of Rhagomys , well-developed musculature is probably implied behind this configuration. Even working with damaged material, Thomas (1886: 250) was able to observe a ‘skull with the cranial portion very large’ characterizing Rhagomys (see also: Luna & Patterson, 2003; Percequillo et al., 2004). The dominance of the braincase is accompanied by a large foramen magnum oriented posteroventrally ( Fig. 11D View Figure 11 ). The latter is better appreciated when viewed from inside, including also the caudal downward orientation of the basioccipital, the middle lacerate foramina present as slits [but not absent, as was stated by Percequillo et al. (2004: 245)] and a long presphenoid ( Fig. 3D View Figure 3 ). Turbinals are comparatively complex ( Fig. 3E View Figure 3 ; cf. Martinez et al., 2018). Interesting to note, although the tegmen tympani abuts, to some degree, a small suspensory process of the squamosal in adults, this condition is not evident in juveniles (Supporting Information, Appendix S13). Special features of the dentary in Rhagomys include a large mandibular foramen ventrally flanked by an unusually bulging and, in cross-section, rounded crista mandibulae (Supporting Information, Appendix S14). While the former indicates a thick mandibular (trigeminal) nerve, the latter reflects the enlarged development of the incisor.
Thomas (1917: 193) emphasized the upper incisor features in Rhagomys indicating ‘… front surface flattened and inclined inwards, so that the resulting relations of the two teeth and the shapes of their tips are about as in the Dormice [ Gliridae ], not as in any of the genera above mentioned [ Oecomys , Rhipidomys , etc.]’. Our inspection reinforces the occurrence of this peculiar kind of incisor frontal surface condition and orientation ( Fig. 5C View Figure 5 ), probably associated with a particular feeding strategy (see below). In addition, at least for R. rufescens , the dentine profile is staggered when the incisor is viewed labially ( Fig. 11B View Figure 11 ). The number of molar roots remains unreported for Rhagomys . According to the 3D-images, at least R. septentrionalis has the M1/m1 anchored by four roots, while the M2-M3 are each three-rooted; all lower molars are two-rooted, a widespread condition in sigmodontines (Supporting Information, Appendix S15). Gyldenstolpe (1932: pl. III, fig. 4a) illustrated a four-rooted M1 and a three-rooted M 3 in the holotype of R. rufescens .
Although mentioned by Thomas (1886) long ago, the soft palate of Rhagomys has never been described. In our examination of R. septentrionalis , the configuration of diastemal and interdental ridges included three and four rugae, respectively. While the former are highly marked and broadened at their midpoints, the latter appear flattened, the first three well directed backwards and leaving an ample medial space ( Fig. 9C View Figure 9 ). The overall morphology of the soft interdental palate appears to facilitate rapid passage of food items more than any action associated with retention or chewing.
Gross morphology of the stomach is unilocularhemiglandular, first described and illustrated for R. longilingua ( Luna & Patterson, 2003: fig. 11) and subsequently mentioned for R. rufescens ( Pinheiro et al., 2004: 7) . Our examination of single specimens each of R. rufescens and of R. septentrionalis confirms this generic pattern, which includes a heart-outlined organ with roughly subequal, cornified and glandular epithelia, a bordering fold not protruding to the left of the oesophageal opening (but directed posteriorly toward the greater curvature), a shallow incisura angularis and a scarcely differentiated prepyloric part ( Fig. 9D View Figure 9 ; Supporting Information, Appendix S16). Finally, the gall bladder was reported as absent in R. longilingua ( Luna & Patterson, 2003: 12) , and the same condition can be asserted for the other species of the genus based on the specimens we dissected ( R. rufescens, MZUFV-CM 3706; R. septentrionalis, MECN 6172).
Nomenclatural remarks
The genus name is probably derived from Ancient Greek ῥαγόω = rhagóō, literally to crack (Supporting Information, Appendix S17) and µυς, mouse. Although not mentioned by Thomas (1917), this name possibly refers to the fact that the soles of the hind and forefeet are dissected by pronounced transverse grooves. The name of the tribe is derived from the addition of the tribal ending -ini, therefore, Rhagomyini .
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