Phoberomys sp.

Boivin, Myriam, Antoine, Pierre-Olivier, Benites-Palomino, Aldo, Marivaux, Laurent & Salas-Gismondi, Rodolfo, 2019, A new record of a giant neoepiblemid rodent from Peruvian Amazonia and an overview of lower tooth dental homologies among chinchilloids, Acta Palaeontologica Polonica 64 (3), pp. 627-642 : 632-634

Phoberomys sp.

Fig. 2 .

Material.— MHNC-MS-001, left mandibular fragment with incisor (portion) and p4 (portion)–m3, from Monte Salvado Native Community, Madre de Dios Department, Peru. Although found as float, based on its matrix, this specimen most likely originates from the lower unit of the Madre de Dios Formation, late Miocene in age (see Geological setting) .

Measurements. —See Table 2.

Description.—Dentary: MHNC-MS-001 is a left mandibular fragment preserving m1–m3 and the distal portion of p4 ( Figs. 2 , 3 ). It is undistorted but fractured at several points. The body of the mandible is anteriorly broken at the level of the posterior part of the lower diastema. Posteriorly, the angular apophysis and most of the ascending ramus, including the mandibular condyle, are missing. The coronoid process is broken at its base posterodorsally.

The mandibular body is robust. The mandibular symphysis is stout, broken anteriorly, and ends at the level of m1. Labially, the notch for the insertion of the tendon of the zygomatico-mandibularis pars infraorbitalis is wide, below m1–m2, and ventrally situated on the labial edge of the mandible. The anterior tip of the masseteric crest and that of the lateral crest end below the m2, and they link the notch for the insertion of the tendon of the zygomatico-mandibularis pars infraorbitalis, at the level of its posteroventral and posterodorsal regions, respectively. The masseteric crest is posteriorly broken. It is posteroventrally directed and prominent in its anterior part. It is sub-horizontal and more reduced toward its posterior region. The lateral crest, posterodorsally directed, is markedly oblique. Its posterior part is not visible. The anterior part of the horizontal crest is absent. By contrast, its posterior part is conspicuous, although broken, and it delimits ventrally the fossa for the insertion of the zygomatico-mandibularis muscle. This fossa is moderately deep. The preserved part of the ascending ramus, which runs toward the coronoid process, begins below the m3. The retromolar fossa, posteriorly located with respect to the m3, is well developed. Lingually, the alveolar sheath of the lower incisor is partially broken, showing the lower incisor at two locations. The bottom of this alveolar sheath is situated at the level of the distal portion of the m3.

Lower tooth row: The p4–m3 of MHNC-MS-001 are damaged mesially and distally. The m3 is also slightly broken on its lingual edge. The four teeth are high-crowned and taeniodont (i.e., absence of anterior arm of the hypoconid). The cuspids/stylids are not visible because they are subsumed within enlarged lophids, thereby forming laminar cristids (i.e., laminae). The latter are mesiolabially directed (slightly oblique forward with respect to the long axis of the tooth row). Compared with the size of teeth, each cristid displays a continuous and relatively thin enamel layer (without noticeable heterogeneous thickness on the leading and trailing edges), coating a thick dentine layer. The inter-cristid regions (i.e., flexids) are laminar and filled by cement.

On p4, despite the damage, three cristids and three inter-cristid cement layers are distinct on this tooth, suggesting that it likely displayed four cristids when it was complete (i.e., the first cristid is missing). Due to the fragmentary state of the p4, the presence or absence of labial connections between cristids cannot be determined. The lower molars display three cristids and two inter-cristid cement layers. Although the cristids have their lingual and labial tips mostly fractured, they seem to be connected neither lingually nor labially with each other. The m2 is slightly longer than m1, and these two teeth have a similar width. The m3 is much longer and slightly wider than m1 and m2.

Remarks. —Taeniodont and high-crowned lower teeth with laminar, oblique, and thick cristids suggest chinchilloid affinities for MHNC-MS-001. The lower teeth have a typical neoepiblemid occlusal pattern characterized by the presence of laminar and thick inter-cristid cement, as well as a continuous enamel layer (i.e., without heterogeneous thickness between leading and trailing edges). The huge size ( Table 2) and the presence of a tetralophodont p4 suggest a generic assignment of the MHNC-MS-001 specimen to Phoberomys . The two included species, P. burmeisteri and P. pattersoni , have a similar dental size ( Carrillo and Sánchez-Villagra 2015; Rasia and Candela 2018). They are differentiated by characters on M3 and p4 (see Mones 1980; Carrillo and Sánchez-Villagra 2015; Rasia and Candela 2018). The M3s of P. pattersoni have straighter laminae than those of P. burmeisteri . There are mesial indentations on the sixth or seventh laminae of M3s in P. burmeisteri , whereas the edges of the distal laminae are straight in P. pattersoni . In P. pattersoni , the two mesial laminae are labially united on p4, and the distal ones are free. Representatives of Phoberomys burmeisteri show this connection but they can also have a labial connection between the second and third laminae. Due to the poor preservation of MHNC-MS-001, the latter feature cannot be assessed. Besides, the specimen does not exhibit the morphological characters that would clearly differentiate it from P. burmeisteri , P. pattersoni , Phoberomys sp. 1 , Phoberomys sp. 2 , and Phoberomys sp. A from the Urumaco Formation ( Venezuela; Bondesio and Bocquentin Villanueva 1988; Horovitz et al. 2006; Carrillo and Sánchez-Villagra 2015), and the UFAC 1817 m 1 or m2 assigned to Phoberomys sp. 4 from the Solimões Formation ( Brazil; Kerber et al. 2017). Teeth of Phoberomys sp. A (and Phoberomys sp. B ) from the Urumaco Formation are smaller in size than those of MHNC-MS-001. In caviomorphs, some groups with hypsodont teeth show a wide range of dental size during ontogeny (i.e., teeth grow in length and width in addition to crown height), often associated with morphological variations (e.g., Kraglievich and Parodi 1940; Vucetich et al. 2005; Fields 1957; Nasif and Abdala 2015). Therefore, based on only one specimen, the size criterion is somewhat useless for differentiating MHNC-MS-001 from other species of Phoberomys ( Carrillo and Sánchez-Villagra 2015; Rasia and Candela 2018). Lastly, MHNC-MS-001 being a fragmentary mandible, comparison with taxa only known by upper teeth or postcranial remains is de facto limited: the neoepiblemid from the Upper Pisqui River, Peru (originally described as Perumys gyulavarii Kretzoi and Vörös, 1989 ; see Kerber et al. 2017), and cf. Phoberomys sp. 1 , cf. Phoberomys sp. 2 , Phoberomys sp. 3 , and Phoberomys sp. B from the Urumaco Formation ( Horovitz et al. 2006; Carrillo and Sánchez-Villagra 2015). In light of these various points, MHNC-MS-001 is provisionally identified here as Phoberomys sp.

Figures

Fig. 2. Photograph of the MHNC-MS-001 attributed to caviomorph rodent Phoberomys sp., from Monte Salvado, Peruvian Amazonia, late Miocene or Pliocene; fragmentary left mandible in occlusal (A1), labial (A2), and lingual (A3) views, p4–m3 in occlusal view (A4).

Fig. 3. Explanatory drawings of the MHNC-MS-001 attributed to caviomorph rodent Phoberomys sp., from Monte Salvado, Peruvian Amazonia, late Miocene or Pliocene; fragmentary left mandible in occlusal (A1), labial (A2), and lingual (A3) views, p4–m3 in occlusal view (A4). The dotted lines indicate incomplete parts.