Pteromys volans (Linnaeus, 1758)

Pang, Li-bo, Chen, Shao-kun, Hu, Xin, Wu, Yan & Wei, Guang-biao, 2024, Fossil flying squirrels (Petauristinae, Sciuridae, Rodentia) from the Yumidong Cave in Wushan County, Chongqing, China, Fossil Record 27 (1), pp. 209-219 : 209-219

Pteromys volans (Linnaeus, 1758)

Fig. 2 ; Suppl. material 1: table S 2

Materials.

As in Suppl. material 1: table S 1, there are four maxillary bones, 27 mandibular bones and one isolated tooth from the layer ②-2 and 23 maxillary bones, 21 mandibular bones and 10 isolated teeth from the layer ④.

Description.

The mandible is very short and its length is close to the height. The incisor is relatively narrow and curves towards the labial side. Its tip is slightly lower than the worn surface of the cheek teeth. Its posterior end lies under the mandibular foramen. The diastema is deep and short. A single large and round mental foramen locates at the buccal side of the lowest part of the diastema, closer to p 4 than the incisor. Both the upper and lower masseter muscle ridges are obvious with the anterior angle at the level of the middle part of p 4. The masseter muscle fossa is very wide and relatively deep. The angular process is particularly marked and wider than the ascending ramus. Its upper part is curved to the buccal side, while the inferior margin is curved to the lingual side. The lower and posterior edges are ridge-like. The pterygoid muscle fossa at the lingual side is very deep; its anterior end is at the level of the middle part of m 3. The mandibular foramen is relatively large, oval in shape and positioned more dorsally than the worn surface of the cheek teeth. The ascending ramus is very thin. The condylar process and coronoid process are rarely preserved.

P 3 is small, single-rooted and cylinder-shaped. Its crown surface is oval, with a central main cusp at the buccal side and one or two small accessory cusps at the lingual side. Viewing from the buccal side, P 3 cannot be covered by P 4 completely.

P 4 is molariformed. Its crown surface is close to trapezoid: the buccal side is longer than the lingual side and the antero-lingual corner is shrunk. In occlusal view, the tooth is mainly composed of four transverse ridges (anteroloph, protoloph, metaloph and posteroloph) and two cusps at the lingual side (protocone and hypocone). The parastyle is large and the anteroloph is very low and short. The protoloph starts at the tip of the paracone and ends at the base of the protocone. The protoconule is not obvious. The metaloph is somewhat short and ends at the developed metaconule. A crista stretches out from the metaconule to the posteroloph. The posteroloph is low and there is a V-shaped groove between it and the hypocone. The hypocone is the smallest amongst the main cusps, isolated when unworn, but connected with the postprotocrista after being worn. The protocone is well developed, occupying 3 / 4 of the length of the lingual side. The anterior valley and the central valley are both V-shaped and much wider and deeper than the posterior valley. The extra anteroloph is absent, while a weak protolophule is permanent. The cingulum is well developed at the lingual side, but cannot be checked at other sides.

The main structure of M 1 is the same as that of P 4, but the anteroloph is more developed, making the occlusal outline almost quadrate. The parastyle is quite degenerate and merges with the anteroloph. The protoloph is high, starting at the tip of the paracone and ending at the preprotocrista. The protoconule is tiny, but clear. The metaloph is short and low, ending at the middle of the metaconule. The metaconule is well developed and its posterior edge connects with the posteroloph. The posteroloph is very low. The protocone is like a longitudinal ridge, occupying 4 / 5 or more of the length of the lingual side. The hypocone is small and only occupies the posterolingual corner of the tooth. The extra anteroloph and the protolophule are present in most of the specimens.

M 2 is very similar to M 1 in size and shape.

The occlusal outline of M 3 is close to a rounded triangle. The anterior lobe is very similar to that of M 1 / 2, but the posterior lobe is much degenerate and shrunk. The metacone is almost isolated and there is no obvious metaloph and metaconule. The posteroloph is very short and low and connects with the posterior base of the metacone. The hypocone is tiny and locates at the middle of the posterior side of the tooth.

The occlusal outline of dp 4 is an irregular quadrilateral with a narrow anterior lobe and a wide posterior lobe. A single anteroconid is obvious and the anterolophid is very weak. The protoconid is near to the metaconid, but somewhat larger. The mesoconid is small and locates behind the protoconid. There is no trace of the metalophid and the mesolophid is very short. The hypoconid is almost the same as the protoconid in size. The posterolophid is curved and connects the entoconid with a small hypoconulid between them. The morphological structure of p 4 is similar to dp 4, except its slightly wider anterior lobe.

The occlusal outline of m 1 is close to an oblique rhomboid. The metaconid and the hypoconid are stronger than the protoconid and the entoconid. The mesoconid is permanent, while the mesostylid is relatively weak. Amongst the four transverse ridges on the occlusal surface, the anterolophid is continuous and highest. The posterolophid is also continuous, but lower than the anterolophid. The metalophid is variable. It may be very short on some specimens, while it may reach to the base of the metaconid on the others. The mesolophid is not well developed. The trigonid basin is much narrower than the talonid basin and not closed. The anterobuccal sinusid is very shallow. The buccal valley is wide.

The occlusal structure of m 2 is similar to that of m 1, except the anterior lobe of m 1 is narrower than that of m 2.

m 3 is the longest check tooth and its anterior lobe is almost the same as m 2. On the posterior lobe, a prominent hypolophid connects the hypoconid and the entoconid. The posterolophid is well developed and convex backwards.

Comparison.

The size (in Suppl. material 1: table S 2) and occlusal structure show these specimens belong to a species of small flying squirrel. Until now, two genera and three species of Pleistocene small flying squirrels have been known in south China, these being Hylopetes electilis , Pteromys huananensis and P. volans .

Hylopetes electilis is an extant species and its fossils were only reported from the Middle Pleistocene deposits of the Wazhuwan Cave and the Tianmen Cave in Tongzi, Guizhou Province ( Zheng 1993). The most typical character for this species is the pitted enamel of its cheek teeth. In addition, this species has weaker entoconid and more developed mesoconid and mesostylid than Pteromys volans .

Pteromys huananensis was only known from the Early Pleistocene Longgupo Cave in Wushan, Chongqing Municipality ( Huang and Fang 1991). The validity of this species is yet to be discussed. Its M 3 has clear metacone, protoconule, mesostyle and metaloph, which is different from the specimens described here.

The fossils of Pteromys volans were excavated from the Longgupo Cave of the Early Pleistocene, the Xinglong Cave, the Puding Cave, the Yanhui Cave, the Tianmen Cave and the Upper Pingba Cave of the Middle Pleistocene and the Xitaiping Cave of the Late Pleistocene ( Zheng 1993; Huang et al. 2002; Tong et al. 2008). The dental diagnosis of this species includes (modified from Li CK et al. (2019 a)): cheek teeth brachyodont; P 3 small, but with differentiated cusps; P 4 molarised; hypocone and metaconule of P 4 - M 2 well developed; the extra anteroloph and the protolophule present; M 3 without metaloph and metaconule; the mesoconid and the entoconid of lower cheek teeth permanent; the trigonid basin not closed; m 3 elongated obviously. Morphologically, the specimens described herein resemble P. volans and should be assigned to this species.

Figures

Figure 2. Pteromys volans from the Yumidong Cave. A. 12 YMDT 4 ② 35.18, right P 3 - M 3, occlusal view; B. 12 YMDT 4 ② 35.286, left p 4 - m 3, occlusal view; C. 12 YMDT 4 ② 4.1, right mandible with dp 4 - m 3, occlusal (1), labial (2) and lingual (3) view. Scale bar: 2 mm (A, B, C 1); 4 mm (C 2, C 3).