Trogopterus xanthipes (Milne-Edwards, 1867)

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

Trogopterus xanthipes (Milne-Edwards, 1867)

Fig. 3 A – C ; Suppl. material 1: table S 3

Materials.

As in Suppl. material 1: table S 1, there are two maxillary bones, 12 mandibular bones and three isolated teeth from the layer ②-2, one maxillary bone and three isolated teeth from the layer ④ and one mandibular bone from the layer ⑥.

Description.

The mandible is high and strong. The tip of the incisor is slightly lower than or as high as the worn surface of the cheek teeth. The transversal section of incisor is close to a triangle. The diastema is deep and short. A single mental foramen, small and open forwards, locates at the buccal side of the lowest part of the diastema. The lower masseter muscle ridge is weak and the upper masseter muscle ridge is almost undetectable. The masseter muscle fossa is relatively shallow. The inferior margin of the horizontal ramus is straight. The vascular notch is extensive. The angular process is usually badly preserved. Based on the preserved part, this process should extend downwards significantly. The pterygoid muscle fossa at the lingual side is very deep and wide with its anterior end terminates at the level of the middle part of m 3. The mandibular foramen is low, relatively small, round in shape and open backwards. The ascending ramus is moderately strong and starts near the m 2 / m 3 boundary. The coronoid process is not preserved on any of the specimens of this species, but should be higher than the condylar process. The condylar process is a transverse axis and its neck is somewhat obvious. The mandibular notch is an obtuse angle.

P 3 is tiny, single-rooted and very near to the anterolingual corner of P 4. P 4 is three-rooted and larger than other cheek teeth. Its occlusal outline is a trapezoid with a narrow anterior lobe and a wide posterior lobe. The anteroloph is formed by four independent cusps which are separated by three small grooves and the anterocone is the highest amongst them. The parastyle is located at the anterobuccal side of the paracone and is the buccal end of the anteroloph. The paracone and the metacone are similar in size. The mesostyle is well developed and connects with the paracone and the metacone. The protocone is very strong, with a small protostyle at its anterolingual side. The hypocone is relatively slender, located at the posterolingual corner of the teeth. The protocone and the hypocone are separated by a shallow groove at the lingual side. The protoconule and the metaconule are both obvious, but the metaconule is more enlarged and prominent. The protoloph is continuous, starting from the paracone to the protocone via the protoconule. The metaloph becomes vestigial for the enlarged metaconule. The posteroloph is continuous and low. There are many cristae in the anterior and the posterior valleys, dividing them into several enamel loops.

The anteroloph of M 1 is low, straight and smooth and the protostyle is absent, which are different from P 4. Other than these, the posterior part of M 1 is very similar with that of P 4. M 2 is very similar to M 1 in size and shape. The anterior part of M 3 is very similar to M 1 / 2, but its posterior part is somewhat shrunk. The hypocone is reduced, but still permanent, connecting with the metaconule by a straight metaloph. The posteroloph is absent and the metacone transfers to the back of the tooth.

The occlusal outline of p 4 is close to a trapezoid, narrow in the anterior part and wide in the posterior part. p 4 is the largest in lower check teeth. Its anteroconid is transversely oval-shaped, connected to the preprotocristid and separated from the metaconid by a narrow groove. The metaconid and the protoconid are higher than the hypoconid and the entoconid. A narrow ridge extends from the posterolingual side of the protoconid to the prehypocristid, forming the ectolophid. The mesoconid is prominent and three short ridges extend from it: the anterior one (the premesocristid) connects with the postprotocristid; the posterior one (the postmesocristid) connects with the posthypocristid; the buccal one (the ectomesolophid) connects with the ectolophid at the tip of the prehypocristid. The mesostylid is somewhat developed and parallel to the teeth row, but does not connect with the entoconid, making the talonid basin open. The transverse section of entoconid is almost round. The entolophid is thin and there are three cristae on it. The hypoconid is L-shaped, located at the posterobuccal corner of the tooth. There are two cusps on the posterolophid: the small one is at the buccal side and is separated from the hypoconid by a shallow groove; the large one is the hypoconulid, which is almost as large as the entoconid and is separated from the entoconid by a very deep groove. dp 4 is similar to p 4 in morphology, except there is no obvious anteroconid. The size of dp 4 does not exceed m 1.

The occlusal outline of lower molars is oblique quadrilateral. The main structure is similar to p 4, except the anteroconid is fused with the anterolophid. At the unworn stage, the posterolophid and the ectolophid are separated by a groove. With moderate wear, the anterolophid, the posterolophid, the ectolophid and a lingual lophid are connected via the mesostylid and the entolophid, forming a complete occlusal outline of the tooth. The anterobuccal corner of the tooth is curved and smooth. There is no trace of the anterobuccal cingulum and sinusid. One or two metalophids and several folds are developed in the talonid basin, making the occlusal surface more complex than p 4. The mesostylid is not well developed and stretches forward obliquely. m 3 is not shrunk. Its posterolophid somewhat curves and the hypoconulid is weaker than that of m 1 / 2.

Comparison.

The size (in Suppl. material 1: table S 3), complex occlusal surface and developed enamel folds show these specimens belong to a species of medium-large-sized flying squirrel. In the Quaternary of south China, four genera of medium-large-sized flying squirrels have been known so far, which are Petaurista , Aeretes , Belomys and Trogopterus .

The cheek teeth of Petaurista and Aeretes are obviously different from the described specimens. Their occlusal surfaces are relatively simple. There are no mesostyles, but well-developed hypoconules on the upper cheek teeth and the transverse ridges are more notable. In addition, M 1 / 2 of Petaurista has no hypocone ( Li CK et al. 2019 a), while it is always present on the Yumidong specimens. The hypocone on P 4 - M 2 of Aeretes is present, but always very near to the protocone ( Zheng 1993; Tong 2007; Tong et al. 2008; Li CK et al. 2019 a).

Belomys resembles Trogopterusion in occlusal structures more than other flying squirrels ( Zheng 1993; Tong 2007). One of the most obvious differences between them is the significantly larger size of Trogopterus . The specimens described here are relatively similar to Trogopterus morphologically and their size matches Trogopterus better than Belomys . Only one species of Trogopterus has been known until now, the extant T. xanthipes . Therefore, the specimens could be identified as T. xanthipes .

Figures

Figure 3. Trogopterus xanthipes (A – D) and Belomys pearsonii (E, F) from the Yumidong Cave. A. 12 YMDT 4 ② 30.2, left mandible with lower incisor and p 4 - m 3, occlusal (1), labial (2) and lingual (3) view; B. 12 YMDT 4 ② 35.1, right dp 4 - m 3, occlusal view; C. 12 YMDT 4 ② 35.7, left P 4 - M 2, occlusal view; D. 12 YMDT 4 ② 60.3, right M 1 – M 3, occlusal view; E. 12 YMDT 4 ② 35.9, left P 3 - M 3, occlusal view; F. 12 YMDT 4 ② 35.8, right mandible with lower incisor and p 4 - m 3, occlusal (1), labial (2) and lingual (3) view. Scale bar: 5 mm (A 1, B – E, F 1); 10 mm (A 2, A 3, F 2, F 3).