Biswamoyopterus sp. M644

Li, Guogang, Lwin, Ye Htet, Yang, Bin, Qin, Tao, Phothisath, Phouthong, Maung, Kyaw-Win, Quan, Rui-Chang & Li, Song, 2020, Taxonomic revision and phylogenetic position of the flying squirrel genus Biswamoyopterus (Mammalia, Rodentia, Sciuridae, Pteromyini) on the northern Indo-China peninsula, ZooKeys 939, pp. 65-85 : 65

Morphological description of Biswamoyopterus sp. M644 Figures 4G, H , 5D , 6D , 7D

Remarks.

Morphometrical data are presented in Table 2 . As a whole, the dorsal pelage is reddish brown, with dense whitish hairs on the shoulders and hips, the ventral pelage is whitish, with yellowish brown on the edge of the membrane, the anus area is dull yellowish, but the base of the tail is brown-grey. The ears are black with few hairs, but with tufts of long, whitish hairs at the base. The feet backs are covered with black hairs, and the tail is cylindrical and reddish brown in its anterior part but gradually tending to blackish brown in its distal part. The skull is large with a GLS of 74.77 mm and a ZOB of 47.09 mm. The bullae are enlarged and each of them includes numerous septa (> 10) in a complex honeycomb pattern. The anterior edge of the nasals is slightly arc-shaped and extends slightly beyond the surface of the incisors. The surfaces of the upper and lower incisors are dull yellowish, without any orange. In the cheek teeth, P3 is relatively enlarged and cone-shaped. The length of P4 slightly exceeds each of the molars; P4 has three well-developed cusps on the labial side and one large cusp on the lingual side. Both M1 and M2 have two well-developed cusps on the labial side and one large cusp and one smaller cusp on the lingual side, and there is a smaller cusp on the posterior transverse ridge of P4, M1, and M2. M3 is smaller than P4, M1, and M2, and its later crown surface becomes a “U” shape, with a slight depression in its center.

The upper surface of the head is deep reddish brown, the muzzle is brown, the rim of the eyes is brown, the cheeks are reddish brown with occasional whitish hairs on their lower parts, the ears are black with few hairs but tufts with long, whitish hairs at the base, the back of the neck is reddish brown, and the throat and chin show whitish grey extending to both sides of the neck.

The back is mainly reddish brown, but is scattered with many white tips, especially on the shoulders and hips; individual hairs are variable in color but usually comprise the following components: whitish at the tip, reddish brown in the mid-part, and whitish grey at the base. The anterior margin of the forearms is black-brown. The chest is yellowish grey, the center of the abdomen is yellowish white, and the anus area is dull yellowish. The upper part of the membrane is reddish brown and the underpart whitish, extending to yellowish brown on the edge. The tail is cylindrical, reddish brown anteriorly, but gradually darkening towards the tip, so its posterior part is blackish brown, and the underpart area of the tail base is brown-grey. The fore and hind feet are covered with black hairs; however, the hind feet have denser hair than the fore feet, and both have dark hairless pads.

The skull is large, the frontal part is significantly depressed, the rostrum is short and wide, the anterior edge of the nasals is slightly beyond the surface of the incisors with a slight arc-shape, the incisive foramen is developed, the palatine posterior edge has an arc-shaped depressed deformation, the pterygoid is strong and the pterygoid fossa wider, the bulla is developed with numerous septa (> 10) in a complex honeycomb pattern, the orbital regions are large and there is an incision on the edge of each orbit, the postorbital process is strong and curves down a little, the zygomatic plate is slant, the zygomatic arch is stronger with lower connection to the squamosal, the mastoid process is comparatively smaller, but the occipital condyle is strong.

The mandible is strong, with the coronoid process developed, and the condylar process has a developed articular surface; the angular process is developed and curved towards the inside at its bottom. The upper incisors are strong and positioned vertically downwards; their outer surfaces are yellowish, without any orange. P3 is cone-shaped and on the inside of the front of P4; overall, the crown surface of P4 appears as a triangle with three well-developed cusps on the labial side and one large cusp on the lingual side, and its labial side length is slightly longer than those of M1, M2, and M3. M1 and M2 are approximately equal in size; both have two well-developed cusps on the labial side, and one large cusp and one smaller cusp on the lingual side. There is a smaller cusp on the posterior transverse ridge of P4, M1, and M2. Compared with P4, M1, and M2, M3 is the smallest; its lingual side cusp is larger than the cusp on the labial side, and its later crown surface becomes a U-shape, with a small depression in its center.

The outer surface of the lower incisors is yellowish, the same as for the upper incisors; however, the inside part of the inner surface sinks deeply, making the outside margin sharp. From P4 to M3, the teeth enlarge gradually, and there are two labial and lingual cusps on each of them (the later lingual cusp of M3 becomes a ridge); there is also a smaller cusp between, and slightly internal to, the two labial cusps on each of them. Different levels of depression occur in the centers of the crown surfaces of P4, M1, M2, and M3, with the largest in M3.

Figures

Figure 4. Comparison of skins of all known Biswamoyopterus specimens A, B B. biswasi, ZSI 20705 C, D B. gaoligongensis, KIZ 034924 A-D were derived from Li et al. (2019) E, F B. laoensis, NUoLFES. MM. 12.163, from Sanamxay et al. (2013) G, H Biswamoyopterus sp. M 644 from this study.

Figure 5. Comparison of ear tufts of all known Biswamoyopterus specimens. The red arrow indicates the anterior tufts, and the yellow arrow indicates the posterior tufts A B. biswasi, ZSI 20705 B B. gaoligongensis, KIZ 034924 A, B were derived from Li et al. (2019) C B. laoensis NUoL FES. MM. 12.163 from Sanamxay et al. (2013) D Biswamoyopterus sp. M 644 from this study.

Figure 6. The skulls (first three rows), left maxillary (the fourth rows) and left mandibular teeth (the last rows) of all known Biswamoyopterus specimens A B. biswasi, ZSI 20705 B B. gaoligongensis, KIZ 034924 A, B were derived from Li et al. (2019) C B. laoensis, NUoL FES. MM. 12.163, from Sanamxay et al. (2013) D Biswamoyopterus sp. M 644 from this study.

Figure 7. The posterior margin of the palatal bones relative to the posterior margin of M 3 (dotted line) and shape of the preglenoid process (arrow) of all known Biswamoyopterus specimens A B. biswasi, ZSI 20705 B B. gaoligongensis, KIZ 034924) A, B were derived from Li et al. (2019) C B. laoensis, NUoL FES. MM. 12.163, from Sanamxay et al. (2013) D Biswamoyopterus sp. M 644 from this study.