Eupetaurus, THOMAS, 1888

Jackson, Stephen M., Li, Quan, Wan, Tao, Li, Xue-You, Yu, Fa-Hong, Gao, Ge, He, Li-Kun, Helgen, Kristofer M. & Jiang, Xue-Long, 2022, Across the great divide: revision of the genus Eupetaurus (Sciuridae: Pteromyini), the woolly flying squirrels of the Himalayan region, with the description of two new species, Zoological Journal of the Linnean Society 194, pp. 502-526 : 508-510

GENUS EUPETAURUS THOMAS, 1888

Eupetaurus are amongst the largest flying squirrels, with fur that is thick, long and soft. Dorsal coloration ranges from pale grey to brownish grey, and ventral coloration is typically whitish grey. The cylindrical, fox-like tail is long and bushy. The palmar and plantar surfaces are thickly furred between naked pads ( Fig. 6 ).

The skull is generally similar to other flying squirrels (reviewed in detail by McKenna, 1962), with the following diagnostic features ( Fig. 7 ): (1) the snout is long and constricted at the base; (2) the incisors are ungrooved and relatively small, with pale yellow enamel faces; (3) the glenoid fossa opens posteriorly, with the opening placed high on the skull; (4) the coronoid process of the mandible is weak, not rising above the condyle; (5) the cheek teeth are large, slow to erupt and strongly hypsodont in younger animals (the occlusal surfaces becoming nearly flat after wear); they are enormously expanded vertically (Supporting Information, Supplementary Data SD7), with cusps and their interconnecting ridges combined into lophs and cross-lophs (this remarkable hypsodonty is unlike any other sciurid); (6) the alveolar cavities are strongly swollen to house the unerupted portion of the hypsodont cheek teeth; and (7) ‘the pterygoid fossae possess three diagonally elongate pits for the origin of powerful pterygoid musculature’ ( McKenna, 1962). These diagnostic features appear to relate to Eupetaurus feeding primarily on abrasive food, with the incisors reduced in functionality and chewing achieved mainly by anteroposterior trituration of the cheek teeth ( McKenna, 1962). Limited available natural history information (mainly for E. cinereus ) suggests that these squirrels might specialize on eating pine needles ( Zahler & Khan, 2003).

The upper cheek-tooth row is strongly convergent anteriorly in adults (although it remains more parallel in juveniles). The P 3 is peg-like and strong. The relative size of the upper cheek teeth can be given as P 4> M 1 ≈ M 2> M 3. The P 4 (and DP 4) and the upper molars have similar structures: the endoloph is strongly folded to form the ‘posterolingual diagonal flexus’ (see below); the anteroloph, hypertrophic preparaconule crista, paraloph, metaloph and posteroloph are finger-like and connected, in turn, to the buccal side of the endoloph. The anterior valley is wide and divided by the preparaconule crista. When worn, the preparaconule crista gradually connects with the parastyle to isolate the lingual part of anterior valley into an anterofossette. The central valley is always open. The posterior valley is always closed into a posterofossette, except in the M 3 in early wear.

The relative size of the lower cheek teeth can be given as P 4> M 1 ≈ M 2 ≥ M 3. The structure of the lower molars is relatively simple, with a ‘ring’ at the front and back, connected by the ectolophid in the middle. The anterior ring is formed by the anterolophid between the protoconid and metaconid, and the protolophid between the protoconid and metastylid. The centre of the anterior ring is one ( E. cinereus and E.tibetensis ) or two ( E. nivamons ) anterior fossettids. The lingual edge of the anterior fossettid is not closed in early wear. The posterior ring is formed by the entolophid and posterolophid that run between hypoconid and entoconid. The centre of the posterior ring is one posterior fossettid (talonid basin remnant). The posterior fossettid of M 3 is not closed in early wear. The mesoconid develops into a buccal cusp as strong as the protoconid and hypoconid on the middle of the ectolophid, which divides the hypoflexid into two folds. The central flexid runs from the posterolingual side of the tooth to the base of the anterior fold of the hypoflexid. Generally, DP 4 is similar to the lower molars, but has two opened anterior fossettids and two extra small cusps:an isolated paraconid in front of the buccal anterior fossettid and an isolated ectostylid on the buccal side of the mesoconid ( Fig. 8 ). Also, P 4 possess two open anterior fossettids and an isolated paraconid, but the paraconid will connect with the metaconid and protoconid as the teeth are worn. The mesoconid is very small on P 4 in Eupetaurus ( Fig. 9F ). The entire crown of P 4 is more diagonally oriented than DP 4 and lower molars.

Figures

Figure 6. Skins of the three taxa of Eupetaurus. Top row, specimen from Pakistan (Eupetaurus cinereus BMNH 14. 2. 10. 1); middle row, Tibet (Eupetaurus tibetensis NML 19524, holotype); bottom row, Yunnan (Eupetaurus nivamons KIZ 034190, holotype).

Figure 7. Skulls of Eupetaurus, from left to right: E. cinereus (FLMNH 28583) from Pakistan; E. tibetensis NML 19524, holotype, from Tibet; and E. nivamons KIZ 034190, holotype, from Yunnan. Scale bars (under each skull series): 2 cm.

Figure 8. Cheek teeth of the nine known skulls of Eupetaurus species: A, Eupetaurus cinereus ZSI 9492, adult from Pakistan; B, E. cinereus BNHS 7108, adult (now reported lost) from Pakistan (McKenna 1962); C, E. cinereus FLMNH 28583, old adult, from Pakistan; D, Eupetaurus tibetensis NML 19524, holotype, young adult, from Tibet; E, Eupetaurus nivamons KIZ 034189, immature, from Yunnan; F, E. nivamons KIZ 034192, immature, from Yunnan; G, E. nivamons KIZ 034190, holotype, young adult, from Yunnan; H, E. nivamons KIZ 035087, adult, from Yunnan; and I, E. nivamons KIZ 035088, adult, from Yunnan. Left side is labial side.

Figure 9. Comparison of cheek teeth between Eupetaurus and related genera: A, Petaurista xanthotis (USNM 144021); B, Aeretes melanopterus (USNM 219205); C, Pteromys volans (AMNH 19534); D, Aeromys tephromelas (USNM 292649); E, Biswamoyopterus biswasi (ZSI 20705); F, Eupetaurus nivamons (KIZ 034190); G, Trogopterus xanthipes (FMNH 39835); H, Belomys pearsonii (FMNH 35443); and I, Pteromyscus pulverulentus (USNM 481186).