Fossil flying squirrels (Petauristinae, Sciuridae, Rodentia) from the Yumidong Cave in Wushan County, Chongqing, China

Flying squirrels are important forest environment indicators. There have been many reports on them from fossil localities of the Late Cenozoic in southwest China, but relatively few detailed studies have been carried out on them. Numerous flying squirrel fossils of the Mid-Late Pleistocene were unearthed from the Yumidong Cave in Wushan County, Chongqing Municipality, China, providing excellent materials for morphological comparison and further research on this group. Four species have been recognised from this locality, including Pteromys volans , Trogopterus xanthipes , Belomys pearsonii and Aeretes melanopterus . P. volans and A. melanopterus are Palearctic species, which adapted to the cold environment and had been completely extinct in the study area since the Holocene Megathermal period. Based on the analyses of paleozoogeography and paleoecology of these four species, it could be concluded that the Yumidong Cave area was dominated by subalpine evergreen coniferous forest or coniferous and broad-leaved mixed forest during MIS 2 and MIS 4 periods, which were colder and had more coniferous forest than now, while the vege - tation landscape of MIS 3 and MIS 5 periods were similar to that of nowadays.


Introduction
The subfamily Petauristinae is also known as flying squirrels for they can use their membrane between the fore-and hind-legs to glide.They are strictly arboreal animals.According to fossil records, flying squirrels firstly appeared in Sihong, Jiangsu Province in the Early Miocene (Qiu 2015) and then gradually radiated to other areas in China.Nowadays there are 7-9 genera and 17-20 species of flying squirrels living in China, mostly in east and southwest China (Huang et al. 1995;Jiang et al. 2015a, b;Li Q et al. 2019bLi Q et al. , 2021)).Zheng (1993) made a systematic description on the flying squirrel fossils from cave and fissure deposits of the Pleistocene in southwest China.However, in later studies, researchers usually just listed them in the fauna or only made limited descriptions -even so, there were still many misidentifications.Therefore, it is very meaningful to conduct a detailed morphological research on this category.Furthermore, flying squirrels can be used as indicators for the forest paleoenvironment due to their typical arboreal habits.
The Miaoyu Basin in Wushan County, Chongqing Municipality, China is located at the northern foot of the Wushan Mountains, having a subtropical monsoon climate now.The zoogeographical zone is of the Sino-India Subrealm of the Oriental Realm.Topographically, the Miaoyu Basin and its surrounding area are dominated by low-medium mountains and the terrain is high in the south and low in the north with altitudes between 200 m and 2046 m.The Wushan Mountains are in the transition zone from the second step to the third step of Chinese topography.During the neotectonic movement, three stages of karst planation were formed here: the Exi planation surface, the Shanyuan planation surface and the Yunmeng planation surface (Xie 1990).Many ancient hominin and mammalian fossil localities, represented by the Longgupo Cave, the Longgudong Cave, the Baotansi Cave and the Yumidong Cave, make the Miaoyu Basin and its surrounding area one of the most important areas for the study of Quaternary paleontology in China (Huang and Fang 1991;Zheng 2004;Chen et al. 2017).
The Yumidong Cave (Fig. 1) is developed in a limestone hill named Dongbao in the southern part of the Miaoyu Basin (30°50'44.39"N,109°38'09.22"E),about 4 km away from the Longgupo Cave and the Baotansi Cave.It is a horizontal karst cave with an elevation about 1100 m at the entrance.The background and lithostratigraphic sections of this locality were described by Wei et al. (2017), Chen et al. (2017) and the Chongqing China Three Gorges Museum (2018).Dating results and faunal comparisons showed an age of Mid-Late Pleistocene for the sections.The interbeds of sandy clay and limstone breccia of the sections represented four warm periods (layers ②-1, ③, ⑤, ⑦) and four cool periods (layers ②-2, ④, ⑥, ⑧ and below), which could be correlated to MIS 8 ~ ,1 respectively (Chen et al. 2017;Wei et al. 2017;Shao et al. 2022).A great number of mammalian fossils were collected from this cave (Chen et al. 2017; Chongqing China Three Gorges Museum 2018), amongst which flying squirrel is one of the groups with the most abundant species and specimens in the fauna, providing excellent materials for the study of this group.
The fossils mentioned in the article (Suppl.material 1: table S1) were excavated from the layer ②-2 and below in the Yumidong Cave and are all stored in the Chongqing China Three Gorges Museum.
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 p4 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 p4.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 m3.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.
P3 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, P3 cannot be covered by P4 completely.
P4 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 M1 is the same as that of P4, 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.
M2 is very similar to M1 in size and shape.
The occlusal outline of M3 is close to a rounded triangle.The anterior lobe is very similar to that of M1/2, 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 dp4 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 p4 is similar to dp4, except its slightly wider anterior lobe.
The occlusal outline of m1 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 m2 is similar to that of m1, except the anterior lobe of m1 is narrower than that of m2.m3 is the longest check tooth and its anterior lobe is almost the same as m2.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 S2) 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 M3 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. (2019a)): cheek teeth brachyodont; P3 small, but with differentiated cusps; P4 molarised; hypocone and metaconule of P4-M2 well developed; the extra anteroloph and the protolophule present; M3 without metaloph and metaconule; the mesoconid and the entoconid of lower cheek teeth permanent; the trigonid basin not closed; m3 elongated obviously.Morphologically, the specimens described herein resemble P. volans and should be assigned to this species.

Trogopterus xanthipes (Milne-Edwards, 1867)
Fig. 3A-C; Suppl.material 1: table S3 Materials.As in Suppl.material 1: table S1, 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 m3.The mandibular foramen is low, relatively small, round in shape and open backwards.The ascending ramus is moderately strong and starts near the m2/m3 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.
P3 is tiny, single-rooted and very near to the anterolingual corner of P4.P4 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 M1 is low, straight and smooth and the protostyle is absent, which are different from P4.Other than these, the posterior part of M1 is very similar with that of P4.M2 is very similar to M1 in size and shape.The anterior part of M3 is very similar to M1/2, 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 p4 is close to a trapezoid, narrow in the anterior part and wide in the posterior part.p4 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.dp4 is similar to p4 in morphology, except there is no obvious anteroconid.The size of dp4 does not exceed m1.
The occlusal outline of lower molars is oblique quadrilateral.The main structure is similar to p4, 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 p4.The mesostylid is not well developed and stretches forward obliquely.m3 is not shrunk.Its posterolophid somewhat curves and the hypoconulid is weaker than that of m1/2.
Comparison.The size (in Suppl.material 1: table S3), complex occlusal surface and developed enamel folds show these specimens belong to a species of mediumlarge-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, M1/2 of Petaurista has no hypocone (Li CK et al. 2019a), while it is always present on the Yumidong specimens.The hypocone on P4-M2 of Aeretes is present, but always very near to the protocone (Zheng 1993;Tong 2007;Tong et al. 2008;Li CK et al. 2019a).
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.

Genus Belomys Thomas, 1908
Belomys pearsonii (Gray, 1842) Fig. 3D, E; Suppl.material 1: table S4 Materials.As in Suppl.material 1: table S1, there are one maxillary bone and 11 mandibular bones from the layer ②-2, one mandibular bone from the layer ③, two mandibular bones from the layer ④, two mandibular bones from the layer ⑤, one mandibular bone from the layer ⑥ and one mandibular bone from the layer ⑩.
Description and comparison.The mandible and teeth of Belomys pearsonii are very similar to Trogopterus xanthipes, but there are still some differences.The most obvious is their size: T. xanthipes is much larger than B. pearsonii and there is almost no overlap of the measurements of their cheek teeth (Suppl.material 1: tables S3, S4).Additionally, there are discernible differences of cheek tooth characteristics between these two species: 1) B. pearsonii is more lowercrowned than T. xanthipes; 2) P3 of B. pearsonii is closer to the protocone of P4 than that of T. xanthipes; 3) compared with B. pearsonii, P4/p4 of T. xanthipes is much larger than upper/lower molars; 4) the protocone of P4-M2 of B. pearsonii is more developed than that of T. xanthipes, but the hypocone is somewhat weaker; 5) the hypocone of M3 of T. xanthipes is permanent and connects with the metaconule by a straight metaloph, but the hypocone and the metaloph of M3 of B. pearsonii are absent.
There are three species in the genus Belomys, B. pearsonii, B. parapearsoni and B. thamkaewi.B. parapearsoni is only known from the Early Pleistocene.It is smaller than B. pearsonii, with lower tooth crown, less developed mesostyle, less developed mesostylid and more developed hypoconid.B. thamkaewi was unearthed from cave deposits of the Late Pleistocene in Thailand and its validity is yet to be discussed.Chaimanee and Jaeger (2000) thought it was very similar to B. pearsonii, except the somewhat larger size.However, their measurements seem doubtful, but even so, the data do not exceed the data range of fossil B. pearsonii from China.
Description.The mandible is robust and the diastemal portion is short.The tip of the incisor is slightly higher than the worn surface of the cheek teeth.Its posterior end lies under the posterior root of m3.The diastema is short and a medium-sized mental foramen is located at the middle part of the buccal side of the diastema.The inferior margin of the horizontal ramus is smoothly curved.A vascular notch is obvious and under the masseter muscle fossa.The masseter muscle fossa is relatively shallow with a weak ridge.Its anterior angle is at the level of the anterior root of m1.The pterygoid muscle fossa at the lingual side is very wide and deep.Its anterior end is at the level of the posterior part of m3.The mandibular foramen is large, oval-shaped and positioned more ventrally than the worn surface of the cheek teeth.The angular process is broken, but the preserved part shows it should be almost as wide as the ascending ramus.The upper part of the ascending ramus is badly preserved, but it seems not very high.The condylar process is a short transverse axis and its neck is long.The coronoid process is thin and higher than the condylar process.
DP4 is smaller than P4 and M1.Its occlusal outline is triangular.The parastyle is fused with the anteroloph, forming a high and isolated anterolingual corner.Besides the parastyle, the paracone, the metacone and the protocone are all well developed on the crown surface.The protostyle is invisible.The hypocone is very weak, close behind the protocone.The protoloph is short and straight.No obvious protoconule.The metaloph bends back at the metaconule.The enlarged metaconule is connected with the posteroloph by a short ridge, forming a small hypoconule on the posteroloph and dividing the posterior valley into two.The posteroloph is low.The posterolingual flexus is narrow and closed after being moderately worn.The anterior valley and the central valley are short and the anterior one is wider than the central one.There are one large root at the lingual side and two small ones at the buccal side.
P3 is not preserved.On the basis of the alveolar, it should be single-rooted, not particularly small and visible from the buccal side.
P4 is molariformed.Its lingual side is much shorter than the buccal side, making its occlusal outline triangular.The lingual wall is wrinkled and higher than the buccal wall.The protocone is small and separated from the developed hypocone by a vertical groove (the anterolingual flexus) which extends to the base of the tooth crown.The anterolingual flexus is shallow, visible after being moderately worn.The posterolingual flexus is quite deep and narrow.There are four transverse ridges on the crown surface (the anteroloph, the protoloph, the metaloph and the posteroloph).
The anteroloph is well developed and the parastyle is fused with it.The anteroloph is connected with the protocone at the slightly worn stage, but separated after being moderately worn by the anterolingual flexus.The protoloph is relatively straight.There are two ridges extending from the protoconule, the smaller one backwards and the larger one forwards.The metaconule is marked and the metaloph bends back at the metaconule.There are three small ridges extending backwards from the metaloph and two of them are connected with the posteroloph.The posteroloph is continuous and is connected with the hypocone only after being very deeply worn.Three roots, one larger at the lingual side and two smaller at the buccal side.
The occlusal outline of M1 is close to a square.The occlusal structure is mainly composed of four cusps (the paracone, the metacone, the protocone and the hypocone) and four transverse ridges (the anteroloph, the protoloph, the metaloph and the posteroloph).The anteroloph and the posteroloph are lower than the protoloph and the metaloph.The protocone is not completely separated from the hypocone and both of them are ridge-shaped.The anteroloph and the protoloph converge with the protocone.The metaloph is connected with the hypocone.The paraconule and the metaconule are small.The main structure of the posterior lobe is similar to that of DP4.
The occlusal outline of p4 is a trapezoid with a narrow anterior lobe and a wide posterior lobe.The anteroconid is small, located at the anterobuccal side of the metaconid.The metaconid is the highest amongst all cusps of the tooth.The metastylid is barely visible and is separated from the well-developed mesostylid by a deep groove.The protoconid is weaker than the metaconid, but larger than other cusps.The anterobuccal sinusid is shallow and V-shaped.The buccal valley is wide.The mesoconid is obvious.The mesolophid divides the talonid basin into two.The protolophulid is tiny and the extra posterolophid is short.The posterolophid is curved.Two roots.
The lower molars have similar occlusal structure with p4.The anterolophid is developed and connected with the anteroconid and the metaconid.The anterobuccal sinusid is much deeper than that of p4.The mesoconid is connected with the protoconid.The metalophid is more developed than that of p4.The posterior lobe of m3 tapers obviously with a weak entoconid.Four roots.
Comparison.Based on the dental dimensions (in Suppl.material 1: table S5), the described fossils should be a large species of flying squirrel, which is most likely to belong to either Aeretes or Petaurista.
In Petaurista, M1/2 lacks the hypocone and the lower cheek teeth have marked anterobuccal sinusid, which can be used for generic diagnosis and are obviously different from the features of the described fossils.
The diagnosis of the genus Aeretes includes (summarised by Li CK et al. (2019a)): P3 small, P4 larger than molars, the hypocone of P4-M2 weak and located close behind the protocone, the protoconule and the metaconule weak, the hypoconule well developed, the posterolingual flexus very deep, the entoconid of lower molars small, the mesostylid developed and separated from the metastylid and entoconid by deep grooves and the mesoconid small.The described fossils resemble Aeretes.
There are three species in the genus Aeretes, A. premelanopterus, A. grandidens and A. melanopterus.The primitive A. premelanopterus has weak protoconule, but strong metaconule; A. grandidens has stronger protoconule and metaconule than A. premelanopterus.The protoconule and metaconule of A. melanopterus from different regions varies in morphology: specimens from Chongqing and Guizhou have a very weak protoconule and developed metaconule (Zheng 1993), while specimens from Beijing have a marked protoconule and metaconule (Tong 2007).The Yumidong specimens are most similar to A. melanopterus from Beijng.Furthermore, comparing the Yumidong specimens with A. melanopterus from the Baotansi Cave in Chongqing, there is another obvious dental morphological difference: the lingual vertical groove of P4 is clear on the Yumidong specimens, but unclear on the Baotansi specimens.
Compared with the Aeretes melanopterus from the Tianyuan Cave and extant specimens (based on Tong (2007)), the morphological structure of the described specimens falls within their variable range.Based on dimensions, the described specimens are much smaller than the Tianyuan Cave specimens and fall within the variable range of the living species.It is reasonable to attribute these fossils to A. melanopterus.Tong (2007) mentioned the diagnosis of this species, including: cheek teeth subhypsodont, enamel slightly rugose, the lingual wall of upper cheek teeth higher than the buccal wall, the posterolingual flexus very narrow and deep.

Paleoenvironmental analysis
The evergreen forest covers the area around the Yumidong Cave and the character of vertical zonality of this area is clear nowadays.In areas below 700 m above sea level, the vegetation type is dominated by evergreen broad-leaved and deciduous broad-leaved mixed forest.With increasing elevation, the proportion of coniferous and broad-leaved mixed forest increases.In areas above 2000 m, with only a limited range of peaks in the Wushan Mountains higher than this altitude, the coniferous and broad-leaved mixed forest becomes dominant.It was thought that the drastic climate changes during the Late Pleistocene intensely affected the distribution patterns of flora and fauna (Zhang 2011a, b).What was the paleoenvironment like in the Late Pleistocene in this area?Fossil flying squirrels from the Yumidong Cave provide a clue for this question.The environmental changes in the Pleistocene undoubtedly impacted the paleozoogeographic process of flying squirrels (Lu et al. 2013).Two species of flying squirrel, Petaurista alborufus and Trogopterus xanthipes, are still surviving in this area and the latter is on the list of fossil flying squirrels excavated from the Yumidong Cave.The other three species of fossil flying squirrels excavated from the Yumidong Cave have completely disappeared in this area now (Fig. 5), which could reflect a different environment in the Late Pleistocene in this area.
Pteromys volans is a Palaearctic cold-adapted species, widely distributed in forest zones from northeast Europe to the Korean Peninsula, even to the Hokkaido Island of Japan nowadays (Nowak 1991;Huang et al. 1995).In China, it is mainly distributed in the north of Xinjiang, north and northeast China and lives in the temperate and cold-temperate alpine coniferous forest or coniferous and broad-leaved mixed forest.A small group of this species was listed in the subalpine coniferous forest of the north-eastern margin of the Qinghai-Tibet Plateau (Hu and Wang 1984;Huang et al. 1995), but it has not been confirmed.It was also listed in the Nanling Mountains of Hunan (Liu and Yuan 1981), but Fu (1987) removed it from the list when he summed up the report of the Nanling mammal expedition.Fossils of this species were described from deposits of several caves in Guizhou, Chongqing and Beijing, ranging from the Early to the Late Pleistocene (Zheng 1993;Huang et al. 2002;Tong et al. 2008).However, inexplicably, fossils of P. volans have never been reported in northeast China, which is one of its current distribution areas, while in other areas of the world, except in southwest China, its fossil records are not earlier than the late Middle Pleistocene (Yalkovskaya et al. 2015).Specimens of P. volans are abundant in the Yumidong Cave, making up 9% of small mammalian fossils in layer ②-2 and 46% in layer ④.This phenomenon indicates the existence of temperate and cold-temperate alpine coniferous forest or coniferous and broad-leaved mixed forest during MIS 2 and MIS 4 in the researched area.
Trogopterus xanthipes is an endemic species in China, living from temperate subalpine coniferous forest to subtropical broad-leaved forest at an altitude of 1360 m-2750 m (Hu and Wang 1984).This species is still living in the researched area at the present time.As a fossil species, it was only known from Guizhou, Chongqing and Beijing (Zheng 1993;Tong et al. 2008).
The extant Belomys pearsonii is widely distributed in south China, north-eastern South Asia and Southeast Asia (Molur 2016), but it has completely disappeared from the researched area.However, the subfossils of this species have been reported from the Neolithic Dahekou site in Chongqing (Wang et al. 2021).As a fossil species, it is one of the most common flying squirrel species in southwest China (Zheng 1993;Wu 2006).
The extant Aeretes melanopterus is mainly distributed in the north-eastern Qinghai-Tibet Plateau, the Hengduan Mountains and the Qinling Mountains and a small population is also known in Beijing.This species inhabits the coniferous and broad-leaved mixed forest at altitudes of 2500 m-3000 m and prefers to nest on high trees (Allen 1940;Hu and Wang 1984;Chen et al. 2002;Zheng and Song 2010).
Throughout the current habitats of extant species of fossil Petauristinae from the Yumidong Cave, all the four species can adapt to coniferous and broad-leaved mixed forest and alpine coniferous forest.Pteromys volans and Aeretes melanopterus have completely disappeared from the researched area and its vicinity.Now they live in temperate and cold-temperate zones at higher latitudes or inhabit areas of middle to high altitudes at latitudes similar to the Yumidong Cave (Fig. 5).All the species of flying squirrel described here were mainly excavated from the layer ②-2 and the layer ④ of the Yumidong Cave.They may indicate a paleoenvironment of subalpine evergreen coniferous forest or coniferous and broadleaved mixed forest during MIS 2 and MIS 4, which was colder and of more coniferous forest than nowadays in this area.It is similar to the vegetation landscape on the peaks of the Wushan Mountains over 2000 m above sea level now.However, the regional extinct species P. volans and A. melanopterus were not found from the layer ③ and the layer ⑤ of the Yumidong Cave, probably representing a vegetation landscape similar to nowadays.

Extinction of cold-adapted Petauristinae in the Three Gorges area
The species combination of the flying squirrels from the Baotansi Cave 4 km away is consistent with the Yumidong Cave.It was thought that the age of the Baotansi deposits was the Middle Pleistocene on the basis of an ambiguous specimen of ?Allocricetus sp.(Zheng 1993), but we believe that its main deposits are from about the latest Pleistocene by our survey at the locality.Another latest Pleistocene fossil locality nearby is the Migong Cave (Pang et al. 2017).Small mammal fossils from this locality also supported a cool paleoenvironment, but the forest would have been relatively sparse because there was no trace of flying squirrel fossils at all (Pang et al. 2017).These two localities are near the Yumidong Cave and their strata were formed at the same period with the layer ②-2 of the Yumidong Cave.The main reason for their different paleoenvironments is the difference in altitudes.The Yumidong Cave is about 1100 m in altitude; the Baotansi Cave is about 820 m; the Midong Cave is about 160 m-200 m.It is concluded that the different habitats in mountains and valleys caused by the vertical zonality resulted in the fauna divergence during the latest Pleistocene in the Three Gorges area.
The distribution pattern of Petauristinae fossils in the strata of the Yumidong Cave might indicate that the cold-adapted flying squirrels spread southwards during the glacial periods and disappeared in this area during the interglacial periods.It is speculated that cold-adapted flying squirrels lived at the middle zone of the Wushan Mountains during the latest Pleistocene or the Last Glacial Maximum.When the Holocene Megathermal period came, their habitats might have contracted to higher peaks, which were very limited ecological spaces and could not support their survival in the following period.As a result, these cold-adapted flying squirrels died out in this area.
Specimens of flying squirrels were also reported from the Holocene Dahekou Site in Fuling District of Chongqing (Wang et al. 2021).However, the specimens originally identified as Trogopterus xanthipes should be Belomys pearsonii.The specimens of Petaurista petaurista and

Conclusion
Numerous flying squirrel fossils of the Mid-Late Pleistocene were unearthed from the Yumidong Cave in Wushan County, Chongqing Municipality, China.According to the morphological characteristics, these specimens can be identified as Pteromys volans, Trogopterus xanthipes, Belomys pearsonii and Aeretes melanopterus, four species of four genera in total, making the Yumidong Cave one of the richest flying squirrel localities of the Quaternary in China.Throughout the current habitats of extant species of fossil Petauristinae from the Yumidong Cave, all the four species could adapt to coniferous and broad-leaved mixed forest and alpine coniferous forest.Their distribution in the strata indicates that the landscapes around the Yumidong Cave during MIS 2 and MIS 4 were dominated by subalpine evergreen coniferous forest or coniferous and broad-leaved mixed forest, similar to the environment of the peaks of the Wushan Mountains above 2000 m nowadays, while the landscapes during MIS 3 and MIS 5 were similar to the environment around the Yumidong Cave at present.

Supplementary material 1
Flying squirrel fossils described in the article from the Yumidong cave

Figure 1 .
Figure 1.Geographical location (A), excavation units (B), stratigraphic sequence of T6 and their age (C) of the Yumidong Cave.The age is based on Shao et al. (2022).

Figure 5 .
Figure 5. Distribution patterns of the living forms of Petauristinae from the Yumidong Cave.Aeretes melanopteurs seem to be an undetermined species of the genus Petauristas.The specimens of Hylopetes alboniger should belong to Sciurinae and resemble Tamiops swinhoei.That is to say, current evidence cannot prove the cold-adapted flying squirrels, such as Pteromys volans, Trogopterus xanthipes and Aeretes melanopteurs, survived through the Last Glacial to the Holocene in the Three Gorges area.

table S1 :
Li-bo Pang, Shao-kun Chen, Xin Hu, Yan W, Guang-biao Wei Data type: docx Explanation note: Flying squirrel fossils described in the article from the Yumidong cave; Authors:

table S2 :
Measurements of Pteromys volans fossils from the Yumidong cave and comparisons with related specimens (in mm); table S3: Measurements of Trogopterus xanthipes fossils from the Yumidong cave and comparisons with related specimens (in mm); table S4: Measurements of Belomys pearsonii fossils from the Yumidong cave and comparisons with related specimens (in mm); table S5: Measurements of Aeretes melanopterus fossils from the Yumidong cave and comparisons with related specimens (in mm).Copyright notice: This dataset is made available under the Open Database License (http://opendatacommons. org/licenses/odbl/1.0).The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.Link: https://doi.org/10.3897/fr.27.115693.suppl1