The reconstruction of skull morphology is based on the almost complete cranium and mandible of IPS56468 from ACM/C5-D1 (Figure 3a–e, Figure 6—figure supplement 1), as well as on the additional information provided by the partial cranium IPS88677 from ACM/C8-Af (Figure 6—figure supplement 2) and the partial mandible IPS87560 from sector ACM/C8-B (Figure 3f–g). The cranium IPS56468 is laterally crushed in an oblique angle, with the rostrum deviated towards the left side. The zygomatic arch is complete but crushed, with the jugal and other fragments pressed against the orbit. On the other hand, IPS88677 is dorsoventraly crushed and most of the premaxillary and frontal bones are not preserved. The virtual reconstruction of the skull (see Materials and Methods; Figure 6 and Video 3) relied primarily on IPS56468h, but used some elements from IPS88677 when the bone was particularly damaged or missing in the former specimen (zygomatic plate, jugal, pterygoid, alisphenoid; Figure 6—figure supplement 1-2, Table 3). The description of cranial morphology relies on the original specimens but also considers aspects of the virtual reconstruction concerning overall shape and proportions of certain structures (i.e., rostrum breadth and height, separation between bullae). The skull was directly compared to that of extant flying squirrels housed within the collections of the Naturalis Biodiversity Center of Leiden (Aeromys tephromelas; Belomys pearsonii; Eupetaurus cinereus; Glaucomys sabrinus; Glaucomys volans; Hylopetes alboniger; Hylopetes phayrei; Hylopetes platyurus; Hylopetes sagitta; Iomys horsfieldii; Petaurista alborufus; Petaurista elegans; Petaurista petaurista; Petaurista philippensis; Petinomys hageni; Petinomys setosus; Petinomys vordermanni; Pteromys momonga; Pteromys volans) and the Muséum National d’Histoire Naturelle of Paris (Iomys horsfieldii, Petaurillus kinlochii, Pteromyscus pulverulentus, Trogopterus xantiphes). Only Eoglaucomys and the poorly-known genera Aeretes and Biswamayopterus could not be directly compared. Comparisons with Aeretes melanopterus are based on published data (Thorington et al., 2002; Thorington et al., 2012). In addition, the posterior region of the skull of P. petaurista was scanned in order to compare its inner ear morphology to that of M. neogrivensis (see Materials and Methods, Video 4). Several cranial and mandibular measurements were taken in the specimens of M. neogrivensis and some comparative material (see Supplementary file 5–6). Measurements were also taken from the virtually reconstructed cranium of M. neogrivensis using Avizo 7.0.

Cranium. The cranium is relatively short and wide, as in most large-sized flying squirrels but Eupetaurus and a few species of Petaurista, such as Petaurista alborufus (Figure 6). The upper incisors are orthodont, as in most flying squirrels except Pteromys and Aeretes, which show opisthodont incisors. The incisors are not as wide and robust as in other large-sized flying squirrels. The rostrum is similar in length to that of Petaurista, Aeretes and Aeromys. It is also relatively wide and high, although not as much as in Aeretes. The nasals are not preserved in any of the specimens, but their suture with the premaxilla is almost complete in IPS88677 (Figure 6—figure supplement 2), showing that they were most likely wide, resembling those in Aeretes and Petaurista. The incisive foramina are slightly longer than in most flying squirrels, approaching the condition seen in Aeromys and Eupetaurus. The masseteric tubercles are somewhat prominent and bulbous, being very similar in size and shape to those of Petaurista. In addition, there is a prominent semicircular scar for the origin of the superficial masseter (Ball and Roth, 1995; Thorington and Darrow, 1996) in front of the cheek tooth row. The palate is not particularly wide, being markedly narrower than in Glaucomys, Pteromys and most Hylopetes and Petinomys species. A short posterior nasal spine is present at the suture between the palatines. This is highly variable in flying squirrels, but the spine is generally reduced in Aeromys, Hylopetes and Glaucomys. The pterygoid ridges diverge distally, as in Aeretes, instead of being parallel as in most flying squirrels or slightly convergent as in Pteromys and Trogopterus. The auditory bullae are inflated and similar in size to those of Petaurista and Aeromys and separated by a wide distance as in Petaurista and most Hylopetes species. This is in marked contrast with Aeromys and, especially, Aeretes, which show a narrower space between the bullae. A μCT scan of the middle ear cavity of IPS88677 shows the presence of two ventral transbullar septa (Video 4), while this feature is not preserved in IPS56468h. Septated bullae occur in the tympanic cavity of certain rodents (Aplodontiidae, Sciuridae, Geomyidae and some Arvicolinae), although their function is unclear (Mason, 2015). Several authors have stressed the taxonomic importance of the presence or absence of these structures and the number of septa (Moore, 1959; Pfaff et al., 2015; Thorington et al., 2002). Most squirrels show transbullar septa, and their number typically ranges from one to three, but some flying squirrels show many more, sometimes even arranged in a honeycomb (as in Petinomys) or in a sliver pattern (as in Trogopterus and Pteromyscus) (Moore, 1959; Pfaff et al., 2015; Thorington et al., 2002). Miopetaurista neogrivensis shows just two septa, as in most flying squirrels including Petaurista (Video 4). The external auditory meatus is large and rounded. The bony process from the opening ear canal is large. As in all flying squirrels but Iomys and Petaurillus, the mastoid process is not inflated.

In dorsal view the postorbital process of the frontal is robust and long, partly enclosing the distal half of the orbit, thus approaching the morphology of large-sized flying squirrels such as Petaurista and Aeromys. Small-sized flying squirrels usually show more slender and shorter postorbital processes. The frontal is depressed on its anterior part. There is a conspicuous semicircular notch just in front of the postorbital processes. This notch is also present in many flying squirrels, including Aeromys and Petaurista, but it is more pronounced in Miopetaurista, thus approaching the morphology of Pteromys, Glaucomys and most Hylopetes species. The zygomatic plate of the premaxillary bone is similar in size and morphology to that of most flying squirrels, except for Pteromys and Petaurillus, which show wider plates bordered by more marked ridges. The infraorbital foramen is relatively wide. The zygomatic arches are wide and approximately parallel to the longitudinal axis of the skull. The jugal is deeper than in most flying squirrels and shows a dorsal process pointing towards the postorbital process of the frontal. This process is also observed in many flying squirrel genera, including Petaurista, Aeromys and Aeretes, but it is more pointed than in Miopetaurista. The origin of the deep masseter on the jugal bone (Ball and Roth, 1995; Thorington and Darrow, 1996) defines a marked scar that is larger and extends somewhat more dorsally than in any other studied squirrel. The cranial vault is relatively flat as in most large-sized flying squirrels, except for Aeromys tephromelas and some Petaurista species. Small-sized flying squirrels generally show more convex cranial vaults, sometimes markedly inflated as in Iomys. The parietal is wide and shows well defined temporal ridges (marking the insertion of the temporalis muscle (Ball and Roth, 1995; Thorington and Darrow, 1996) that approach each other towards the posterior part of the skull. Even though this occurs to some degree in certain flying squirrels (Pteromys, Hylopetes), the ridges come so close only in Eupetaurus.

Mandible. Both hemimandibles of IPS56468 are damaged in their posterior part, with the mandibular process partially broken and displaced (Figure 3a–e). The articular process is completely missing in the left hemimandible (IPS56468j), which also lacks the tip of the coronoid process. On the contrary, both processes are preserved in the right hemimandible (IPS56468i), although detached and moved from their anatomical location. The partial hemimandible IPS87560 preserves the articular process in its original position as well as the angular process (Figure 3f–g). A complete mandible was assembled using three-dimensional models of these three specimens digitized by means of photogrammetry (see Materials and Methods; Figure 1, Videos 1 and 3D model in Supplementary file 1). However, the description of mandibular morphology relies on the original specimens, the three-dimensional model being only considered for assessing the shape of the mandibular ramus between the coronoid and articular processes.

The mandible is relatively deep and short, with the lower incisor projecting slightly above the occlusal plane of the cheek teeth. The corpus is strong and deep. The symphysis is strong and the diastema is relatively short. The masseteric ridges converge at the level of the distal root of the m1. The point where the ridges meet does not form a bump or a tubercle as in other sciurids. The masseteric fossa is quite deep. The coronoid process is robust and hook-shaped, resembling that of Aeretes, Aeromys, Petaurista, Belomys, Petinomys and some Hylopetes species such as Hylopetes sagitta. The coronoid is also noticeably higher than the articular process as in all these genera, but rises in an angle closer to that of Hylopetes and Petinomys and not as steep as in Petaurista, Aeretes, Belomys or Trogopterus. IPS56468i partially preserves the outline of the ramus between the coronoid and articular processes. These two processes are not as separated as in Petaurista, Aeretes, Belomys or Trogopterus, being closer as in Aeromys, Glaucomys and some species of Hylopetes and Petinomys. IPS87560 preserves the articular process in its original position and shows that it is long and projects distally. Aeretes, Glaucomys and Hylopetes show similarly long articular processes, while most large-sized flying squirrels such as Aeromys, Eupetaurus and Petaurista show shorter processes. The articular condyle is longitudinally elongated. The angular process is of intermediate length and distally directed, not extending far ventrally. As in Petaurista, Aeromys and Hylopetes, for example, the angular process does not reach farther posteriorly than the articular one. The inner surface of the angular process shows a marked fossa for the insertion of the medial pterygoid partially divided by a marked longitudinal ridge. The ventral edge of the angular process presents a well-defined area for the insertion of the superficial masseter bordered by faint ridges. Overall, mandibular morphology and robusticity recall Aeromys and Hylopetes, being slightly more gracile and elongated than in Aeretes and Petaurista, for example.

The skull of Miopetaurista resembles that of other large-sized flying squirrels, particularly Aeromys and Petaurista, which are characterized by their short and wide rostrum, moderately inflated bullae and relatively wide posterior region of the skull. Other details are strikingly similar to these extant genera, including the robust and long postorbital process that partially encloses the orbit, the well-developed jugal process in the zygomatic arch, or the presence of two septa in the tympanic cavity. Most of the smaller flying squirrels show more elongate muzzles, slender or shorter postorbital processes and, in some cases, a higher number of transbullar septa. However, the mandible of Miopetaurista is somewhat slenderer than in most large-sized flying squirrels, showing a more elongated articular process resembling that of Hylopetes and other small-sized flying squirrels instead of the lower and shorter process of Petaurista. The characters of the skull agree with our phylogenetic results (see main text and Figure 7) in showing a close relationship between Miopetaursita and extant large-sized flying squirrels, particularly Petaurista. However, we must note that none of these characters distinguishes flying squirrels exclusively. Indeed, overall skull morphology of some large-sized tree squirrels such as Ratufa and Protoxerus (Ellerman, 1940; Moore, 1959) is similar to that of large-sized flying squirrels.