Vilevolodon

Based on dental morphological characteristics, Vilevolodon is inferred have had an herbivorous diet, likely consisting largely of seeds and soft plant tissues. Cusp likeness to sciurid rodents, which have a diet of nuts, seeds and fruits, and supplemented by insects, suggests some potential for omnivory. Luo posits that Vilevolodon was not a folivore however, as the teeth lack strong crests which are characteristic of folivorous taxa, including modern marsupial gliders.[4][1] All extant mammalian gliders are primarily herbivorous, thus an herbivorous dietary inference for Vilevolodon is consistent with modern, analogous gliders.[1] Overall, the distinctive teeth of Vilevolodon expand the known dental morphology of mammaliaforms, especially when compared to Triassic haramiyids of the same clade, whose teeth are often much less transformed with reduced basins, and lie in straight, organized rows within the jaw.[1]

Vilevolodon is interpreted as volant and capable of gliding. Meng et al cited the relative proportions of limb bones, and the acromion-clavicle joint which allowed for significant shoulder mobility as supportive of a stance required for gliding locomotion. The acromion-clavicle joint structure is persistent throughout eleutherodonts and represents an adaptation for volant locomotion running convergent to the increased mobility of the sternum-clavicle joint observed across therian gliders.[3] The shortened shape of the astragalus and calcaneus resemble those of a modern bat, and the proximal pedal phalanx has a pronounced groove used for tendon attachment which is observed in mammals with enhanced grasping capabilities. Elongated necessary for roosting and hanging behavior.[3] When considered together, the gliding capabilities and modified feet with climbing and roosting specializations suggest an arboreal lifestyle for Vilevolodon. This is consistent with other members of the clade Eleutherodontia, of which the taxa, like Maiopatagium, were a majority arboreal.

Preserved alongside skeletal elements is what Luo et al has interpreted as a patagium, or a membranous structure used in gliding or flight assistance. Propatagium, plagiopatagium and uropatagium sub-structures have been identified as well. Morphometric analyses carried out contiguously with another gliding Haramiyid, Maiopatagium furculiferum, from the same locality are consistent with the gliding adaptations of extant mammals and other fossil taxa.[5] The pelage of Vilevolodon is preserved as a mat of carbonized fur and long guard hairs which was compressed upon patagial membranes. Among therian gliders, this patagium is most comparatively similar to the gliding sciurid rodents based upon similar proportions of propatagium, plagiopatagium, and uropatagium structures. This structure, and other gliding structures seen among eleutherodonts, differs from modern anomalurids in that the plagiopatagium extends to reach the ankle and wrist, and they also differ from modern rodent gliders and the marsupial glider, Petauroides volans, in that they lack a styliform structure from the wrist or ulnar olecranon. Vilevolodon also exhibits strongly developed uropatagia and propatagia, where these structures in marsupial gliders are much less prominent.[1]

The dental morphology of Vilevolodon is characterized by a dual “mortar-and-pestle” occlusion of molars in which the tallest distal cusp of the upper molar occludes into the deepest distal basin of the opposite, lower molar. The tallest mesial cusp of the lower molar occludes into the mesial basin of the opposite upper molar, simultaneously. Analysis using STL models and tomography scans suggest that this occlusal pattern allows for a complex mechanism of dual crushing and grinding capabilities.[1] A similar occlusal pattern is seen in other Haramiyids, Arboroharamiya[6] and Xianshou.[2] CT scans of Vilevolodon reveal no replacement premolars within the mandible, and that the molars are fully erupted and occluded with closed root tips. However, the upper and lower incisors are captured in mid-replacement. This pattern of tooth replacement, with off-set or potentially heterochronical replacement and eruption of incisors as compared to the molars and premolars is unique in mammaliaforms.[7] Luo posits that Vilevolodon either had an unusually accelerated completion of molar eruptions, or the incisor replacement captured in the holotype represents a paedomorphic adult feature.[1]

Like other euharamiyidans.[8] the ear bones of Vilevolodon had not achieved full separation from the mandible.[1] As the transition of the middle ear away from the dentary via the modification of the quadrate and articular bones into and incus and malleus respectively is a hallmark for mammalian recognition, the preservation of an ear structure in the Vilevolodon holotype is not only crucial to its placement as a euharamiyidan, but has important phylogenetic implications as well. The holotype features a malleus connected anterior to Meckels's cartilage, and the ectotympanic features an anterior limb and a straight reflected lamina. The anterior ectotympanic limb is nestled in a post-dentary trough on the internal edge of the mandible. This groove occupies a similar location in other mammaliaforms,[8] however, it is narrower and much shorter in Vilevolodon, and does not extend posteriorly to the dentary condyle. Based upon CT scans and proximity of preservation, the Meckel's cartilage and the ectotympanic contact each other loosely. Luo et al reconstructed these two elements as contiguous, in a similar fashion to cynodonts and other mammaliaforms.[1] While Morganucodon and other docodontans have modern-mammal-like diphyodont (two sets of teeth) replacement which is completed during early ontogeny. Luo et al posit that this suggests that the diphyodont tooth replacement had heterochronical variation, and is likely a homoplasy in early mammaliaforms.[1]