Main

Clade Mammaliaformes1

Clade Haramiyida (emended by ref. 11)

Family Eleutherodontidae4,11

Vilevolodon diplomylos gen. et sp. nov.

Etymology. Vilevol (Latin): glider; don (Greek): tooth, and a common suffix for mammalian taxon names; diplo (Greek): double; mylos (Greek): grinding; diplomylos refers to the dual mortar–pestle occlusion of opposing upper and lower molars.

Holotype. Beijing Museum of Natural History PM002942A and B (abbreviated as BMNH2942A and BMNH2942B) are the main slab and counter slab, respectively, preserved with carbonized residues of patagial skin membranes associated with a postcranial skeleton that is 70% complete. The skull is preserved with teeth in occlusion and a middle ear associated with the mandible (Figs 1, 2, Extended Data Figs 1, 2, 3).

Figure 1: Dentition of mammaliaform Vilevolodon diplomylos (Haramiyida, Eleutherodontidae) (BMNH2942 holotype).
figure 1

a, b, Mandibles and teeth on BMNH2942A from computed tomography scans (top and under sides); lower M2 (inset) from BMNH2042B. c, Upper teeth in lingual and occlusal views. d, Lower teeth in lingual and occlusal views. e, Posteromedial view of the mandible to demonstrate the M2/M2 occlusal alignment and the middle ear contact to mandible. f, Dual mortar–pestle occlusion. The tallest cusp ‘A1 pestle’ of lower M2 occludes into the anterior fusiform mortar basin of M2 while the tallest cusp ‘A1 pestle’ of upper M2 occludes into the posterior fusiform mortar basin of lower M2. g, M2/M2 dual mortar–pestle occlusion in posteromedial view as in e. L, left; R, right; P, premolars; M, molars. Lower incisors I1A and I1B are two successive incisor generations of lower locus 1; incisors I1A and I1B are two generations at upper incisor 1 locus. The whole slab and counter-slab with carbonized patagia are shown in Extended Data Fig. 1.

PowerPoint slide

Full size image
Figure 2: Disparate tooth occlusal patterns in haramiyidans.
figure 2

a, Haramiyavia with lower A1 of M3 occluding into lingual embrasure of upper M2–M3. Upper tooth outlines flipped and superimposed on lower molar M3 (ref. 6). b, Occlusal view of right M3. c, Occlusal view of right upper M2–M3. d, Megaconus. Lower M3 A1 cusp occludes into the lingual embrasure between upper M2–M3. e, f, Right M3 occlusal view (e) and right M1–M3 occlusal view (f). g, Maiopatagium right upper P3–M2 in occlusal view. h–j, Vilevolodon right lower M1–M2 (i) and right upper P4–M2 (j). The opposite upper molars (transparent) over the lower molars (grey) in dual mortar-pestle occlusion. k–m, Eleutherodon putative right M2 (l) and right M2 (m)3,9. M2 (transparent) over M2 (grey)3,4. Node 1, ancestral occlusal pattern of the haramiyidan Haramiyavia: thoroughfare occlusal furrow for ortho-palinal occlusal movement. Node 2, clade of Megaconus and Vilevolodon: shortened Meckel’s sulcus and reduced postdentary trough. Node 3, ventro-flexed rostrum. Node 4, cusp coalescence within cusp row, thoroughfare occlusal furrow for ortho-palinal occlusal movement. Node 5, clade of Vilevolodon and Eleutherodon (including Xianshou, Arboroharamiya and Sineleutherus) with dual mortar–pestle occlusion. Phylogenetic analyses presented in Supplementary Information.

PowerPoint slide

Full size image

Geological age. Vilevolodon diplomylos type specimen is from the Nanshimen site of the Tiaojishan Formation in Qinglong County, Hebei Province, China, stratigraphically correlated by the regional index fossil Qaidamestheria13, and estimated to be 161–160 million years old. The Tiaojishan Formation has yielded additional mammaliaforms at the same site and elsewhere9,10,11,14,15,16.

Diagnosis. Among haramiyidans, Vilevolodon is most similar to Arboroharamiya10 and Xianshou11 in having dual mortar–pestle molar occlusion in which the tallest distal cusp A1 of the upper molar (upper ‘pestle’) occludes into the distal basin (lower ‘mortar’) of the opposite lower molar. The tallest mesial cusp A1 of the lower molar (lower pestle) occludes into the mesial basin (upper mortar) of the opposite upper molar (Figs 1, 2, Extended Data Figs 3, 4, 5, 6; Supplementary Video 1). It differs from Maiopatagium15 and Shenshou11 in that upper molars are arranged in successively imbricated series in which M2 is oblique to M1, and M1 is oblique to P4. This is mirrored by imbrication of the lower teeth in which M2 is medial (and oblique) to M1, and M1 is medial to P4 (Fig. 1). This differs from the straight tooth rows of Maiopatagium and Shenshou (Fig. 2). Among eleutherodontids (see Supplementary Information), V. diplomylos is most similar to species of Xianshou in having a hypertrophied P4 and pronounced flexure of upper P3–P4 that is correlated with rostroventral bending of the maxilla. Differs from Xianshou species in having larger teeth but a shorter mandible. Differs from paulchoffatiid multituberculates16 in that its molars have a confluent root in a single alveolus, not double roots in separate alveoli. Differs from eutriconodonts and spalacotherioids of crown mammals in that Meckel’s sulcus is vestigial and Meckel’s element is shortened17,18. Full differential diagnosis of V. diplomylos from other mammaliaforms is provided in the Supplementary Information.

The unique dental occlusion of Vilevolodon includes dual crushing and grinding functions. Our simulation analysis using STL models from computed tomography scans shows evidence of complex chewing movement in two separate occlusal cycles (Supplementary Video 1 and Extended Data Fig. 6). The trenchant cusp of P4 contacts the P4 basin simultaneously with the dual mortar–pestle occlusion of M1/M1 and M2/M2 in the same chewing cycle (Supplementary Video 1 and Extended Data Fig. 6). The main trajectory of cusp A1 (pestle) of the lower molar is ortho-palinal, similar to that of Haramiyavia4,6, but the palinal movement has a strong ventral vector, as constrained by the deep basins of upper and lower molars. It is not possible for the lower P4–M2 to move posteriorly and horizontally (that is, fully palinal). This differs from multituberculates, Megaconus and Maiopatagium, in which the lower teeth can have full palinal movement4,9,19 (Fig. 2). The dual mortar–pestle occlusal contact and two distinctive occlusal cycles are unique to eleutherodontids (probably also applicable to Arboroharamiya), but absent in Haramiyavia, Megaconus, Maiopatagium and other stem mammaliaforms.

We infer that Vilevolodon had an herbivorous or omnivorous diet consisting largely of seeds and soft plant parts. Tooth crown complexity of Vilevolodon and Xianshou is comparable to derived multituberculates with plant-dominated diets20. Their tooth crenulations and creases resemble those of sciurid rodents that have diets of nuts, seeds, fruits and young leaves, supplemented by insects21. Vilevolodon was not a folivore, as it lacks strong tooth crests that are characteristic of primarily folivorous and volant taxa such as dermopterans, anomalurids and marsupial gliders. All modern mammalian gliders are primarily herbivorous and none are primarily insectivorous22, and we interpret Vilevolodon to be a glider based on the presence of carbonized patagia and skeletal morphometric analyses15. Thus, our herbivorous dietary inference for Vilevolodon is consistent with modern, analogous gliders.

The peculiar teeth of Vilevolodon expand the known dental morphological disparity of mammaliaforms. Its densely ornamented and partly basined teeth are markedly distinct from the simplified teeth of Maiopatagium and from the straight cusp rows and straight furrow of Megaconus. Jurassic eleutherodontids are highly transformed, even compared to Triassic haramiyids of the same clade (Fig. 2). Their disparate tooth morphologies suggest resource partitioning among omnivorous and herbivorous feeding guilds of eleutherodonts. Furthermore, eleutherodonts diversified contemporaneously with early multituberculates16 and ecomorphologically diverse docodonts14,23, offering compelling evidence that clade divergence and ecological diversification are coupled in adaptive diversifications in multiple Mesozoic mammaliaform clades9,10,11 (Fig. 2).

Eleutherodonts diversified in a pre-angiosperm biota of the Middle to Late Jurassic. Their diets would probably have included seeds, reproductive parts such as strobili and cones of ferns, plus soft meristem tissues and young leaves of seed and gymnosperm plants, which have all been hypothesized to be probable dietary sources for animals24. The association of herbivorous and volant eleutherodonts with pre-angiosperm plants in the Jurassic is analogous to the association of herbivorous volant therians (that is, dermopterans, and multiple clades of rodents and marsupials) with angiosperm plants of the Cenozoic era. Eleutherodontids show a marked similarity to the primate Daubentonia in the ventrally bent rostrum and deep mandible, and both features are interpreted to be reinforcement for incisor gnawing25 (Extended Data Fig. 7). Daubentonia has a dietary mixture of fruits, exudates and insect larvae. Eleutherodonts and Megaconus (Extended Data Fig. 6) show a zigzag tooth row profile in lateral view, convergent with that of frugivorous bats26 (Extended Data Fig. 7). The highly ornamented teeth of eleutherodontids (Eleutherodon, Sineleutherus, Arboroharamiya, Xianshou and Vilevolodon) are convergent with some extant sciurid rodents with granivorous and frugivorous diets and to herbivorous phyllostomid bats21,26. These observations suggest that haramiyidans differentiated into several feeding guilds during their evolutionary diversification in the Jurassic.

Computed tomography scans of the Vilevolodon holotype confirm that premolar loci have no replacing tooth. The right M2/M2 are fully erupted and occluded, and these have closed root tips. M1/M1 show occlusal wear (Extended Data Fig. 4). Cheek teeth of the Vilevolodon have attained adult status27 according to the individual dental age system detailed in ref. 28. However, its upper and lower incisors are still undergoing replacement, either by a prolonged, or a delayed replacement. Thus, Vilevolodon shows a heterochronical pattern of incisor replacement versus premolar and molar eruption, which is unique in mammaliaforms (except Sinoconodon)27. Either Vilevolodon had an unusually accelerated completion of molar eruptions as a juvenile (indicated by ongoing incisor replacement) or the ongoing incisor replacements at I1/I1 loci are a paedomorphic adult feature (indicated by complete molar eruption). By contrast, Morganucodon and docodontans have modern-mammal-like diphyodont replacements in which replacement of antemolars is completed during juvenile growth stages, well before complete eruption of adult molars23,29. This suggests that diphyodont tooth replacement, a hallmark feature for determinate skull growth of modern mammals27,29, had heterochronical variation, and may be homoplastic in early mammaliaform lineages.

In Vilevolodon the malleus is anteriorly connected to a short ossified Meckel’s cartilage (as in refs 17, 18) (which could be interpreted to be the prearticular bone; see Supplementary Information). The ectotympanic (homologous to the angular bone of non-mammalian cynodonts) has an anterior limb and a straight reflected lamina (Fig. 3 and Extended Data Figs 8 and 9). These ear structures are notably similar to those of tritylodontid cynodonts30. The anterior limb of the ectotympanic and short Meckel’s element are nestled in a triangular depression between the inflected angular process and vertical plate of the mandible. The homologous part of the postdentary trough in the mandibular angle in eleutherodonts represents only a small, reduced part of the full postdentary trough in Haramiyavia and other mammaliaforms (Fig. 3 and Extended Data Fig. 9). We confirm that eleutherodonts lack the anterior section of Meckel’s sulcus along the ventral margin of the mandible (beyond the mandibular foramen), as noted in earlier studies11,12. Meckel’s cartilage is much smaller in eleutherodonts than in eutriconodonts and spalacotherioids. We infer that the middle ear would be nestled in the inflected mandibular angle (Extended Data Fig. 9). Vilevolodon differs from multituberculates in that multituberculates completely lost the anterior limb of the ectotympanic and the Meckel’s element (Fig. 3).

Figure 3: Transformations of mandibular and middle ear structures among mammaliaforms.
figure 3

a, b, Multituberculate Sinobaatar mandible (a) and middle ear (b). The Meckel’s element and the anterior limb of ectotympanic are absent from the middle ear, and the mandible has no structure for middle ear connection (BMNH1145 and other specimens). This represents one of at least three evolutionarily independent ear–jaw disconnections in crown mammals. DMME, definitive mammalian middle ear. c, Haramiyavia6 mandible. d, e, Haramiyidan Megaconus mandible (d) and middle ear (e)9. f–h, Vilevolodon middle ear removed from right mandible to show attachment structure (f); the middle ear restored as on the mandible (g); and the middle ear reconstruction showing Meckel’s element continuing with the malleus and the ectotympanic with an anterior limb (h). Details of fossil and comparison are provided in Extended Data Figs 2, 8 and 9. i, The cynodont Kayentatherium30, showing morphological similarities to Vilevolodon. j–m, Mammaliaforms Morganucodon (j, k) Hadrocodium (l) and Agilodocodon (m), and their mandibular structure for middle ear attachment. The haramiyidan clade (c–h) represents an independent evolution in the size reduction of Meckel’s element and shortening of the anterior ectotympanic limb, but these are still attached to a vestigial Meckel’s sulcus and a reduced postdentary trough. Vilevolodon resembles Kayentatherium (i) and other mammaliaforms but differs from multituberculates with regards to the middle ear (a, b, f–h) (see also Extended Data Figs 2, 8 and 9).

PowerPoint slide

Full size image

In the phylogenetic context that haramiyidans (including eleutherodontids) are a clade of stem mammaliaforms that exclude multituberculates1,5,6,15, the shortened Meckel’s element and reduced postdentary trough are separately derived features among haramiyidans, as the Late Triassic Haramiyavia and other mammaliaforms have a full Meckel’s sulcus and postdentary trough (Fig. 3). Size decrease of the middle ear among haramiyidans is convergent with the multiple evolutions of size reduction in crown mammal clades, except that the ear elements had not achieved full separation from the mandible in a dead-end side-branch lineage of mammaliaforms.

The unique mosaic of characters related to tooth replacements and the middle ear of eleutherodonts adds to growing evidence of complex transformations of mammalian characteristics. Their complex dentitions and occlusal patterns are probably adapted for omnivory and herbivory, showing that the volant and herbivorous lifestyle, previously known only in therian gliders, was also part of mammaliaform evolutionary experimentation during the Jurassic (Fig. 2 and Extended Data Fig. 7).

Data Availability

All specimens of this study have been deposited at the Beijing Museum of Natural History. Graphics and phylogenetics data are provided in the Supplementary Information. Life Science Identifier (LSID): the new genus and species are registered with Zoobank (http://zoobank.org): urn:lsid:zoobank.org:pub:10917E53-A185-48A5-8C53-A30C5D7474A5.