Stem mammaliaforms are forerunners to modern mammals1, and they achieved considerable ecomorphological diversity in their own right2. Recent discoveries suggest that eleutherodontids, a subclade of Haramiyida, were more species-rich during the Jurassic period in Asia than previously recognized3,4,5,6,7,8,9,10,11,12. Here we report a new Jurassic eleutherodontid mammaliaform with an unusual mosaic of highly specialized characteristics1,2,3,4,5,6, and the results of phylogenetic analyses that support the hypothesis that haramiyidans are stem mammaliaforms. The new fossil shows fossilized skin membranes that are interpreted to be for gliding and a mandibular middle ear with a unique character combination previously unknown in mammaliaforms. Incisor replacement is prolonged until well after molars are fully erupted, a timing pattern unique to most other mammaliaforms. In situ molar occlusion and a functional analysis reveal a new mode of dental occlusion: dual mortar–pestle occlusion of opposing upper and lower molars, probably for dual crushing and grinding. This suggests that eleutherodontids are herbivorous, and probably specialized for granivory or feeding on soft plant tissues. The inferred dietary adaptation of eleutherodontid gliders represents a remarkable evolutionary convergence with herbivorous gliders in Theria. These Jurassic fossils represent volant, herbivorous stem mammaliaforms associated with pre-angiosperm plants that appear long before the later, iterative associations between angiosperm plants and volant herbivores in various therian clades.
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We thank A. Shinya for fossil preparation; S. Bi, S. Gatesy, L. Heaney, H.-J. Li, Z.-J. Gao, T. Martin, B. Patterson, N. Shubin, X.-T. Zheng and C.-F. Zhou for access to comparative specimens. Funding supported Q.-J.M. (Beijing Scientific Commission) and Z.-X.L. (UChicago-BSD). Full acknowledgments are provided in the Supplementary Information.
Extended data figures
Part 1: Comparison of distinctive cycle 1 versus cycle 2 of premolars and molars: cycle 1 has full contacts of molars and incisors, but no contact between P3/p4; cycle 2 has full contact of P3/p4 but minimal or no contacts of molars and incisors. Part 2: Molar cusp and basin identification, and cycle 1 of occlusal movement of M1/m1 and M2/m2 during (see Fig. S6). During occlusal cycle 2, the P3/p4 are the only teeth that can fully occlude; the dual mortar-pestle contacts of upper and lower molars do not occur. Thus, cycle 2 is not shown here in animation analysis Part 2 of the video. For better clarity, the un-erupted deciduous incisors are not shown in the video.