Abstract

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 recognized^{3,4,5,6,7,8,9,10,11,12}. Here we report a new Jurassic eleutherodontid mammaliaform with an unusual mosaic of highly specialized characteristics^1,2,3,4,5,6,  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.