Abstract

Among extant vertebrates, mammals are distinguished by having a chain of three auditory ossicles (malleus, incus and stapes) that transduce sound waves and promote an increased range of audible, especially high frequencies1. In contrast, the homologous bones in early fossil mammals and relatives also functioned in chewing through their bony attachments to the lower jaw2 (Fig. 1a,b). Recent discoveries of well-preserved Mesozoic mammals have provided glimpses into the transition from the dual (masticatory and auditory) to the single auditory function for the ossicles, which is now widely accepted to have occurred at least three times in mammal evolution3–6. Here we report a new haramiyidan from the Middle Jurassic (160 my) with remarkable preservation of the malleus, incus and ectotympanic (which supports the tympanic membrane) (Fig. 2) and compare it with other Mesozoic and extant mammals. We propose that the overlapping incudomallear articulation found in this and other Mesozoic fossils, in extant monotremes, and in early ontogeny in extant marsupials and placentals is a novel morphology that evolved in multiple groups of mammals in the transition from the dual to single function for the ossicles.