Among extant vertebrates, mammals are distinguished by having a chain of three auditory ossicles (the malleus, incus and stapes) that transduce sound waves and promote an increased range of audible—especially high—frequencies¹. By contrast, the homologous bones in early fossil mammals and relatives also functioned in chewing through their bony attachments to the lower jaw². 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 evolution^3,4,5,6. Here we report a skull and postcranium that we refer to the haramiyidan Vilevolodon diplomylos (dating to the Middle Jurassic epoch (160 million years ago)) and that shows excellent preservation of the malleus, incus and ectotympanic (which supports the tympanic membrane). After comparing this fossil 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 morphology that evolved in several groups of mammals in the transition from the dual to the single function for the ossicles.

在现存的脊椎动物中，哺乳动物的特点是拥有一连串的三个听小骨（锤骨、砧骨和镫骨），它们可以传导声波并促进可听频率（尤其是高频）范围的扩大¹。相比之下，早期化石哺乳动物及其亲属中的同源骨骼也具有咀嚼下颌²骨附着的功能。最近对保存完好的中生代哺乳动物的发现让我们瞥见了听小骨从双重（咀嚼和听觉）功能到单一听觉功能的转变，现在人们普遍认为这种转变在哺乳动物进化过程中至少发生过三次^{3,4， 5,6}。在这里，我们报告了一个头骨和后颅骨，我们指的是 haramiyidan Vilevolodon diplomylos（可追溯到侏罗纪中期（1.6 亿年前）），这表明锤骨、砧骨和外鼓膜（支持鼓膜）保存完好。在将这块化石与其他中生代和现存哺乳动物进行比较后，我们提出，在这块化石和其他中生代化石、现存单孔目动物和现存有袋类动物和胎盘动物的早期个体发育中发现的重叠板颌关节是在听小骨从双重功能转变为单一功能。