The middle ear ossicles in modern mammals are repurposed from postdentary bones in non-mammalian cynodonts. Recent discoveries by palaeontological and embryonic studies have developed different models for the middle ear evolution in mammaliaforms. However, little is known about the evolutionary scenario of the middle ear in early therians. Here we report a detached middle ear preserved in a new eutherian mammal from the Early Cretaceous Jehol Biota. The well-preserved articulation of the malleus and incus suggest that the saddle-shaped incudomallear joint is a major apomorphy of Early Cretaceous eutherians. By contrast to the distinct saddle-like incudomallear articulation in therians, differences between the overlapping versus the half-overlapping incudomallear joints in monotremes and stem mammals would be relatively minor. The middle ear belongs to the microtype by definition, indicating its adaptation to high-frequency hearing. Current evidence indicates that significant evolutionary innovations of the middle ear in modern therians evolved in Early Cretaceous.

现代哺乳动物的中耳小骨是由非哺乳动物犬齿兽的齿后骨改造而来的。古生物学和胚胎研究的最新发现已经开发出哺乳动物中耳进化的不同模型。然而，人们对早期兽类中耳的进化情况知之甚少。在这里，我们报道了来自早白垩世热河生物群的一种新的真兽类哺乳动物中保存的分离的中耳。锤骨和砧骨保存完好的关节表明，马鞍形的砧踝关节是早白垩世真兽类的一个主要非变型。与兽类中独特的马鞍状砧踝关节相比，单孔类动物和干哺乳动物中重叠与半重叠的砧踝关节之间的差异相对较小。中耳从定义上属于微型型，表明它适应高频听觉。目前的证据表明，现代兽类中耳的重大进化创新是在白垩纪早期进化出来的。