一件来自中国白垩纪地层已经绝灭的小哺乳动物化石标本的发现，填补了哺乳动物中耳形成之谜中的一个重要环节。爬行类的下颌骨由若干不同的骨片构成，而在哺乳动物中下颌仅由一块骨片构成（即附着牙齿的齿骨）——大部分其余的骨片已经演化为中耳的骨片（外鼓骨、锤骨）。然而，却一直没有直接的化石证据记录这一转变。
最近（4月14日）出版的《自然》杂志中，中国科学院古脊椎动物与古人类研究所客座研究员孟津等描述了一件化石标本，代表了三尖齿兽类的一个新种——胡氏辽尖齿兽(Liaoconodon hui)。其下颌成分已经开始转变为现生哺乳动物中耳的听小骨。该种的外鼓骨和锤骨已经不再和齿骨接触，但仍然通过一细长的骨化麦氏软骨与下颌相连。研究者们认为在哺乳动物的演化过程中，麦氏软骨应该起着保持稳定的作用，连接齿骨和已经分离的听小骨。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09921
Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont
Jin Meng,1, 2 Yuanqing Wang2 & Chuankui Li2
The transference of post-dentary jaw elements to the cranium of mammals as auditory ossicles is one of the central topics in evolutionary biology of vertebrates. Homologies of these bones among jawed vertebrates have long been demonstrated by developmental studies; but fossils illuminating this critical transference are sparse and often ambiguous. Here we report the first unambiguous ectotympanic (angular), malleus (articular and prearticular) and incus (quadrate) of an Early Cretaceous eutriconodont mammal from the Jehol Biota, Liaoning, China. The ectotympanic and malleus have lost their direct contact with the dentary bone but still connect the ossified Meckel’s cartilage (OMC); we hypothesize that the OMC serves as a stabilizing mechanism bridging the dentary and the detached ossicles during mammalian evolution. This transitional mammalian middle ear narrows the morphological gap between the mandibular middle ear in basal mammaliaforms and the definitive mammalian middle ear (DMME) of extant mammals; it reveals complex changes contributing to the detachment of ear ossicles during mammalian evolution.
胡氏辽尖齿兽(Liaoconodon hui)正型标本（IVPP V16051）（王元青供图）。
