Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China.
Division of Paleontology, American Museum of Natural History, New York, NY, USA.
Nature. 2024 Apr;628(8008):576-581. doi: 10.1038/s41586-024-07235-0. Epub 2024 Apr 3.
The dual jaw joint of Morganucodon consists of the dentary-squamosal joint laterally and the articular-quadrate one medially. The articular-quadrate joint and its associated post-dentary bones constitute the precursor of the mammalian middle ear. Fossils documenting the transition from such a precursor to the mammalian middle ear are poor, resulting in inconsistent interpretations of this hallmark apparatus in the earliest stage of mammaliaform evolution. Here we report mandibular middle ears from two Jurassic mammaliaforms: a new morganucodontan-like species and a pseudotribosphenic shuotheriid species. The morganucodontan-like species shows many previously unknown post-dentary bone morphologies and exhibits features that suggest a loss of load-bearing function in its articular-quadrate joint. The middle ear of the shuotheriid approaches the mammalian condition in that it has features that are suitable for an exclusively auditory function, although the post-dentary bones are still attached to the dentary. With size reduction of the jaw-joint bones, the quadrate shifts medially at different degrees in relation to the articular in the two mammaliaforms. These changes provide evidence of a gradual loss of load-bearing function in the articular-quadrate jaw joint-a prerequisite for the detachment of the post-dentary bones from the dentary and the eventual breakdown of the Meckel's cartilage during the evolution of mammaliaforms.
摩根齿兽的双颌关节由侧向的齿骨-鳞状骨关节和内侧的关节-方骨关节组成。关节-方骨关节及其相关的后齿骨构成了哺乳动物中耳的前身。记录从这种前体向哺乳动物中耳过渡的化石很少，导致对哺乳动物形类演化早期这个标志性器官的解释不一致。本文报道了来自两个侏罗纪哺乳动物形类的下颌中耳：一个新的摩根齿兽样物种和一个假三尖齿兽类 shuotheriid 物种。摩根齿兽样物种表现出许多以前未知的后齿骨形态，并具有表明其关节-方骨关节丧失承载功能的特征。shuotheriid 的中耳在接近哺乳动物的状态，因为它具有适合纯粹听觉功能的特征，尽管后齿骨仍然附着在齿骨上。随着颌关节骨的尺寸减小，在这两个哺乳动物形类中，方骨相对于关节在不同程度上向内侧移位。这些变化提供了证据表明关节-方骨颌关节的承载功能逐渐丧失——这是后齿骨与齿骨分离以及在哺乳动物形类演化过程中 Meckel 软骨最终解体的前提。
