Mammals articulate their jaws using a novel joint between the dentary and squamosal bones. In eutherian mammals, this joint forms in the embryo, supporting feeding and vocalisation from birth. In contrast, marsupials and monotremes exhibit extreme altriciality and are born before the bones of the novel mammalian jaw joint form. These mammals need to rely on other mechanisms to allow them to feed. Here, we show that this vital function is carried out by the earlier developing, cartilaginous incus of the middle ear, abutting the cranial base to form a cranio-mandibular articulation. The nature of this articulation varies between monotremes and marsupials, with juvenile monotremes retaining a double articulation, similar to that of the fossil mammaliaform Morganucodon, while marsupials use a versican-rich matrix to stabilise the jaw against the cranial base. These findings provide novel insight into the evolution of mammals and the changing relationship between the jaw and ear.

中文翻译：

---

中耳在哺乳动物中作为下颌支撑的短暂作用


哺乳动物利用齿骨和鳞状骨之间的新型关节来连接下颌。在真兽类哺乳动物中，这种关节在胚胎中形成，支持从出生起的进食和发声。相比之下，有袋动物和单孔类动物表现出极端的晚熟性，并且出生在新的哺乳动物颌关节形成之前。这些哺乳动物需要依靠其他机制来进食。在这里，我们表明这一重要功能是由中耳早期发育的软骨砧骨执行的，该砧骨邻接颅底形成颅下颌关节。这种关节的性质在单孔类动物和有袋类动物之间有所不同，幼年的单孔类动物保留双关节，类似于化石哺乳动物摩根古龙，而有袋类动物则使用富含多功能蛋白聚糖的基质将下颌稳定在颅底上。这些发现为哺乳动物的进化以及下巴和耳朵之间关系的变化提供了新的见解。