Mao Fangyuan, Jiang Shan, Liu Jun, Ren Jicheng, Ye Yong, Liu Yu, Shen Xin, Wang Tao, Wang Guofu, Wang Ping, Chen Juan, Meng Jin
Key Laboratory of Vertebrate Evolution and Human Origins, 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. 2025 Sep 24. doi: 10.1038/s41586-025-09572-0.
The evolution of a single-dentary-boned lower jaw and its secondary craniomandibular articulation between the dentary condyle and the squamosal glenoid has been regarded as a pivotal vertebrate innovation and defining mammalian trait. Here we report two mammaliamorphs with novel shapes of secondary jaw joint, offering insight into the evolution of the mammalian jaw. The first, Polistodon, a Middle Jurassic herbivorous tritylodontid with a relatively large body size and a lifestyle that is likely to have been fossorial, uniquely evolved a dentary-jugal articulation. The second, an Early Jurassic morganucodontan, exhibits a dentary-squamosal joint that lacks a bulbous condyle, supporting the hypothesis that the mammalian dentary condyle was formed by expansion of the lateral ridge of the dentary. These diverse joints reflect repeated evolutionary experimentation in advanced cynodonts, in which secondary jaw joints arose independently, and in which the load-bearing dentary-squamosal joint is a synapomorphy of mammaliaforms. Although body miniaturization might have driven this transformation, our findings indicate that other factors were involved, such as jaw-muscle reorganization, feeding ecology and masticatory behaviour. The ecomorphological diversity of these taxa suggest that phenotypic plasticity and environmentally induced morphological changes could have shaped jaw-joint evolution, emphasizing how ecological pressures and developmental flexibility guided the diversification of jaw structures in mammalian ancestors.
单齿骨下颌的演化及其在齿骨髁与鳞状骨关节盂之间的次生颅下颌关节，被视为脊椎动物的一项关键创新和哺乳动物的决定性特征。在此，我们报告了两种具有新型次生颌关节形状的似哺乳类动物，这为了解哺乳动物颌骨的演化提供了线索。第一种是Polistodon，一种中侏罗世的食草三列齿兽类，体型相对较大，生活方式可能为穴居，它独特地演化出了齿骨-颧骨关节。第二种是早侏罗世的摩根齿兽类，其齿骨-鳞状骨关节缺乏球状髁，支持了哺乳动物齿骨髁是由齿骨侧嵴扩展形成的假说。这些不同的关节反映了进步犬齿兽类中反复出现的进化试验，在这类动物中，次生颌关节独立出现，且承重的齿骨-鳞状骨关节是哺乳形类的共有衍征。尽管身体小型化可能推动了这种转变，但我们的研究结果表明还有其他因素参与其中，如颌部肌肉重组、摄食生态和咀嚼行为。这些类群的生态形态多样性表明，表型可塑性和环境诱导的形态变化可能塑造了颌关节的演化，强调了生态压力和发育灵活性如何引导哺乳动物祖先颌骨结构的多样化。
