Convergent evolution of diverse jaw joints in mammaliamorphs

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^1-7. Here we report two mammaliamorphs with novel shapes of secondary jaw joint, offering insight into the evolution of the mammalian jaw. The first, Polistodon⁸, 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 dentary⁹. These diverse joints reflect repeated evolutionary experimentation in advanced cynodonts, in which secondary jaw joints arose independently^7,10, and in which the load-bearing dentary-squamosal joint is a synapomorphy of mammaliaforms. Although body miniaturization might have driven this transformation¹¹, our findings indicate that other factors were involved, such as jaw-muscle reorganization, feeding ecology and masticatory behaviour^7,12-17. The ecomorphological diversity of these taxa suggest that phenotypic plasticity and environmentally induced morphological changes^18-20 could have shaped jaw-joint evolution, emphasizing how ecological pressures and developmental flexibility guided the diversification of jaw structures in mammalian ancestors.