Convergent evolution of diverse jaw-joints in mammaliamorph

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 shape of secondary jaw joints, offering new insight into the evolution toward the mammalian jaw. The first, Polistodon, a Middle Jurassic herbivorous tritylodontid with a relatively large body size and a likely fossorial lifestyle, uniquely evolved a dentary-jugal articulation. The second, a new Early Jurassic morganucodontan, exhibits a dentary-squamosal joint lacking 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, with secondary jaw joints arising independently of which the load-bearing dentary-squamosal joint is a synapomorphy of mammaliaforms. While body miniaturization may have driven this transformation, our findings support additional factors, such as jaw muscle reorganization, feeding ecology, and masticatory behavior. The ecomorphological diversity of these taxa suggest that phenotypic plasticity and environmentally induced morphological changes could have shaped the jaw-joint evolution, emphasizing how ecological pressures and developmental flexibility guided diversification of jaw structures in mammalian ancestors.