Origin of the avian predentary and evidence of a unique form of cranial kinesis in Cretaceous ornithuromorphs

The avian predentary is a small, rarely preserved skeletal structure found only in toothed Mesozoic ornithuromorph birds. The origin and function of this enigmatic bone is poorly understood. Based on skeletal tissue identification in a specimen of the Early Cretaceous Yanornis martini, we propose developmental, structural, and functional hypotheses for this element. It consists of trabecular bone with a caudal articular surface for the dentary covered by cartilage. Tissue mechanobiology suggests this contact was kinetic, representing a previously unknown type of avian cranial kinesis. We hypothesize that the avian predentary was a biomechanically induced sesamoid that coevolved with an edentulous portion of the premaxilla, together representing a successful morphotype that combined a small beak with a proprioceptive system.