Functional anatomy, jaw mechanisms, and feeding behavior of Dunkleosteus terrelli (Placodermi, Arthrodira).

The comparative and functional anatomy of the cleaver-like jaws of the Late Devonian arthrodire Dunkleosteus terrelli remains poorly understood, despite significant advances in our understanding of arthrodire anatomy. Here, we re-investigate the jaw morphology of D. terrelli from a comparative anatomical perspective. Rare cartilaginous elements, preserved attachment sites, and comparisons with other arthrodires suggest the skull of Dunkleosteus contained significantly more cartilage than previously assumed, with many major jaw muscle attachments and cranial articulations located on cartilages. The m. adductor mandibulae likely did not originate on the skull roof, contra previous reconstructions, but instead either partially or entirely on the palatoquadrate, which in D. terrelli has an unusual, asymmetrical shape. A large external groove spanning the suborbital plate suggests the presence of a large m. preorbitalis originating on the enlarged ethmoid ossification, likely playing a role in jaw closure. The submarginal plate is unusually elongate compared to other arthrodires and lacks bony connections to the skull roof or the rest of the cheek, suggesting a specialized, loosely consolidated posterior cheek. The symphyseal cartilages of the lower jaw have a well-developed medial facet, which would have created a stable joint with limited symphyseal flexibility. Many features of D. terrelli appear to reflect broader evolutionary trends characterizing dunkleosteoids, and some appear to be convergent specializations for macropredation shared with some aspinothoracidans. Prior claims of suction feeding in D. terrelli are questionable on comparative anatomical and ecological grounds, and previous estimates of bite force need to be re-examined based on the revised myology.