High resolution XCT scanning reveals complex morphology of gnathal elements in an Early Devonian arthrodire

Arthodire placoderms, as a possible sister group of Chinese ‘maxillate’ placoderms plus crown gnathostomes, provide important information regarding early evolution of jaws and teeth. High-resolution computed tomography and digital dissection on a unique articulated 400 million-year-old buchanosteid arthrodire permitsa detailed description of the three types of gnathal elements in basal arthrodires for the first time, giving insights into their morphology and the organization of the associated dentition. In displaying numerous denticle rows (dental fields), the gnathal element morphology is very different from the much-reduced denticulation of higher brachythoracid arthrodires, even though the latter have been used recently to interpret origin and early evolution of teeth. Ossification centres are anterolateral on the anterior supragnathal (attached to the braincase), anteromesial on the posterior supragnathal (attached to the palatoquadrate), and in the central part of the biting portion of the infragnathal (attached to the meckelian cartilage). The latter bone shows no evidence of two ossification centres as has been interpreted for more advanced arthrodires. Denticle rows radiating from the ossification centre form dental fields in all three elements, and are more similar to the gnathal elements of phlyctaeniid and actinolepid arthrodires than to advanced brachythoracids. The new evidence gives insights into the primitive arthrodire condition for comparison with the dermal jaw bones of Chinese ‘maxillate’ placoderms that have been homologised with the premaxilla, maxilla, and dentary of osteichthyans. The new details will help clarify the sequence of character acquisition in the evolution of marginal jaw bones in basal gnathostome groups.