Complex Morphology of Gnathal Elements in Devonian Placoderms from the Early Devonian of Australia Sheds Light on the Evolution and Development of Teeth

The evolution and development of teeth is one of the most important events for gnathostomes. However, how and when teeth evolved remain debatable. The extinct Devonian arthrodire placoderms, as a possible sister group of Chinese ‘maxillate’ placoderms plus crown gnathostomes, provide important information regarding early evolution of jaws and teeth. Synchrotron X-ray tomographic microscopy on several gnathal elements of arthrodire placoderms from Australia permits a detailed description of different types of gnathal elements in basal arthrodires. In displaying numerous denticle rows, gnathal element morphology is very different from the much reduced denticulation in higher brachythoracid arthrodires, even though the latter have been used recently to interpret the origin and early evolution of teeth. The gnathal element ossification centers of basal arthrodires have radiating denticle rows forming dental fields. They resemble those in phlyctaeniid and actinolepid arthrodires rather than advanced brachythoracids, and indicate the primitive arthrodire condition for comparison with the dermal jaw bones of Chinese ‘maxillate’ placoderms. These have been homologized with the premaxilla, maxilla and dentary of osteichthyans. The CT-data from this study demonstrates much more new morphological information than previously obtained from vertebrate fossils, and clarifies the sequence of character acquisition in the evolution of marginal jaw bones in basal gnathostome groups.