Skeletal histology of Bothriolepis canadensis (Placodermi, Antiarchi) and evolution of the skeleton at the origin of jawed vertebrates.

We used light microscopy and scanning electron microscopy to compile a complete histological description of the dermal skeleton of the antiarch placoderm, Bothriolepis canadensis. Placodermi is most often cited as the sister group of crown group Gnathostomata, but some recent authors propose that placoderms instead represent a paraphyly of forms leading to the crown. In either phylogenetic scenario, comparative analysis of placoderm and gnathostome histological data allows us to address the primitive condition of both the gnathostome skeleton and the jawed vertebrate skeleton. The results of this work support the interpretation that the external skeleton of Bothriolepis canadensis is comprised exclusively of cellular dermal bone tissue. The unique stratification of the antiarch thoracic skeleton that has led to controversial interpretations in the past is explained by the nature of the articulations between adjacent elements. Skeletal features long thought to be gnathostome innovations are instead discovered to arise along the gnathostome stem. These innovations include secondary osteons, the systematic reconstruction of the skeleton in response to growth, and unfused, overlapping joints that enable marginal growth while maximizing the area of the articulation surface. The extensive evidence for spheritic mineralization agrees with a model of the skeleton as one capable of a high growth rate and active remodeling. Dermal skeletal development in both placoderms and osteichthyans is primarily skeletogenetic with only a minor odontogenetic contribution in some taxa. This demonstrates the problem inherent with assuming a broad application for those hypotheses of dermal skeletal evolution that are based on a chondrichthyan model. Our results highlight the importance of anatomical and ontogenetic context in the interpretation of fossil tissues.