By offering new histological data on the placoderm species, Bothriolepis canadensis, this dissertation addresses skeletal development in placoderms and the question of the primitive condition of the jawed vertebrate skeleton. Placodermi is the most basal clade of jawed vertebrates and sister group to the living group of jawed vertebrates, Gnathostomata. Comparative analysis of placoderm and gnathostome histological data provides a singular opportunity to address the condition of the common ancestor of these clades and therefore the common ancestor of all jawed vertebrates. This dissertation reaches important conclusions. First, the external skeleton of Bothriolepis canadensis is comprised exclusively of cellular dermal bone tissue. Though the tissue is organized in multiple ways, the data support the identification of each according to the bone tissue classification scheme of de Ricqles et al. (1991). Second, microstructural and topological data are found to be inconsistent with the hypothesis that the middle cancellous tissue and basal lamellar tissue represent contributions of the endoskeleton. The stratification of the antiarch thoracic skeleton that has long puzzled researchers is explained by the nature of the articulations between adjacent elements. Independent development of these two tissue units allowed for the different pacing of marginal growth in each, a necessary condition for overlapping articulations. In addition, the growth series of specimens shows that mineral spherites become more abundant with growth and may represent a mechanism employed to keep apace with a high growth rate and extensive remodeling of the skeleton in response to growth. Finally, skeletal features long thought to be gnathostome innovations are instead discovered to arise along the gnathostome stem and act as synapomorphies for jawed vertebrates, a more inclusive clade than Gnathostomata. These innovations include 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. As the most basal clade of jawed vertebrates, placoderms enable inferences regarding the primitive condition of the jawed vertebrate skeleton. The combination of skeletal innovations that can be tied to the origin of vertebrates is hypothesized to coincide with a shift in growth rate.
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