Structure and histogenesis of tooth plates inSagenodus inaequalisOwen considered in relation to the phylogeny of post-Devonian dipnoans

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Abstract
The histology of tooth plates of S. inaequalis was investigated to obtain information on the histogenesis of the tissue. The histological mechanisms of growth and replacement of the tooth plate are described in terms of an increase in area of the tooth plate by addition of denticles to the lateral margins of the ridges, replacement of worn dentine at the tritoral surface by basal growth of dentine and invasive growth of dentine into the surrounding bone at the anterior and medial margins. The histogenesis of a specialized character for dipnoans is described, namely hypermineralized dentine, or petrodentine, within the tooth plates. This has placed an advanced character further back in the phylogenetic organization of dipnoans than was previously recognized. Proposed phylogenies and previous observations on tooth plates of other dipnoans were discussed. S. inaequalis probably shared a common ancestor with both the ceratodontids and the lepidosirenids. A sequence is proposed for the phyletic relationship of tooth plate-bearing dipnoans. From a consideration of the features of histogenesis of this specialized dentine, the alternative terminologies were reviewed and petrodentine chosen for the hypermineralized dentine and syndentine as the collective term for the entire mass of dentine in the tooth plate.