Histologic Studies of Ostracoderms, Placoderms and Fossil Elasmobranchs

An account is given of the structure and growth of certain ptyctodontid tooth plates, especially those of Ctenurella gladbachensis , based on study by light microscopy and the SEM. These tooth plates consist of a framework of acellular bone tissue and an interior hard tissue referred to as pleromin. It is concluded that the pleromin is neither dentine nor enameloid in the strict sense, being distinguishable from the former by its hypermineralization and lack of lamellation, and from the latter by its continuous growth, its inclusion of bony trabeculae, sometimes even vascular canals, during this growth, and its process of mineralization taking place without any participation of epidermis cells or secretion products of such cells. It is also different from the semidentine or other dental hard tissue in the dermal skeleton of arthrodires such as e.g. the brachythoracids. Because of their structural peculiarities, ptyctodontid tooth plates can hardly have been derived phyletically from arthrodiran gnathalia of the kind met with in e.g. arctolepids, brachythoracids or holonematids.