Comparative morphology of tooth plates in extant holocephalans

Tooth plates are not only a synapomorphy of holocephalans, but they are also one of the few holocephalan tissues that fossilizes well. Consequently, they have traditionally been viewed as essential for phylogenetic studies of holocephalans, but their level of significance within Holocephali has not been established. Detailed comparative studies of tooth plates in extant forms are a prerequisite. Once a basis for comparison is established among the living taxa polarization of characters of the tooth plates is possible for both fossil and living taxa. To do this I examined tooth plates of all six genera of living chimaeriforms (Callorhinchus, Rhinochimaera, Harriotta, Neoharriotta, Chimaera , and Hydrolagus ) using dissection, LM and SEM. Gross morphology of tooth plates varies within a species and with age; therefore, as generic or specific characters they may be of limited value. However, tooth plates do provide valuable characters for higher level phylogenetic analyses. For example, the presence of compact hypermineralized dentine (= pleromin) is a derived character uniting the families Rhinochimaeridae and Chimaeridae with at least two genera of Jurassic holocephalans.