Distribution and variation in enamel structure in the oral teeth of sarcopterygians: Its significance for the evolution of a protoprismatic enamel

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Abstract
The property of tooth enamel to resist alteration during fossilization, is used to analyse the unique arrangements of biological crystallites amongst genera of Paleozoic sarcopterygians, with both polarized light and s.e.m. Previous concepts of crystallite organization in reptiles and mammal‐like reptiles are evaluated. Two of the Devonian sarcopterygians, are shown to exhibit a protoprismatic pattern, identical with that of a stem group therian. The patterns of crystallites, together with the arrangement of incremental lines establish that this tissue is solely an ectodermal product; monotypic enamel, in contrast to bitypic enamel with two cell products contributing to it as in enameloid or acrodin. Each genus examined has a different pattern, of significance in considering relationships amongst sarcopterygians. Recent information on ganoine and some new findings on enamel in extant lungfishes have led to the conclusion that types of monotypic enamel are present in both actinopterygians and sarcopterygians, and challenges the use of monotypic enamel as a synapomorphy of sarcopterygians in cladistic analyses.