| Summary: | The prioniodontids were the first to develop a functionally differentiated apparatus. The apparatuses of selected prioniodontid taxa are reconstructed by direct comparisons between elements from disjunct collections and those preserved in natural assemblages of Promissum pulchrum. These comparisons and evidence of internal morphology have enabled the recognition of homologous elements. Examination of prioniodontid hard tissues has provided a clearer understanding of conodont hard tissues. Polished sections have provided evidence of intergradation between white matter and hyaline tissues confirming their synchronous deposition. Also surface ornamentation is shown to be directly related to internal structure. Apparatus reconstructions and studies of internal and external wear patterns also reveal new evidence for element function. Elements occluded in a way broadly comparable to those of the ozarkodinid elements (Donoghue and Purnell, 1999a), though interlocking occlusion only occurs in association with a well formed blade. Where a blade is not developed, elements simply worked against each other, wearing down the oral surfaces. Phragmodus inflexus represents the second prioniodontid apparatus preserved as a natural assemblage and its architecture, presented here, differs markedly from that of Promissum. This has important implications for prioniodontid apparatus architecture, as the elements of Phragmodus are comparable to those typical of many prioniodontids not included with the balognathids. This may suggest that apparatuses of most prioniodontids had an ozarkodinid-like apparatus and that Promissum possesses a more derived plan. This new evidence is important for understanding phylogenetic relationships between prioniodontids. Realistic cladistic studies should be based on clearly understood homologous characters and it is hoped that in the future, cladistic analyses will draw from detailed data, such as those presented here.
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