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Studies of scutellid echinoid teeth in living specimens reveal calcite-impregnated dental tissues which closely resemble the hydroxyapatite-impregnated dentin, cementum, and enamel characteristic of primitive vertebrate teeth and scales.
In the echinoids, flexible, scalelike plates are added continuously to the aboral base of each tooth, and fuse to form an imbricate series on emergence from the dental sac. A tubular, dentinlike packing develops between the plates, forming most of the keel. At mid-tooth, "cementum" encloses the "dentin" and plates, producing a thin surficial zone with distinctive lacunae and canaliculi. "Cementum" grades into a protective "enamel" coating (over a consolidated "dentin" core) where the tooth passes through the epidermis.
Among extant invertebrates, only echinoids possess such cellular dental tissue. Although teeth have been reported in Ordovician echinoids, and all modern lantern elements appeared by Late Silurian time, the microstructure of fossil echinoid teeth is unknown. In contrast, the microstructure of Ordovician vertebrate scales and teeth has been studied in detail. The highly evolved state of vertebrate dental tissue in the Ordovician has remained unexplained because of a lack of feasible ancestral forms. Despite chemical differences, the tissue-type structural similarities between modern scutellids and lower vertebrates are remarkable. Microstructures of early Paleozoic echinoid dental tissues may yield some clues to Precambrian ancestral relations between the echinoid and vertebrate evolut onary lines.
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