The Structure of the Teeth of some Mammal-like Reptiles

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Abstract
The principal changes in calcified tissues which have fossilized are the loss of organic material and the conversion of hydroxyapatite into fluorapatite. Since these two minerals are very similar and because, initially, mammalian enamel has a low organic content, its optical properties are hardly affected by fossilization. On the other hand, the loss of optically active collagen results in a modification of the characters of dentine. The teeth of synapsid reptiles possess dentine similar to that of recent reptiles. Occasionally the dentine contained globuli, but unlike mammalian dentine, there appeared to be no ‘spheritic’ orientation of crystallites within the globuli. Certain dicynodont tusks consisted of dentine only and it is possible that enamel was missing even in the original condition. Other synapsid teeth possessed a thin, well-defined enamel layer made up of incremental lamellae but lacking true prisms. Nevertheless, cylindrical groups of crystallites exist throughout this enamel within which the orientation of the crystallite axes varies regularly. Between crossed nicols of a polarizing microscope this crystallite arrangement gives the enamel a prismatic appearance. However, the enamel is quite homogeneous, for these pseudo-prisms are not physically separated from each other. Furthermore, this regular prismatic appearance and an irregular Saulengliederung, such as occurs in crocodile enamel, may exist in the same tooth. There is, therefore, no evidence that the prismatic enamel characteristic of mammals existed in the pre-mammalian reptiles.