Incremental markings of enamel in ectothermal vertebrates.

The deposition of enamel is marked by the formation of growth lines, which reflect incremental growth. Although periodic markings have been observed in enamel of non-mammalian vertebrates, the cross-striation interval and the pattern of enamel deposition have not been formally investigated. Here a structural study was made of the enamel in four non-mammalian vertebrates, with emphasis on periodic markings. Teeth from Rana catesbeiana, Tropidurus torquatus, Caiman crocodilus and a Canadian carnosaur were analysed. Enamel of T. torquatus and R. castebeiana was aprismatic; that of C. crocodilus and the carnosaur was formed by large, prism-like structures. Conspicuous incremental lines were observed in the enamel of the three living species, which presented a cross-striation repeat smaller than the prism cross-striations of mammalian enamel. Incremental lines of carnosaur enamel had a mean repeat interval similar to that of mammalian prism cross-striations. As metabolic activity in ectotherms is influenced by environmental conditions, the analysis of incremental markings of enamel is a potentially valuable source of information in the study of living and fossil reptiles and amphibians.