Steinar Risnes Department of Oral Biology, Faculty of Dentistry, University of Oslo, P.O. Box 1052 Blindern, N-0316 Oslo, N orway E-mail: steinarr@odont.uio.no Received 18 March 1997 revision received 6 January 1998 and accepted 7 March 1998 Keywords : dental enamel, crystal orientation, prisms, prism cross-striations, Retzius lines. Growth tracks in dental enamel The present paper evaluates the enamel growth tracks as tools in the chronological mapping of dental development, with special reference to hominids. Dental enamel consists of tightly packed hydroxyapatite crystals organized by differential orientation into a pattern of prisms and interprisms. The crystal organization is probably under the influence of both cellular and physico-chemical factors. The structure of mature enamel testifies to events that took place during enamel formation. The prisms are the fossilized tracks traced out by amelo- blasts. The tangential diameter of ameloblasts and the central distance of prisms increase from the enamel-dentine junction to the enamel surface. Available evidence suggests that prism cross-striations are light microscopic expressions of prism varicosities and/or compo- sitional variations, that these are due to a rhythm in enamel formation, and that this rhythm is diurnal. In human enamel the mean daily rate of enamel production is about 3.5 m, but increases from inner to outer enamel and decreases from incisal/cuspal to cervical enamel. Conclusive evidence has shown that Retzius lines are incremental lines. Evenly spaced Retzius lines probably represent a 6–11 day rhythm in enamel formation, while other Retzius lines may be due to various types of stress. The geometry of the enamel growth tracks and their chronological significance are valuable tools in chronological mapping of dental development and for understanding temporal and spatial patterns in tooth morphogenesis. The taxonomic significance of prism packing patterns, prism decussation and enamel thickness should be clarified through further systematic descriptive research.  1998 Academic Press Journal of Human Evolution (1998) 35, 331–350 Article No. hu980229 Introduction Morphological and developmental aspects of teeth and dentitions represent an import- ant line of research in the study of primate and human evolution ( Beynon & Dean, 1988 ; Dean, 1989 ; Mann et al. , 1990 ; Winkler & Swindler, 1991 ). Teeth consti- tute an important part of the fossil record. The length of the childhood dependency period, which is a key issue in hominid evolution, appears to be prolonged in mod- ern humans ( Mann et al. , 1990 ). The length of the childhood dependency period may be inferred from estimates of the length of the period of dental development. Enamel is a nonvital and durable dental tissue within whose microstructure resides fossilized tracks of its growth process ( Boyde, 1964 ; Osborn, 1973 ; Risnes, 1986 a). Some of these growth tracks reflect the incremental growth of enamel, i.e., Retzius lines, periky- mata, and prism cross-striations. The incre- mental growth tracks represent an internal record of time which, linked to a time scale, may serve as a valuable tool in determining the rate of enamel formation and the crown formation time ( Boyde, 1964 ; Newman & Poole, 1974 ; Bromage & Dean, 1985 ; Risnes, 1986 b). The use of enamel growth tracks as a record of time depends on a thorough understanding of their structure and mor- phogenesis. The structure of mature enamel and the process of enamel development serve as reciprocal frames of reference; data 0047–2484/98/100331+20$30.00/0  1998 Academic Press