Development of the upper dentition in Alligator mississippiensis was investigated using a close series of accurately staged and aged embryos, hatchlings, and young juveniles up to 11 days posthatching, as well as some young and old adult specimens. Studies from scanning electron microscopy, light microscopy, acetate and computer reconstructions, radiography and macroscopy were combined to elucidate the details of embryonic dental development, tooth initiation pattern, dentitional growth, and erupted functional dentition. The results were compared with those from the lower jaw and related to the development of other craniofacial structures. Approximately 17 early teeth in each jaw half develop as surface teeth, of which 13 project for 1 to 12 days before sinking into the mesenchyme. The first three teeth initiate directly from the oral epithelium at Ferguson stages 14-15 (days 15-19 after egg laying), before there is any local trace of dental lamina formation. All other teeth develop from a dental prolamina or lamina; and with progressive lamina development, submerged teeth initiate from the aboral end leading to the formation of replacement teeth. All teeth form dentin matrix, but 12 early teeth do not form enamel. Approximately 20 embryonic teeth are resorbed, 6 are transitional, and 42 function for longer periods after hatching. The embryonic tooth initiation pattern (illustrated by defining a tooth position formula) does not support the previous models of Odontostichi, Zahnreihen, and Tooth Families, each of which postulates perfect regularity. Up to three interstitial tooth positions develop between sites of primary tooth initiation, and families with up to five generations at hatching are at first arbitrarily defined.

Development of the dentition in a close series of accurately staged and aged embryos of Alligator mississippiensis was studied by macroscopy, light microscopy, scanning electron microscopy (SEM) and detailed reconstructions. Dental development in the lower jaw up to stage 18; day 26 is described in this paper. In the lower jaw, the first formed tooth germ appears at stage 14 in tooth family 3; the second at stage 15 in family 6; the third, fourth, fifth, sixth and seventh at stage 17 in families 12,2,4,9 and 16, respectively. Neither ‘Zahnreihen’ nor ‘perfect alternation’ theories of tooth initiation explain the data. Rather the present data suggest that initiation of teeth is related to jaw growth, the distance between existing teeth and the size and developmental maturity of the latter. Each tooth is postulated to have a ‘zone of inhibition’ around it. The first formed embryonic teeth are discernible by macroscopy and SEM as surface elevations (projecting tooth germs). The dental tissues of these are poorly differentiated and lack enamel. Projecting tooth germs sink into the jaw mesenchyme, their dental epithelia degenerate and they persist as non‐functional dentine rudiments for varying periods before becoming completely resorbed or shed. Formation of the dental lamina occurs by an overgrowth of the operculum (on which a distinct line of cobblestoned cells is visible by SEM) and closure of the dental furrow, as well as by the formation of a dental prolamina. The scaled fate maps and descriptive appearances of the developing alligator dentition highlight its value for future experimental studies of pattern formation and positional information.

With 5 plates and 5 figures in the text) Development of the lower dentition from stage 19 (day 26) embryos to 11 days after hatching (day 76) was studied in a close series of accurately aged (and staged) specimens of Alligator mississippiensis using macroscopy, light microscopy, scanning electron microsc~py, radiography and detailed reconstructions. This study complements our earlier investigations of lower dentition development in alligator embryos from initiation to stage 18, day 26 (Westergaard & Ferguson, 1986) and so provides the first careful documentation of dentition development in any non-mammalian tetrapod from the first initiated tooth to the first erupted and functional dentition. In this way, it is possible to test previous models of dental development, e.g. Woerdeman (1919) suggested that four Odontostichi would be resorbed from initiation to eruption, and Edmund's (1962) Zahnreihe theory predicts that 84 embryonic teeth would be resorbed in this time period. Both are wrong: approximately 19 early teeth (resorptive group) are resorbed or shed without becoming functional; seven teeth (transitional group) function for a short period (less than two weeks) or are sometimes resorbed or shed without becoming functional and 36 teeth (functional group), initiated during embryonic life, function for longer periods. All teeth produce dentine, but a differentiated enamel organ is absent in teeth 1-8, 9b, 10, 12 and 14: these teeth never produce enamel. Teeth 9a and 11 have a poorly Merentiated enamel organ and form only a thin layer of enamel; the remaining teeth develop a typical mammalian enamel organ and produce enamel. As several resorptive teeth produce enamel, there is no simple relationship between dental competence and function.Tooth families are rather arbitrarily defined before separation by connective tissue, thus supporting the Tooth Position theory (Westergaard, 1980(Westergaard, , 1983. Merential jaw growth enables the establishment of up to five intervening tooth families between families of the first tooth row. Growth is most pronounced in the middle of each jaw half. Except for teeth 1 and 3, the teeth from odd-and even-numbered families can be fitted into fairly smooth initiation curves. The most active initiation phase occurs between embryonic days 21 and 33. By hatching, families founded by teeth 1 to 5 (3, 6, 12, 2 and 4) have developed four generations of teeth, families 17, 19 and 20, two generations and all other families, three generations of teeth. The interval of time between initiation of successive teeth from the same tooth family ranges from 6-26 days. One clutch of embryos showed a disappearance of tooth family 6 from one jaw side and another clutch exhibited an extra tooth family 0: both these clutch-related anomalies were associated with differences in jaw growth. The embryonic dental system seems to be evolutionarily very plastic, changes of jaw growth enabling the creation or removal of tooth positions anywhere in the odontogenic area. A new progress zone model of dentition ...

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