Comparative dental development and microstructure of ... - UCL

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192 A. D. BEYNON ET AL. Figure 14. The upper part of the Figure relates to the juvenile P. heseloni specimen. The line ‘‘Birth’’ runs through the neonatal lines in the M 1 and dm 2 . The lower line ‘‘Death’’ runs through the last dentine formed beneath the tallest cusp in I 1 ,I 2 and M 2 (d). Accentuated lines in the I 1 and I 2 germs and M 1 germ allowed these teeth to be tied together developmentally. The times between birth and death in each of these teeth allow a timescale to be placed on this sequence. The time of M 2 crown formation exceeds that of the other germs and therefore cannot belong to the same specimen. The lower part of the Figure relates to the adult P. heseloni individual. Crown completion in M 1 is indicated by a dashed line. An accentuated line just above crown completion and close to the cervix of the adult M 1 corresponds with a line in the cusp of the M 2 and is shown just above the dashed line in M 2 . The same line can be seen in P 4 just above the dotted line that indicated crown completion in M 1 . Another accentuated line in the cervix of the M 2 (shown just below the dashed line in M 2 ) matches a line in the P 4 cervix. The dashed line which represents adult M 1 crown completion time was located in both the M 2 and P 4 by counting the number of striae beyond the first accentuated line in each tooth. M 3 cannot be sequenced with these teeth.
DENTAL DEVELOPMENT IN PROCONSUL 193 Table 6 Summary of crown completion times Using perikymata Using enamel Using dentine Maximum value of means Tooth type P. nyanzae P. heseloni P. nyanzae P. heseloni P. nyanzae P. heseloni P. nyanzae P. heseloni dm1 0·25 0·25 dm2 0·46 0·46 I1 2·5 2·4 2·5 2·4 I2 2·5 2·4 2·5 2·4 C 3·0–4·7 2·5 2·4 3·0–4·7 2·5 P3 1·8 1·8 P4 1·5 1·7 1·7 1·7 M1 1·8 1·0 1·9 1·2 2·0 1·2 2·0 1·2 M2 2·0 1·3 2·2 1·4 2·3 1·6 2·3 1·4 M3 1·7 1·7 1·7 Summary of data for crown formation times derived in three different ways in this study presented by tooth type. (i) Perikymata counts combined with cuspal enamel formation times derived from the histological sections of each tooth have been averaged for upper and lower teeth of the same tooth type. (ii) Estimates made from histological sections only when tooth crowns were complete. (iii) Estimates made from counts and measurements of incremental markings in dentine. In the last column the greatest values for crown formation times in both species of Proconsul are summarized. The estimates that appear here for P. heseloni were used in the bar chart of dental development (Figure 19). development in Proconsul. The first is the presence of both a strong and a fainter accentuated line that recur in the enamel of the P 4 and second M 2 of the adult P. heseloni specimen and the same strong line in the cervical enamel of the M 1 . These lines tie these teeth together developmentally. The second line of evidence comes from the accentuated lines in the incisor tooth germs and in the M 1 germ belonging to the infant P. heseloni individual. The third line of evidence comes from the distribution of linear hypoplastic bands on the buccal enamel surface of KNM-RU 7290. One strong band with two fainter bands either side of it, can be cross-matched across all the anterior teeth, across the premolars and across the M 2 s. The positions of these accentuated lines within the enamel and of the hypoplastic bands combined with the perikymata counts on the surface of the teeth of KNM-RU 7290 allow us to reconstruct a preliminary sequence of tooth development for P. heseloni from this specimen. The timing of the initial mineralization of the third molar is conjectural but we have simply presumed here that it follows the pattern and sequence known from histological studies on dental development for certain other primates where initial mineralization overlaps with M 2 crown formation (Beynon et al., 1991; Chandrasekera et al., 1993; Reid & Dirks, 1997; Reid et al., 1998; Swindler & Beynon, 1992) but this of course may turn out not to be so in Proconsul. Summary of the results The following ten key points summarize the results of this study. (i) Enamel thickness on the lingual aspect of the permanent lower incisor crowns in P. heseloni most resembles that in extant New World monkeys and hominoids and differs from that in extant cercopithecoids in that it is not absent or exceptionally thin. (ii) There is no evidence, from this study that P. heseloni or P. nyanzae had relatively thin molar enamel. The measure of ‘‘relative enamel thickness’’ for posterior teeth in P. heseloni and P. nyanzae falls above and beyond the range previously recorded for P. major and P. africanus,
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