Dental development of known-age chimpanzees Pan troglodytes (primates pongidae).код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 86229-241 (19911 Dental Development of Known-Age Chimpanzees, Pan troglodytes (Primates, Pongidae) ROBERT L. ANEMONE, ELIZABETH S. WATTS, AND DARIS R. SWINDLER Department of Anthropology, State University of New York, Geneseo, New York 14454 (R.L.A.);Department of Anthropology, Tulane University, New Orleans, Louisiana 70118 (E.S.WJ; Department of Anthropology, University of Washington, Seattle, Washington 98195 (D.R.S.) KEY WORDS Dental standards, Mandibular dentition, Growth and development ABSTRACT Interpretation of dental development of fossil hominids requires understanding of and comparison with the pattern and timing of dental development among living humans and pongids. We report the first study of crown and root calcification in the lower permanent molar teeth among chimpanzees (Pan troglodytes) of known chronological age. A series of 99 lateral head radiographs of 16 captive-born chimpanzees were analyzed. Radiographs were taken at irregular intervals throughout the entire postnatal period of dental development from birth to 13 years of age. Permanent mandibular molars were rated on a n eight-point maturation scale from initial radiographic appearance through crown and root calcification and apical closure of the root canals. In addition, we were able to document initial crown calcification and completion, as well as root completion and apical closure in incisors, canines, and premolars. Our results show several differences from the widely cited developmental schedule for pongid dentitions of Dean and Wood (Folia Primatol. 361111-127, 1981). We found a much greater degree of temporal overlap in calcification of the crowns of adjacent molars, a pattern very unlike that usually seen in human dental development, which is characterized by delays between the onset of crown calcification in the molar series. Also, the ages and durations of crown and root formation in our chimp sample differ from the estimates proposed by Dean and Wood. By more clearly establishing the nature of developmental schedules and the timing of major events in the pongid dentition, these results should aid in the ongoing controversies concerning the human or pongid nature of dental development among Plio-Pleistocene hominids. During the past few years there has been a resurgence of interest among physical anthropologists in understanding the pattern and timing of dental development among Plio-Pleistocene hominid fossils (Lewin, 1987; Dean, 1987a, Beynon and Dean, 1988). Attention has been focused on ascertaining whether early hominids resembled modern great apes or humans in their characteristic schedules of dental development, including tooth calcification and eruption sequences (Dean, 1985; Smith, 1986,1987;Grine, 1987; Conroy and Vannier, 1987, 1988; Mann et al., 1987; Mann, 1988; Wolpoff et al., 1988). @ 1991 WILEY-LISS,INC Independent approaches utilizing the presence of incremental growth markings in dental enamel have been used to ascertain the timing of events in the dental development of fossil hominids (Bromage and Dean, 1985; Bromage, 1985, 1987; Dean et al., 1986; Beynon and Wood, 1986,1987; Dean, 1987b; Beynon and Dean, 1987). The point of departure for much of this research is Mann’s Received March 5,1990; accepted November 21,1990. Address reprint requests to Robert L. Anemone. Department of Anthropology, Tulane University, New Orleans, LA 701 18. 230 R.L. ANEMONE ET AL. (1975) monograph on the dental remains of the Plio-Pleistocene hominids of southern Africa. Relying heavily on the sample of immature dentitions of Paranthropus robustus from Swartkrans, Mann (1975) argued that the shared patterns of delayed development and eruption of the permanent molar sequence between both robust and gracile australopithecines and modern humans strongly suggest a prolonged period of childhood dependency among these fossil taxa. Elsewhere, Mann (1972:382) defined human culture as a “set of learned behaviors that modify the environment and are crucial to the survival of the species.” Reasoning that a long period of childhood dependency is best explained as a n adaptation for learning the skills involved in tool manufacture and usage, Mann(1975:84)stated that the evidence of prolonged dental development among australopithecines is “the first direct morphological data for the presence of culture in this hominid group.” Mann’s (1975) opinion that fossil hominids shared basically human schedules of growth and development has only very recently been reexamined by a series of workers (Bromage, 1985, 1987; Smith, 1986, 1987; Conroy and Vannier, 1987,1988). Bromage (1987) questions the argument that the presence of prolonged dental development necessarily indicates a culture-bearing existence. A more serious problem is Mann’s (1975) apparent conclusion that the presence of a human pattern of dental development among australopithecines implies human timing of developmental events (e.g., eruption of M1 a t 6 years, rather than a t 3.5 years, a s in apes), and Smith’s (1986)counterargument that a n apelike pattern implies a pongid rate of development. We see no valid basis for inferring rate from pattern, recognizing that these two aspects of development may be independent of each other. An underlying theme in much of the discussion of dental development among fossil hominids is the question of standards of dental development for living humans and great apes (Smith, 1986, 1987; Mann et al., 1987; Lewin, 1987).While standards for both the pattern and the timing of dental development among modern humans have been well established in a number of longitudinal studies (Schour and Massler, 1940; Gleiser and Hunt, 1955; Fanning, 1961; Moorrees, 1959; Moorrees e t al., 1963; Demirjiran, 1986),the same is not true for nonhuman primates (Swindler, 1985). Longitudinal studies of dental development involving animals of known chronological age have been published for very few primate taxa, notably Macaca mulatta (Hurme and Van Wagenen, 1961; Swindler and Gavan, 1962), Cebus albifrons (Fleagle and Schaffler, 19821, and Macaca nemestrina (Sirianni and Swindler, 1985). Published longitudinal studies of the development of the pongid dentition are restricted to the emergence of the deciduous (Nissen and Riesen, 1945) and permanent (Nissen and Riesen, 1964) dentition of the chimpanzee. The pongid standards for tooth development cited in much of the recent literature (e.g., Smith, 1986,1987;Bromage, 1987)are from a single cross-sectional radiographic study of museum specimens of unknown chronological age (Dean and Wood, 1981). Recently we have had the opportunity to study a longitudinal series of lateral head radiographs of chimpanzees (Pan troglodytes) of known chronological age. In this paper we present findings on the development of the mandibular dentition in a n attempt to improve the published standards for pongids. MATERIALS AND METHODS The subjects used in this study were 16 (eight male and eight female) chimpanzees born and reared at the Yerkes Laboratories of Primate Biology. Fifteen of these animals were raised specifically for a longitudinal study of growth and development (Nissen, 1942). Their growth (Gavan, 1953, 19711, skeletal maturation (Nissen and Riesen, 1949a), and dental emergence (Nissen and Riesen, 1945, 1964) have been reported in previous publications. Information on the parentage and treatment of the animals, as well as the radiographic procedures, may be found in these papers (see especially Nissen and Riesen, 1949a). In addition to the 15 animals known as the Normative Group, we have included data on one female, animal 120, who was used in a n experimental study of the effects of light deprivation. Her skeletal development was found to be delayed (Nissen and Riesen, 1949b). We include a single X-ray of this animal, taken a t 48 months, in our sample because her dental development appears to have been unaffected. Comparing her molar calcification stages to those of the only other chimp with a radiograph a t that age (male 61), she shows identical stages in M1 and M2 and is actually advanced in initial calcification of the M3 crown. 231 DENTAL DEVELOPMENT IN CHIMPANZEES Although the lateral head radiographs were collected a s part of the longitudinal study, they are analyzed mainly in crosssectional fashion here due to certain inherent limitations of the data. While radiographs were taken for most animals every 2 to 3 months during the first few years of life and yearly after about 4 years of age, lateral head radiographs were not taken for each animal at each examination. This resulted in, for our purposes, many missing data points (see Table 1and Fig. 1).The frequency and length of these gaps make it difficult to track longitudinally the development of individual teeth in individual animals from initial calcification to crown and root completion. In those individuals for whom sampling is relatively complete, however, the longitudinal nature of the data allows estimation of the rate a t which certain events in the dental development of individual chimpanzees typically occur. For example, we can sometimes determine the crown or root calcification times for various teeth in individuals, a s well a s the amount of time individual animals remain in a given dental stage for various teeth a t various different stages. Where possible, these kinds of inferences are drawn in our analyses. The eight dental stages used to describe the development of chimp molar teeth were modified versions of those defined by Demirjiran (1986) and Demirjiran et al. (1973) for human molar development. They are illustrated in Figure 2 and described in Table 2. The first four stages describe the development of the crown of the tooth from initial calcification (A) through complete calcification to the level of the cementoenamel junction (D). The final four stages reflect increasing growth in length of the roots, culminating in apical closure (H). We were able to apply this model of dental development only to the permanent molar teeth because anterior teeth were not easily observed on the lateral radiographs. Rather than use the attainment of dental stages as a basis for a dental maturity scoring system (Demirjiran, 1986; Demirjiran et al., 19731, we simply describe the calcification schedule of the lower molar teeth in our chimpanzee sample. We have also included, where the data allow, observations on age a t initial calcification as well as crown and root completion for the antemolar dentition. All radiographs were scored by one of us (R.L.A.), while another of us (D.R.S.)examined nearly all of the available radiographs and verified the identifications. All radiographs of animals younger than 120 months were scored a second time (by R.L.A.), a t the end of the study, to verify the reliability of stage identification. There were very few differences in the rescoring of radiographs, and none of more than one dental stage. The age a t which a dental stage was attained in a n individual animal was recorded a s the age a t the first radiographic appearance of that stage. This procedure was used, rather than the standard procedure of using the mean age of the TABLE 1 . Lateral head radiographs for individuals of Pan troglodytes Individual' M/55 M/57 MI59 M/61 M/65 M/67 M/71 M/79 F/90 F/94 F/98 FA00 F/102 FA04 F/ 106 F/120 Total Aee2 120/156 120/132/144/156 84/96/108/120/132 8/24/36/48/108/120/132/144/156 15/18/27/42/96/120 10/12/96/108/120/132/144 1 day/2/4/7/84/96/108/120/132 1 day/72/84/96/120 24/36/108/120/132/144/156 3 weeks/21/108/120/132/144/156 12/108/120/132/144 12/18/96/108/120/132/144 15/33/96/108/120/132 1 day/2/4/6/8/12/72/84/96/108/120/132 1 day/2/10/18/21/24/96 48 Number 2 4 5 9 5 7 6 12 7 1 99 'M designates a male;F designates a female. The numbers are the animal identification numbers from the Yerkes Primate Laboratories. 21n months, except where noted. 232 R.L.ANEMONE ET AL. * Number of Radiographs O 10 m - 0 Fig. 1. Histogram depicting the sample of lateral head radiographs of chimpanzees (Pan troglodytes 1 on which this study is based. A total of 99 radiographs were examined. All ages are in postnatal months except 1D = 1 day and 3W = 3 weeks. roots of M1 can be said to begin development a t approximately 24 months. All radiographs of individuals 72 months of age (n = 1) and older (84 months, n = 2; 96 months, n = 5; 108 months, n = 4)show the terminal stage of first molar development (stage H) in which the cessation of root growth is marked by apical closure. These data might suggest that M1 roots take between 4 and 7 years to grow, but a closer RESULTS examination indicates otherwise. The atLower permanent molars tainment of stage G in two animals a t 48 Figures 3-5 depict the calcification sched- months (F/120 and W61) suggests that apiule for the permanent lower molar teeth in cal closure takes place closer to 60 months. our sample of Pan troglodytes. Initial calcifi- I n both M2 and M3, stage G is almost always cation (stage A) of the crown of M1 was followed in the next year by the attainment observed radiographically in all one day old of stage H. If this is also true for M1, we can (n = 3) and three week old ( n = 1)chimpan- estimate a period of 3 years growth for the zees (Fig. 3). The crown of M1 is completely roots of M1 in these animals. Due to the lack calcified (stage D) by about the end of the of radiographs available for animals 60 second year of life. None of the animals months of age (Fig. 1 ), this suggestion must observed had a complete M1 crown a t 18 remain speculative. months ( n = 3, see Fig. 6), while all animals Calcification of the second molar was first older than 18 months had M1 in at least observed in one of two animals a t 15 months stage D, including two animals at 21 months, of age (Fig. 4).By 18 months of age, two other two at 24 months, and one a t 27 months. All individuals show initial calcification of M2 animals older than 27 months a s well as one (Fig. 6). By 21-27 months of age, all individindividual at 21 months (Fig. 7 ) had, in uals observed ( n = 5) have M2 in stage B addition to a complete M1 crown, significant (Fig. 7). Eighteen months then appears to be M1 root formation (Le., stages E-H). Since a good estimate for the onset of radiographiroot formation of molar teeth begins as calci- cally visible calcification of M2. Crown comfication of the crown reaches completion, the pletion first occurs at 48 months (n = 2) and first radiographic appearance of a stage and the last previous radiograph for t h a t individual (Swindler and Gavan, 1962; Fleagle and Schaffler, 1982; Sirianni and Swindler, 1985),due to the sometimes significant gaps between radiographs for individual animals. Consequently, our results for the age at attainment of various dental stages must be considered a s maximum values. - DENTAL DEVELOPMENT IN CHIMPANZEES 233 TABLE 2. Description of dental deuelopmental stages of molar teeth in Pan troglodytes Stages A 11 c Stane DescnDtion Initial cusp calcification is visible as one or a series of small inverted cones in the upper part of the crypt; there is no fusion of these points Initial points of calcification are fused, forming a regularly outlined occlusal surface Enamel formation is completed at the occlusal surface with extension towards the cervical region; dentine deposition has begun below the enamel crown, and the pulp chamber has a smoothly curved superior border Crown is completely calcified to the cementoenamel junction, and the beginnings of root formation are present in the form of a spicule Radicular bifurcation is visible, while root length is always less than crown height Radicular bifurcation is further developed, giving the roots a funnel shape; root length is equal to or greater than crown height Root canals are parallel sided with open apices Completion of root length marked by apical closure D E F G H Fig. 2. Develo mental stages used for molar teeth of Pun troglodytes.&e Table 2 for description of criteria for these stages. is not evident at 42 (n = 1)and 36 months (n = 2) of age. Two and one-half years is thus a reasonable minimum estimate for the duration of calcification of the crown of M2. The roots of M2 complete their growth (stage H) at an age of 96 months (two of nine, 108 months (six of nine), or 120 months (six of six). Second molar root formation can be conservatively estimated as ranging between 3 and 6 years, with a more likely range of 4-5 years. The early developmental stages of M3 are, unfortunately, poorly represented in this data set (Fig. 5). The earliest calcification of M3 appears in 1 individual at 42 months (Fig. 8) and in one of two a t 48 months. The fact that M3 is present in stage B or C in two individuals at 72 months indicates that it would almost certainly be present in these individuals by at least 60 months. The best suggestion that can be made from these data is that M3 is first calcified at close to 48 months of age. Completion of the crown of M3 occurs in two of three radiographs at 84 months and in all seven radiographs taken at 96 months. A maximum estimate of 4.5 years for crown completion times of M3 can be derived from one animal who was in stage A a t 42 months and in stage D at 96 months (W65). This value is a maximum (for this individual), because no radiographs are available between the ages of 42 and 96 lndividu F190 Fl94 F198 F11 0 0 F l l 02 FllO4 FI106 F1120 MI55 MI57 MI59 Mi61 MI65 MI67 MI71 MI79 1D 3 W 2 4 7 6 8 10 12 15 18 21 24 27 33 36 F D A 42 48 72 84 E B c B C A A A ? B B B E B 9 6 108 H H H H H H B C D D H G H D B B B A A E C G D H H F H E B A H Y Fig. 3. Attainment of dental stages for M, o f P a n troglodytes. The letters A-H represent the eight dental stages used in this study (illustrated in Fig. 2 and described in Table 2),and ? indicates that the area in which the tooth is normally found was not present on the radiograph. lndividua F/90 Fi94 Fig8 F11 0 0 F11 0 2 F l l 04 Fl106 FI120 MI55 MI57 MI59 M/61 MI65 MI67 MI71 MI79 12 15 18 21 24 27 33 36 c B 42 48 72 84 E F B X A X C X A B B 96 108120 G H H H G G H G H G H G D H H B A B A F G G F H G H G H G D C C X E H H H Fig. 4. Attainment of dental stages for M, o f P a n troglodytes. The letters A-H represent the eight dental stages used in this study (illustrated in Fig. 2 and described in Table 2);? indicates that the area in which the tooth is normally found was not present on the radiograph, and x indicates the absence of any calcification for the tooth. Individual 1 8 F190 F194 Fl98 FIIOO X FllO2 Fl104 FllO6 F/120 MI55 MI57 MI59 MI61 MI65 MI67 MI71 MI79 21 24 27 33 36 42 48 72 84 C D X X X X 9 6 108 1 2 0 1 3 2 1 4 4 1 5 6 E E F G H F G G H E F H D D E F G D E F G D E F G D A C D X X A B E C D D E D E E E F G F E F F F G F H G ? F G H H H G H H Fig. 5. Attainment of dental stages for M, ofPan troglodytes. The letters A-H represent the eight dental stages used in this study (illustrated in Figure 2 and described in Table 21; ? indicates that the area in which the tooth is normally found was not present on the radiograph, and x indicates the absence of any calcification for the tooth. DENTAL DEVELOPMENT IN CHIMPANZEES Fig. 6 . M/65 a t 18 months. Note that M1 is in stage C while M2 is in late stage A, with molar cusps well developed but not quite fused to form a completely outlined occlusal surface. All permanent teeth except M3 can be seen in their crypts in this radiograph. The entire deciduous dentition is in occlusion. Fig. 7. F/94 at 21 months. Note the beginnings of radicular bifurcation in M1, indicating the attainment of stage E. M2 is in stage B, and all the other permanent teeth except M3 are clearly present in their crypts. 235 236 R.L. ANEMONE ET AL Fig. 8. M/65 at 42 months. M1 is in occlusion in stage F, M2 is in stage C, and M3 is in stage A. months. Another animal (IW79) had M3 in already present in stage A in four of five stages B, C, and D a t ages 72, 84, and 96 radiographs (Fig. 6). Similarly, M3 is present months, respectively. Assuming 1 year for in stage A in one individual a t 42 months the transition from stage A to stage B indi- while M2 is still in stage C (Fig. 8). cates 3 years of crown growth. The available Lower permanent antemolar dentition data do not rule out M3 crown completion times anywhere in the range of 3-4.5 years. Calcification of both permanent lower preThird molar root completion occurs at 132 molar teeth begins at 18 months in our sammonths (one of nine animals), 144 months ple of chimpanzees. Except for two radio(four of seven), or 156 months (four of four). graphs of poor quality in which the premolar Simply looking a t the range in starting age region cannot be seen (FA06 a t 18 and 21 (84-108 months) and age at completion months of age), all animals of 18 months (132-156 months) of the root gives a n over- (n = 21, 21 months (n = 11, and 24 months estimate of the actual time involved. Looking (n = 3) of age show initial calcification of a t these radiographs in a longitudinal fash- both P3 and P4. Time of crown completion of ion, it is clear that most individuals remain the premolars is difficult to determine due to in a single stage between D and H for a t most gaps in our data. One animal at 42 months 2 years, and usually for 1 year. This allows us ( W 6 5 ) has crowns of P3 and P4 one-half to to estimate minimum and maximum values three-quarters complete, while another at 72 months (W79) has complete crowns of for M3 root growth of 4-5 years. These results indicate a substantial unerupted P3 and P4 as well as some initial amount of overlap in the crown development root formation on P3. These few data suggest of adjacent molars among chimps. In our crown completion of P3 and P4 between 5 sample of chimpanzees, M1 is radiographi- and 6 years of age. Apical closure of the cally present a t birth and reaches crown premolar roots occurs in all individuals completion a t 2 years of age o r slightly ear- (n = 14) a t either 9 or 10 years of age. Permanent lower incisor teeth first aplier. By 15-18 months of age, however, when M1 is in either stage B or stage C, M2 is pear radiographically in one individual (F/ DENTAL DEVELOPMENT IN CHIMPANZEES 104)at 6 months (11)and 8 months (12).Both permanent incisors are, however, absent in two other animals aged 7 months (W71) and 8 months (W61). All animals 10 months of age and older have both permanent lower incisors present. Incisal crown completion is attained by about 3 years of age, as seen in animals with crowns of both I1 and I2 complete a t 33 months (F/102),36 months (F/90), and 42 months (W65). The condition of the permanent lower incisors in another 36month-old animal W 6 1 ) is, unfortunately, impossible to determine. Apical closure of the incisor roots has apparently occurred by about 7 years of age ( n = 4). The permanent canine is visible in one (F/104) of four animals a t 12 months and in one (W65) of two a t 15 months of age. By 18 months of age and older, the permanent canine is always present in our sample. The canine crown is completely calcified by about 7 years of age ( n = 4) or slightly older. The canine root reaches apical closure at about 12 years of age ( n = 7). DISCUSSION The question of which standards of human dental development should be used in attempting to describe the patterns and rates of dental development among fossil hominids has recently been the subject of serious debate (Smith, 1986, 1987; Mann et al., 1987; Lewin, 1987). It is therefore surprising, considering the many fewer available data, that there has been little discussion of the suitability of the standards used to describe dental development among pongids. While there is a tendency to view the dentitions of fossil hominids in a dichotomous fashion’as sharing either a “human” or a “pongid” pattern of dental development (Mann, 1975), most recent investigators (e.g., Smith, 1986; Conroy and Vannier, 1987; Beynon and Dean, 1987,1988; Beynon and Wood, 1987) are considering more seriously the possibility that some or all of the fossils have had unique patterns and/or rates of dental development. In any event, longitudinal data on animals of known age are needed to characterize pongid dental development accurately, and until now these have been lacking. The data presented here allow a clear characterization of the developmental pattern and rate of the chimpanzee dentition. The most commonly cited reference on pongid dental development is the pioneering contribution of Dean and Wood (1981).These 237 authors made a radiographic analysis of a cross-sectional museum sample of the skulls and jaws of 175juvenile wild-shot apes (Pan troglodytes, Pongo pygmaeus, and Gorilla gorilla). This paper was the first reasonably successful attempt at developing standards of dental development among apes for comparison with the well known human standards. The developing ape teeth were scored on a nine-point maturation scale very similar to the eight-point scale used in this study. Using published data on gingival emergence of the chimpanzee dentition (Nissen and Riesen, 1945, 1964), a series of extrapolations were made to estimate the age a t which dental development events occur among pongids. Since these animals were obviously of unknown age a t death, Dean and Wood (1981) made the following assumptions: 1) crown calcification time for all molars is 2.5 years and 2) roots of similar tooth types calcify in the same period of time. Dean and Wood (1981:113) clearly understood the limitations inherent in their use of cross-sectional data, and stated that “without longitudinal data i t is impossible to establish the variability of dental development with chronological age; only the relative dental developmental age of a n ape skull can be established from cross-sectional data such as these.” In any event, their work, and especially their Figure 2 (Dean and Wood, 1981:116; adapted as part of our Fig. 9), has been cited as a n important reference on dental development among the pongids and the crux of the comparison of fossil hominid dentitions with those of modern pongids (Smith, 1986,1987; Bromage, 1987; Dean, 1987a, b). The results reported here, a s well a s our reading of the literature, indicate that both of the above assumptions may be invalid and that the time scale of pongid dental development needs recalibration. In support of their assumption that all three molars calcify in approximately the same amount of time among pongids, Dean and Wood (1981) cite two longitudinal studies ofhuman dentition. In the first, Moorrees et al. (1963) estimate the time for crown calcification of the human M1 a s 2.1 years and for M2 and M3 a s 2.8 years. In the second, Gleiser and Hunt (1955)present no data on crown calcification times for M2 and M3. Their article concerns the first molar, for which they estimate crown completion times of 41.5 months sd = 5.6) for boys and 39.3 months (sd = 4.2/months) for girls, and which they describe as “the tooth which un- 238 R.L. ANEMONE ET AL Human I I I I I I I I I I I I I I I I I I I ]Years 1 2 3 4 6 5 7 9 10 11 12 13 14 15 16 17 18 19 20 8 Pongid I1 I2 B. C P, p4 M, M M I c- I: C P P M M M I I I I I I I I I I I I I I I I I I I /Years 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Fig. 9. Crown and root calcification and gingival emergence schedule for human ( A ) )and pongid (B)samples (After Dean and Wood, 19811,and forPun troglodytes (C).Plottedvalues are means or estimated means. Solid lines show crown formation; dashed lines show root formation; E, gingival emergence. Emergence data in both B and C are from Nissen and Riesen, (1964). DENTAL DEVELOPMENT IN CHIMPANZEES dergoes the fastest amelogenesis . . . in the permanent dentition” (Gleiser and Hunt, 1955:270).Both Fleagle and Schaffler (1982) and Sirianni and Swindler (1985) found a progressive lengthening of the period of crown calcification from the first to the third permanent molar in their longitudinal studies of Cebus albifrons and Macaca nemestrina, respectively. In examining our data, we find no support for the assumption of equal periods of calcification for each of the permanent molars in Pan troglodytes. Our best estimates for molar crown calcification times are 2 years for M1,2.5 years for M2, and 3.5 years for M3. Lacking sufficient temporal information on root development, Dean and Wood (1981) made the assumption that the roots of similar tooth types (e.g., all molars, all premolars) develop in the same period of time. Moorrees et al. (1963) indicate that the situation is somewhat more complex among humans, where M1 root calcification time is estimated a s 3.6 years, M2 as 4.8 years, and M3 a s 4.5 years. The data on human premolar root calcification times indicate little difference between P3 and P4, but a significant sexual difference, with males ranging from 5.3 to 5.4 years and females from 4.6 to 4.9 years (Moorrees et al., 1963). Fleagle and Schaffler’s (1982) data on C. albifrons similarly show differences in P3 and P4 root development times of males (P3 106.03 weeks, P4 105.95 weeks) and females (P3 93.86 weeks, P4 96.32 weeks) but no intrasexual differences between the P3 and P4. Their data also indicate a lengthening period of root calcification from the first to the third molar in both male and female C. albifrons (Fleagle and Schaffler, 1982).Likewise, Sirianni and Swindler’s (1985)study ofM. nemestrina also documents a n increase in root calcification time from the first to the third molar in both males and females, but no significant differences in the root calcification times of P3 and P4. Similarly, the data presented here suggest variation in the root calcification times of the molar teeth, but not the premolars, among Pan troglodytes. We estimate that the root of M1 develops for a period of 3-4 years, M2 and M3 roots develop for approximately 4-5 years, and the roots of both P3 and P4 calcify in 4-5 years. In spite of the problems with these two assumptions (crown calcification times of 2.5 years for all molars and equal period of root development for similar tooth types), they form the basis for much of Dean and Wood’s 239 (1981) proposed schedule of dental development among pongids. Our data suggest a substantially different schedule of dental development for P. troglodytes (Fig. 9).Perhaps the most significant difference between our results and those of Dean and Wood (1981) can be seen in the period of crown calcification of the molar dentition and in the degree of overlap in development of adjacent molars. Dean and Wood (1981) apparently found no overlap between the developing crowns of adjacent molars and indicated sequential 2.5 year periods of crown calcification for these teeth, beginning a t birth (Ml), 2.5 years of age (M21,and 5 years of age (M3). Our results show a substantial amount of overlap in the crown development of adjacent molars. This is a particularly interesting finding in that it accentuates the major difference between human and pongid schedules of molar development, that of delayed onset of calcification of the molar series in humans (Moorrees et al., 1963).Fanning and Moorrees (1969) suggest that temporal overlap in calcification of second and third mandibular molar crowns occurs at low frequencies in some modern human populations, including Europeans (4%) and aboriginal Australians (14%). While suggestive, these data do not strongly contradict the evidence for important differences between pongid and human schedules of molar calcification. Our results serve to distinguish more easily between human and pongid developmental schedules by indicating a greater degree of temporal overlap in the calcification of the crowns of adjacent molar teeth in chimpanzees than apparent in earlier published studies (e.g., Dean and Wood, 1981). These results are consistent with, and to some extent explain, the progressive delay in eruption of the permanent molar teeth among humans compared with apes relative to chronological age, to eruption of the anterior teeth, and also to epiphyseal union noted by Schultz (1956). Differences between our estimates and those of Dean and Wood (1981)in the period of molar root formation appear to be of only minor importance (Fig. 9). While our results for the antemolar dentition are less compelling than those for the molars, we were in most cases able to estimate the onset and completion of crown and, to a lesser extent, root calcification. Premolar development in our sample begins somewhat earlier than estimated by Dean and Wood (1981).We have clear evidence of ini- 240 R.L. ANEMONE ET AL tial calcification of both P3 and P4 at 18 months, while Dean and Wood (1981) published ages of slightly greater than 2 years for these events. Premolar crown completion in both their estimate and ours occurs at about 5 years of age for both P3 and P4. Conversely, our data indicate a slightly later onset of calcification of the incisors (6-10 months) than those of Dean and Wood (1981) ( 2 4 months). No major differences are apparent in the development of the canine crown or root or in the development of the roots of any other permanent mandibular teeth. In summary, our analysis of a mixed longitudinal series of lateral head radiographs of chimpanzees of known chronological age has allowed us t o modify published standards of dental development for modern pongids. Concerning the development of the lower permanent molar teeth, for which our data are most complete, our results indicate the following: 1)The duration of crown calcification of adjacent molar teeth overlaps to a considerable extent. This contrasts with the usual human schedule of delayed onset of crown calcification among adjacent molar teeth. 2) The duration of crown and root development appears to increase from the first to the last molar tooth. 3)Ages at initial calcification and completion of the crowns and roots of the lower permanent molars, as well as some of the antemolar teeth, differ from the previously published estimates of Dean and Wood (1981). ACKNOWLEDGMENTS Funding for collection of the original growth data and radiographs, as well as maintenance of the chimpanzees was provided by the Samuel s.Fels Fund and grant RR00165 from the Yerkes Laboratories of Primate Biology (Orange Park, Florida). 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