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Investigation into the relationship between perikymata counts and crown formation times.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 86:175-188 (1991)
Investigation Into the Relationship Between Perikymata Counts
and Crown Formation Times
ALAN E. MA", JANET M. MONGE, AND MICHELLE LAMPL
Department of Anthropology, University of Pennsylvania, Philadelphia,
Pennsylvania 19104
KEY WORDS
Paleoanthropology, Dental development, Fossil
hominines, Hominid
ABSTRACT
This study reports on a sample of 12 modern human incisors
(from two archaeological sites) that were viewed with a scanning electron
microscope and whose perikymata were counted. These 12 incisors more than
doubles the previously published sample size of modern human incisors that
have served as the published standard for perikymata number in human
incisors and have been employed to define taxonomic relationships in fossil
hominids. All previously published fossil specimens fall within the expanded
range of modern human perikymata counts and can no longer be considered
distinctively nonhuman in dental formation time.
Five neandertal incisors from the Krapina site in Croatia, Yugoslavia, were
also examined. These incisors substantially expand the previous data base for
counts of perikymata in Homo sapiens neanderthalensis, likewise overlapping
the previously published modern human range.
Finally, the validity of methods that have been employed for deriving crown
formation times from perikyrnata counts in fossil hominines is called into
question. Utilizing the presently known perikymata ranges for modern humans, these methods do not predict the range of known crown formation times
of modern humans as assessed from studies of living children.
For almost half a century there have been
discussions of the importance of reconstructing the pattern of maturation in earlier
hominines' (e.g., Dart, 1948; Dobzhansky,
1962;Mann, 1972,1975; a u l d , 1976; Lovejoy,
1981; Holloway, 1983). In these discussions,
there was a recognition that the prolongation
of childhood dependency represented a crucial
shiR in human evolution; one that provided
the time necessary for complex behaviors and
detailed information about the environment
to be learned and internalized by a new
generation.
In 1975, Mann presented data based on
gross details of dental eruption and development in South African australopithecines, in
particular, Australopithecus robustus from
Swartkrans, that suggested these early
hominines conformed t o the modern human
'In this paper, we follow Weiss (1987) and Weiss and Mann
(1990)in employing the subfamily Homininae rather than the
family Hominidae in describinghumans and our close immediate
ancestors.
@ 1991 WILEY-LISS, INC.
pattern of a prolonged childhood (Mann,
1972). At the time that this evidence was
presented, there was an accepted view that
the australopithecines' had crossed an evolutionary threshold and had become more
human than apelike. This view was based on
studies of endocranial details, as well as on
archaeological traces from East African sites
(e.g., Holloway, 1972; Isaac, 1972, 1978). In
that climate of opinion, prolongation of australopithecine dependency periods was not
unexpected and tended to confirm the prevailing perspective.
More recently, there has been a critical
reconsideration of these early hominines,
one including a reassessment of their behavior and cultural repertoire and their overall
similarity to modern humans. Reevaluations
'The term australopithecine is used here in an entirely descriptive manner to referto all members of the genus Australopithecus
(which, for us, continues to included. mbustus and A. boisei),as
well as specimens referred to as Homo habilis or described
generally as "earlyHomo."
Received March 5, 1990; accepted January 15,1991.
176
A.E. MA” ET AL
(Fig. 1). These authors argued that each
perikymata represented approximately 7
days of enamel deposition, a conclusion
based on previously published observations
of dental microstructure that reported seven
to eight regularly spaced cross striations
between striae of Retzius and thus, by association of these structures to their surface
manifestations, perikymata. The cross striations, it was maintained, represented the
daily activity of a single enamel-forming cell
(Boyde, 1976).
STUDIES OF DENTAL MICROSTRUCTURE AND
On this foundation, Bromage and Dean
THEIR IMPLICATIONS FOR PAmERNS OF
(1985) counted perikymata on a sample of
HOMINID GROWTH AND DEVELOPMENT
nine australopithecine juvenile incisors, a
In 1985, Bromage and Dean published combined sample of maxillary and mandibuobservations on the microstructure of dental lar, central and lateral incisors. Bromage
enamel that seriously challenged the notion and Dean found a range of between 57 and
of a modern humanlike prolongation of 135 perikymata on these fossil incisors and
growth in australopithecines. They pre- contrasted these results with a sample of 10
sented evidence suggesting that enamel sur- modern human incisor teeth (specific idenface manifestations, termed perikymata, of tity not noted), with perikymata counts
internal enamel bands, termed striae of Ret- ranging between 165 and 202 (Bromage and
zius, could be employed to determine the Dean, 1985). These differences in perikylength of time that a tooth crown took to form mata counts have been interpreted to indi-
of earlier hominine brain endocasts (e.g.,
Falk, 1987) and archaeological evidence and
inference (e.g., Potts, 1984a,b; Potts and
Shipman, 1981; Shipman, 1983, 1986; Binford, 1981, 1984, 1985) have led some to
suggest that the essential core of humanness
was not to evolve until the evolutionary origin of modern humans. In this climate of
opinion, the patterns of growth and maturation in early hominines are also being reexamined.
Fig. 1. The labial surface of an incisor from the site of Hasanlu at x20. Perikymata are
visible on the surfaces ofthe incisors as bands. While many of the perikymata are quite distinct,
others are difficult to distinguish even at the higher magnifications at which these structures
were counted.
177
PERIKYMATA AND CROWN FORMATION TIMES
cate different total incisor crown formation
times between australopithecines and modern humans (Bromage and Dean, 1985; Beynon and Dean, 1988).
Bromage and Dean (1985) based their conclusions on their calculation of total crown
formation time in the fossil incisors: This
was taken as the time of‘enamel formation,
documented by the visible increments of
growth (the perikymata) plus an additional
estimation of the time recorded in hidden
enamel increments and not expressed on the
surface. Bromage and Dean (1985) estimated that there were 20-30 hidden increments not observable as surface perikymata,
which were said to represent 6 months of
additional enamel formation unrecorded by
observable surface manifestations. Therefore, the total time of incisor enamel crown
formation was derived from the total number
of perikymata observable on the tooth surface, each perikymata accounting for 1
week‘s growth, added to 6 months of hidden
growth. The perikymata counts and derived
crown formation times originally published
by Bromage and Dean (1985) are provided in
Table 1.
Bromage and Dean (1985) concluded that
the total time for enamel formation in all the
australopithecines including early Homo
was significantly shorter than that in modern humans, and, based on the ages at death
derived from these crown completion times,
“the growth periods for Plio-Pleistocene
hominids were similar to the modern great
apes” (Bromage and Dean, 1985526).
These original conclusions are interesting
to review. The important aspect of these
conclusions is that early hominines had a
foreshortened and apelike growth period.
This idea was based on the perikymata data,
which were said to document fossil hominines’ shorter incisor crown formation times
relative to modern human dental development: “Earlyhominids evidently had shorter
periods of dental development than modern
humans and therefore a less prolonged infancy” (Beynon and Dean, 1988).
At the time Bromage and Dean (1985)
published their work, no enamel histological
data for modern great ape incisor formation
comparable to that available for modern humans were known. This made direct comparisons of enamel formation times between
apes and humans difficult. Likewise, it was
difficult to evaluate the inferential interpretation of an apelike growth period based on
incisor crown formation times. The reference
TABLEI. Perikymatacountsandcrownformationtimes
from Bromage and Dean (1985)
Taxonomic ~
O U D
SamDle no.
A. afarensis
A. africanus
A. robustus
5
Early Homo
2
H. s. sapiens
10
1
1
Range of crown
formation times
3 years
3 years 1.1 months
1 year 7.2 months
2 years 1.8 months
2 years 6.2 months
2 years 7.4 months
3 years 7.9 months
4 vears 4.4 months
cited by Bromage and Dean (1985) for data
on ape incisor crown formation times was a
previous study by Dean and Wood (1981) in
which radiographs of unaged ape skeletal
specimens had been used to predict great ape
crown formation times. However, in this
1981 reference, Dean and Wood had published the observation that the extant evidence “suggests that there is no difference in
the basic rate of enamel formation between
humans and pongids” (Dean and Wood,
1981:123). In fact, the earlier reference of
Dean and Wood (cited by Bromage and Dean,
1985) actually shows only a 0.25 year differende in incisor development time between
humans and apes (Dean and Wood, 1981:
116). Thus it is unclear how the conclusions
of an apelike growth period for fossil hominines could have been distinguished on the
basis of comparative incisor crown formation
times between the taxa.
Since the original Bromage and Dean
(1985) study was published, additional fossil
specimens have been analyzed that require
modification of the original conclusions. Additional perikymata counts from fossil hominine incisors (three A. afarensis, four A.
boisei, and two early Homo) have been published that expand the hominine incisor
perikymata count range from 57 to 180, as
well as extend the respective crown formation times (Dean, 1987a; Beynon and Dean,
1988).For example, among the australopithecine specimens are the incisors from Laetoli
Hominid 3 (LH 3). The maxillary central
incisor has a perikymata count of 170 and
the maxillary lateral incisor has a perikymata count of 180, translating to crown formation times of 3.8 and 4.0 years, respec-
178
A.E. MA”
tively (by the method of Bromage and Dean,
1985). Both the perikymata counts and the
derived crown formation times for the LH 3
specimen are within the originally published
range of modern human incisor crown formation times reported by Bromage and Dean in
their 1985 paper, although no explanation
has ever been published for this contradiction of their conclusions (Dean, 1989).
Furthermore, in 1986, Dean et al. reported
on the analysis of an upper right central
incisor from the neandertal youngster from
Devil’s Tower, Gibraltar. They found 119
perikymata on this tooth, or an estimated 2.8
years for total crown formation. This total of
119 perikymata is notable, for it is significantly below the range of 165-202 perikymata that is the modern human incisor
perikymata range given by Bromage and
Dean (1985) and is also below the perikymata results obtained for australopithecines
such as LH 2 (with 130 perikymata on the
mandibular right central incisor), Sts 24a
(135 perikymata on the maxillary right central incisor) (Bromage and Dean, 19851,LH 3
(170 and 180, as noted above), and very close
to the early Homo specimen from East Turkana, ER 808 (123 perikymata on the maxillary lateral incisor [side unspecified])
(Beynon and Dean, 1988).Dean et al. (19861,
however, concluded that, by reference to
data on modern human crown formation
times, “the time of crown completion derived
from the perikymata counts in this study for
the Gibraltar child are likely to be accurate,
as the figure obtained falls within the known
ranges of crown completion in Homo sapiens” (Dean et al., 1986:306).
Why were 119 perikymata deriving from a
Homo sapiens neanderthalensis child accepted as within the modern human range of
crown formation times when as many as
124-180 perikymata deriving from australopithecines were excluded and determined
specifically apelike? There is an inconsistency in the acceptance of 119 perikymata
and a 2.8 year crown formation time as
humanlike for the Devil’s Tower neandertal
child (Dean et al., 1986) while rejecting australopithecine incisors including early Homo
specimens as humanlike, even when several
of these, e.g., Sts 24a, possess comparable
numbers of perikymata (Bromage and Dean,
1985; see Table 1).
Review of these previous studies identifies
methodological differences that lead to these
conflicting interpretations. A source of these
discrepancies is found in the selective use of
ET AL
data from modern humans: In the work of
Bromage and Dean (1985) on the australopithecines, only an estimation of a mean was
employed to represent incisor crown formation times in modern H . sapiens while in the
analysis of Dean et al., (1986) on the Neandertal juvenile data on the ranges of crown
formation times for modern humans were
employed.
The debate concerning human and ape
dental maturation has been recently extended by Dean, who notes that the “great
apes may take up t o 8 or 9 years to form
enamel on their incisors and canines”
(1989:171). Dean (1989:171) continues by
observing that perikymata counts of australopithecine anterior tooth crowns do not take
“as long as great ape anterior crowns take to
form.” Thus it is now reasonable to examine
critically the claim, based on incisor crown
formation times, presented in the original
1985 paper that “Plio-Pleistocene hominids
had markedly abbreviated growth periods
relative to modern man, similar to those of
the modern great apes” (Bromage and Dean,
1985525).
PRESENT STUDY
The difficulties in the interpretation of
enamel surface structures, and the often
conflicting claims made about their significance in earlier hominines, suggest that an
important current objective ought to be the
enlargement of the reference sample of
enamel surface structure counts for living
and extinct members of Homininae.
Materials and methods
Twelve modern human incisor teeth and
six Upper Pleistocene hominine teeth were
chosen for this study. All 18 teeth were unworn and unerupted. Fourteen teeth were
crown complete with little o r no root development. Four teeth, all fossil specimens, possessed not quite complete crowns.
Because fossil specimens cannot themselves undergo the preparation necessary for
electron microscopy, which is the preferred
technique for viewing enamel surface details, replicas of the fossils must be produced.
The six fossil hominine teeth were reproduced using a dental silicone impression material, and the resultant epoxy casts were
imaged by the scanning electron microscope
(SEM). A critical determinant in the accuracy of reproduction, and thus in the quality
of detail obtained in the casts, is the choice of
molding and casting media. The advantages
PERIKYMATA AND CROWN FORMATION TIMES
and problems with various materials has
been frequently discussed in the literature
(Gordon, 1984; Barnes, 1978, 1979; Pameijer, 1978; Scott, 1982). As the data base in
studies of fossil dental microstructure have
been replicas, a control study for protocol
was undertaken prior to the collection of
data in the present study.
The techniques used for replication of fossil specimens used by Bromage and Dean
(1985) and by Dean et al. (1986) were outlined by Bromage (1985). The use of the
materials suggested by Bromage (19851, especially the use of hardware epoxies, met
with little success in the present study.
As a control, prior to data collection in the
present study, two of the modern human
incisors were employed t o examine the
amount of detail captured by the replication
process. A Reprosil Light Body mold was
made of these teeth. One mold was cast in
low viscosity (2,000 centipoise), slow-setting
Hysol-brand epoxy (TE 6345 with HD 0111);
the other mold was cast in a medium viscosity, but fast-cure epoxy, the sort that can be
purchased in hardware stores. Both of the
original specimens, and both replicas, were
sputter-coated with gold palladium at the
same time and mounted in the SEM chamber
as matched pairs to be viewed simultaneously at magnifications of x20, x40, and
~ 8 0 These
.
were chosen because the only
previously published magnification level cited
for perikymata counting was x50 (Dean
et al., 1986). Perikymata counts were made
on the matched pairs.
The differences between the detail quality
of the two types of casting resins were dramatic. The use of Reprosil as a molding
media, with Hysol epoxy as the casting resin,
was an excellent combination for the production of highly accurate replicas of perikymata. Perikymata numbers on the replica
were within +5 of the original specimen. The
castings were not effected by pitting or bubbling a t these magnifications as outlined by
Gordon (1984).
In contrast, the use of Reprosil with the
fast-cure hardware epoxy produced inaccurate perikymata counts on the replica. The
number of perikymata observed on the replica were one-third less than on the original
tooth. These results are similar to those
described by Hillson and Jones (1989) and
indicate that the techniques used to produce
replicas for study is crucial in the analysis of
enamel surface microstructure in fossils. Although Bromage (1985) tested these materi-
179
als on various structures within dentin, it is
possible that the porosity differences between dentin and enamel is critical here.
Gordon (1984) noted that the production of
replicas of very smooth surfaces, such as the
enamel surface, can create a distinct series of
problems not encountered in the reproduction of other hard tissues.
Samples
Modern human incisors. The samples of
12 modern human incisor teeth came from
two sites. Ten specimens came from the Iranian site of Hasanlu, a walled city sacked a t
about 3000 B.C. These Hasanlu specimens
were discovered in the citadel, described by
the excavators as the location of a mass
murder of the city’s inhabitants by invaders
(Muscarella, 1991). None of these children,
therefore, died as the direct result of illlness.
The other two modern human specimens
came from the 1200-year-old Island Field
site in the State of Delaware.
These 12 modern human incisors were
chosen because each tooth was unerupted
and crown complete with a portion of root
formation. Perikymata could be viewed over
the entire crown surface at low magnifications using a binocular microscope.
Neandertal incisors. Krapina 90, 91, 93,
94, and 95 (RadovEiC et al., 1988) were
molded as described above with the permission of Dr. Jakov RadovEid of the Croatian
Natural History Museum, Yugoslavia, in the
summer of 1988. An additional mold was
made of one unerupted incisor in the crypt of
Krapina mandible A. To prevent any effects
from mold shrinkage, positive replicas were
cast using Hysol-brand epoxy within 48
hours of making the molds.
Perikymata could be counted on the entire
labial surfaces of all of these dental casts
except the incisor of mandible A. Two of the
Krapina specimens, 91 and 93, were less
than two-thirds crown complete, and Krapina
95 was only one-third crown complete.
Krapina 94 was assessed as three-fourths
crown complete, and Krapina 90 had about
one-third root development. Thus only
Krapina 90 is precisely comparable with previously collected data. Data from the other
Krapina teeth are, however, presented (see
Table 2).
Microscope protocol
All 12 modern human specimens, as well
as the epoxy casts of the fossils, were sputter-
180
A.E. MAXN ET AL
TABLE 2. Perikymata counts and estimated crown formation times in early hominines
Taxonomic group specimen
A. afarensis
LH2l
LH32
LH32
LH62
A. africanus
Sts 24a'
A. robustus
SK 62l
SK 62'
SK 63l
SK 71'
SK 73l
A. boisei
OH 302
ER 8122
ER 14772
ER 18202
Early Homo
SK 74b'
ER 820'
OH 62
ER 80@
H. sapiens neanderthalensis
Gibraltar3
Krapina 90
Krapina 9 l t
Krapina 93t
Krapina 94t
Krapina 95'
Homo sapiens sapiens
Hasanlu
Perikymata counts
130
170
180
116
3.0
3.8
4.0
2.7
135
3.1
57
64
86
>6 1
>79
1.6
1.7
2.1
>1.7
>2.0
101
86
92
82
2.4
2.2
2.3
2.1
110
105
95
123
2.6
2.5
2.3
2.9
119
205 f 10*
>loo f 4
>lo7 2
>144 f 7
>50
2.8
4.4
124 f 1
134 f 4
99 f 5*
128 f 4
157 f 12*
9 0 f 11
75 7*
134 2
103 t_ 1
93 f 1
148 f 7
113 3
2.9
3.1
2.4
2.9
3.5
2.2
1.9
3.1
2.5
2.3
3.3
2.7
+
*
+
Island Field
Estimated crown formation time
+
The present study data are listed as the mean and i 1 SD of the counts: all teeth were counted by three individuals with the exception of
Krapina 90 and three Hasanluspecimens (noted by *), which were counted by four observers. The Krapinaspecimens (noted by +)are crown
incomplete.Thereferencesforperikymatacountdatainclude1BromageandDean(1985),2BeynonandDean(1988),and3Deanetal.(1986).
coated with gold palladium and viewed directly by SEM. All teeth were viewed a t x20,
~ 4 0and
, x80 magnifications at 6,12 and 20
KeV. The labial surface of the entire crown
surface was recorded on Polaroid film. A
montage made up of the several pictures that
cover the entire crown surface was produced.
At least three observers counted perikymata on each tooth, and the results were
averaged. It should be pointed out that
rarely did the counters record exactly the
same number of perikymata on any one
tooth. The reasons for this are obvious:
Perikymata are found on the enamel surface
on many of the specimens in a very complicated pattern. Some perikymata are clear
and distinct, others are packed together so
densely that counting individual structures
is extremely difficult. Occasionally, perikymata are so indistinct that identification
becomes subjective. The location on the labial surface where perikymata were counted
PERIKYMATA AND CROIWN FORMATION TIMES
181
RESULTS
Modern human incisors
Each of the 12 immature modern human
specimens used in this study showed perikymata over the entire crown surface. Additionally, they showed the pattern of compactness of perikymata similar to that previously
reported in the literature for normal human
teeth (Dean, 1987b), specifically, a gradual
increase in the density of perikymata towards the CEJ.
Table 2 details the perikymata counts on
the 12 modern human incisor teeth. The
range of perikymata counts in this sample is
75-157, with a mean of 116, a median of 118,
and an SD of 25.
Neandertal incisors
Table 2 also includes the individual
perikymata counts from the prepared fossil
epoxy casts. Only Krapina 90 has a fully
formed crown and therefore is the only suitable specimen for direct comparison with
previous perikymata counts on crown complete incisors. Krapina 90, a lower right
lateral incisor, has 205 10 perikymata.
Figure 2, at x20, shows that perikymata are
present over the entire crown surface with
progressive compactness of perikymata a t
the CEJ. Perikymata were counted at x40.
Krapina 94, an upper right central incisor
about three-fourths crown complete, and
therefore without the area of greatest compactness of perikymata, possesses 144 2 7
perikymata.
*
Fig. 2. Montage ( ~ 2 0of
) the entire labial surface of
Krapina 90. Even though the tooth was excavated nearly
100 years ago and shows some abrasion, perikymata
could be viewed over the entire tooth crown. Perikymata
counts were performed at ~ 4 0 .
DISCUSSION
Neandertals
The data from Krapina 90 begin to establish a range of perikymata counts for H.S.
neanderthalensis incisors, one that must extend at least from 119 (Devil’s Tower,
Gibraltar) to 205, the present specimen.
Crown formation times calculated from
these perikymata numbers range from 2.8 to
4.4 years (according to the method of Bromage and Dean [1985]). These data provide an
interesting perspective for interpretations of
also effected the number of observable neandertal dental development.
Krapina 90 has over 80 more perikymata
perikymata. Whereas previous researchers
have reported only a single perikymata than the Gibraltar child and falls at the high
count number on fossil specimens, we be- end of the previously published range of
lived it is appropriate that perikymata perikymata counts for modern human inciranges, means, and standard deviations be sor teeth. The Krapina 90 specimen reprereported for each of the specimens. As in all sents an estimated crown formation time of
scientific work, some error is to be expected 4.4 years, well within the modern human
range for mandibular incisors by comparison
on the basis of chance alone.
182
A.E. MA" ET AL.
normal modern humans from 75 to 157 and
expand the known range of perikymata
numbers in modern human incisors from 75
to 202 (with estimated crown formation
times of 1.9 to 4.4 years, according to the
methods of Bromage and Dean [19851)).
The data from this study are notable for
their lack of correspondence to the previously published range for modern humans165-202 (Bromage and Dean, 1985twhile
including most of the published fossil hominines-57-180
perikymata (Bromage and
Dean, 1985; Dean, 1987a; Beynon and Dean,
1988).
The perikymata numbers we report here
for human incisors are similar to those found
in an independent study of modern human
incisors (i.e., a range of 111-179, N = 23)
(Bacon, 1987).
To understand the sampling differences in
perikymata counts from modern humans, it
is necessary to consider the effects of the very
small sample sizes in both studies, as well as
the potential variability in the biological feature being examined. The small samples
(N = 10, Bromage and Dean [19851;N = 12,
the present study) likely come from genetically distinct populations, given the specific
archaeological context of those used in the
present study (Bromage and Dean [19851do
not specify the provenience of their sample).
The features under comparative investigation, perikymata, are said to represent a
biological feature (crown formation time)
that is used to document one of the most
genetically (and environmentally) variable
processes known in humans: growth and
maturation. Sampling effects are to be expected and narrow ranges to be suspect.
The limitation of the previously established modern human perikymata range was
already implicit in the published analysis of
Dean et al. (1986) on the Gibraltar specimen:
119 perikymata, a number clearly outside of
the previously established human perikymata range, was found by Dean et al. to be
compatible with the range of crown formation times in modern humans. Thus the
perikymata range found in the present
study, 75-157, while previously regarded as
nonhuman or apelike, actually overlaps with
other published analyses of perikymata
Modern humans
counts leading to human crown formation
The data presented here also substantially times. This line of reasoning leads to a reasenlarge the published range of perikymata sessment of the estimates of Bromage and
numbers in modern human incisors. These Dean (1985) regarding what it is to be huresults document a range of perikymata man, at least in terms of perikymata numnumber in a small sample of apparently bers and crown formation times.
with radiographic studies from modern children. These radiographic studies have published mean and median ages of dental developmental stages for samples of modern
human children: The Krapina 90 specimen is
considerable above the median age of 3.6
years for lateral incisor crown completion
published by Fass (1969) for a sample of
modern human children and is likewise
above the means of Nolla (1960) for both the
males in her sample, 4.3 years of age for
completion of lateral incisors, and the females, 4.0 years.
Krapina 94, while not a complete crown, is
nonetheless informative. An incomplete
crown, without the area of greatest compactness of perikymata, it contains 25 more
perikymata than the fully formed incisor
crown of the Devil's Tower, Gibraltar neandertal child specimen (with 119 perikymata). Thus the Krapina 94 specimen, a
right upper central incisor, like the Devil's
Tower specimen, has 21% more perikymata
on a tooth that is 25% less complete.
Krapina 94 (a maxillary central incisor a t
about three-fourths crown complete), with
3.4 years already documented according t o
perikymata assessment, is already within
the modern human range for total crown
completion. For example, the earliest maxillary central incisor crown completion time
found by Fass (1969)in his sample of modern
human children was 2.5 years, while the
same sample had a median of 3.85 years. A
number of other radiographic studies further support the human developmental time
period of this Krapina tooth, with maxillary
central incisor crown completion ages from
different samples of children that include a
mean of 3 years (Haavikko, 1970),a mean of
4.25 years (Nolla, 1960) and ranges of 3.56.25 years at the 2 SD level (Moorrees et al.,
1963).
These data do not support the generalization made by Dean et al. (1986) regarding
relative dental developmental acceleration
in incisor crown formation times in neandertals in general. Perikymata counts on
Krapina 91, 93, and 95, all with only partially formed enamel, are presented in Table 2.
PERIKYMATA AND CROWN FORMATION TIMES
183
TABLE 3. Comparative perikymata number and perikymata-derived crown formation times for hominines'
Taxonomic group
A. afarensis
A . afrieanus
A. robustus
A . boisei
Early Homo
H. s. neanderthalensis
H. s. sapiens
Perikymata no.
Crown formation (years)
116-180
135
57-86
82-101
95-123
119-205
75-202
2.7-4.0
3.1
1.6-2.1
2.1-2.4
2.3-2.9
2.8-4.4
1.9-4.4
'These data invalidate previous claims of meaningful distinction between modern humans and all early hominines. The sample sizes
within each of these groupings make statistical tests of intergroup significant differences meaningless: All groups are significantly
different from one another (ANOVA P < 0.01). While the previous human data of Bromage and Dean (1985)cannot be tested against the
present data (only mean and range were published), they would show significant mean differences with the present human group, thus
making interhominine comparisons on these small samples questionable.
Crown formation times: Perikymata us.
radiographic evidence
Perikymata are said to represent temporal
markers of crown formation time and to gain
importance as they accurately reflect rates
of growth and maturation. The significance
of perikymata counts as data bases for the
derivation of crown fonntion times and age
at death have been recently reviewed
(Beynon and Dean, 1988; Dean, 1989). Neither the validity of perikymata counts, and
the associated estimates of both the hidden
increments (said to document time before
the perikymata reach the surface) and the
time before the onset of calcification (3
months) as predictors of crown formation
time, nor the claims that crown formation
times thus derived in the fossil hominines
could provide "good evidence of a great apelike developmental growth period" (Dean
et al., 1986:308) have ever been seriously
questioned.
The perikymata counts documented in the
present study, taken together with the inconsistencies in the previously published
analyses, make it necessary to reestablish
the range of perikymata counts in modern
humans and to clarify the implications this
has for the interpretation of data from fossil
hominines. Implicit in this analysis is an
evaluation of the presently postulated relationships between perikymata counts and
crown formation times.
The crown formation times derived from
the perikymata counts as outlined by Bromage and Dean (1985) and subsequently employed (Beynon and Dean, 1988) must be
recognized as hypothetical, based on theoretical ideas regarding dental development. As
hypotheses, they are amenable to testing. If
the proposed method for calculating crown
formation times from perikymata is accurate, the range of perikymata postulated
to represent all modern humans, together
with the perikymata-derived crown formation times, will predict the known times of
crown formation radiographically documented for modern humans. An overlap between
perikymata-derived crown formation times
and radiographically documented crown formation times was proposed by Bromage and
Dean (1985) to verify their methods.
Their claim of validation between their
perikymata numbers and modern human
incisor crown formation times deriving from
radiographic studies has two complications:
1) radiography tends to lag behind actual
calcification time by some months (e.g., Hess
et al., 1932); and 2) an age-independent factor, crown formation time as derived here, is
being compared with age-dependent data
(crown formation times from radiologic inspecition derive from the chronological ages
of living children).
Bromage and Dean (1985) did not initially
comment on the radiographic problem and
postulated that the gap between crown calci-
184
A.E. MA” ET AL.
fication time and chronological age could be
resolved by estimating the age of initial
crown calcification. They stated that central
and lower lateral incisors are “well known t o
begin calcification about 3 4 months after
birth” (1985526).Thus, to derive chronological age, 3 months were added t o the perikymata-derived crown formation times.
Employing this method, Bromage and
Dean (1985526) stated, referring to Nolla
(1960) and Fass (19691, respectively, that
“estimates for the duration of crown formation in the modern human sample (mean
number of perikymata 188, range 165-202);
mean crown formation times (4.2 years)
agree with estimates of crown formation
times calculated from longitudinal and
cross-sectional studies.”
A review of this aspect of their methodology is important in evaluating the valid relationship between perikymata and crown formation times. While Bromage and Dean
(1985) are not incorrect that their human
perikymata range agrees with estimates of
modern human crown formation times, it
should be noted that their figures substantially underrepresent both the modern human range as well as the mean ages for
incisor crown completion that they cite.
Data from Fass (1969)document a range of
crown formation times from 2.5 years (or
2.75-2.8 chronological years, when the 3
months between birth and predicted onset of
crown calcification are added on), the earliest observed crown completion of maxillary
and mandibular central as well as mandibular lateral incisors, to medians of 3 . 0 5 4 . 5
years (or 3 . 3 4 . 7 5 chronological years).
These crown completion times translate to
theoretical perikymata counts ranging from
104 to 209, with medians of 133-209 perikymata. These ranges far exceed those found
by Bromage and Dean (1985) for humans
and in fact encompass most of the fossil
hominines they investigated.
The earliest reported incisor crown formation times in modern humans are those from
this sample reported by Fass (1969), which
predicts a perikymata-count of 104, for the
children whose dentition was observed to be
completed by this age. Such a perikymata
count is 61 perikymata below the human
range according to the original propositions
of Bromage and Dean (19851, directly in the
range of most of the fossil hominines.
As it is well known that radiography underestimates actual calcification by several
months, the actual human ranges must be
even broader on the lower end than calculated above and theoretically substantially
lower perikymata counts in modern humans
than proposed by Bromage and Dean in
1985. The addition of the information from
the present study to these previously published perikymata data assists in bridging
this lower range gap.
Alternatively, data from Nolla (1960) give
mean times for incisor crown completion
ranging from 3.25 to 5.25 years (chronological ages of 3.5-5.5 years) or theoretical
perikymata counts of 143-248. This translates to perikymata counts 46 greater than
the previously published upper range. The
data from the present study add no information to this upper range gap. Again, as radiographic studies tend to lag behind actual
calcification, it is difficult to predict theoretically the extent of divergence between histological and x-ray data.
In addition t o the studies cited by Bromage
and Dean (1985) as providing data to validate their propositions of the relationship
between perikymata counts and crown formation times, published data on the ages of
incisor crown completion in samples of modern humans include the maxillary data of
Moorrees et al. (1963). Mean crown formation times range from 4.5 to 5.7 years (4.95-6
chronological years), with the 2 SD range
extending from 3.5 years (3.8 chronological
years) for the maxillary central incisor in
females to 7.15 (7.4 chronological years) for
the lateral incisors in males, or theoretical
perikymata counts ranging from 159-347.
This is again far beyond the range of perikymata numbers proposed as the modern human range by Bromage and Dean (1985);the
present data add no information to the upper
end of this range.
The data of Moorrees et al. (1963) emphasize a previously ignored aspect of incisor
development: All of the earlier perikymata
studies have treated central and lateral
(maxillary and mandibular) incisors as identical teeth. This is entirely inappropriate in
terms of the actual time of formation according to the data collected by Moorrees et al.
(1963) and deserves careful attention in future analyses. It is unlikely that all of these
teeth initiate calcification simultaneously at
3 postnatal months of age, and it is clear that
they do not complete calcification in the
same amount of time.
Thus a review of some of the studies documenting ages of incisor crown completion in
modern humans amply illustrates the lim-
*
PERIKYMATA AND CROWN FORMATION TIMES
ited nature of the human perikymata range
as originally published. This original perikymata range, translating to crown formation
times of 3 . 7 4 . 4years represents only 15%of
the known human incisor crown formation
range: The documented range for completion
of modern human incisors, both central and
lateral, is 2.5-7.2 years. The original proposition of Bromage and Dean (1985), that
their perikymata range (165-202) and the
crown formation times derived from them
(3.7-4.4years), were good estimates for the
known crown formation times of modern
humans is a considerable generalization and
overstatement. These figures actually represent only 15% of the documented time required for incisor crown formation by even a
small sample of modern human children.
The remaining 85%variation, previously unaccounted for, is an unacceptably high oversight.
Based on radiographic evidence, the theoretical range of perikymata counts for modern human incisors must be at least 104347, using the method outlined by Bromage
and Dean (1985). Thus the observed human
incisor perikymata counts found in the
present study are within theoretically predicted limits. However, the new perikymata
range, 75-202, still fails to accommodate the
known range of incisor crown completion
times. Only 61% of the upper range is estimated, or 4.4 vs. 7.2 years; and a 1.9 year
crown formation time is predicted on the
lower end, or 0.6 years below the earliest
documented crown formation times by radiographic data. Some of the latter may be
explained by the lag in radiographic evidence of actual calcification. But a 1.9-4.4
year range of crown formation time as derived from perikymata counts does not overlap with the known range of 2.5-7.2 years
documented radiographically.
185
formation times in humans by as much as 3
years. Both the original study and the
present one are based on very small sample
sizes. To accommodate all humans, a much
larger sample size of a number of populations is required; however, this would have
to expand the perikymata number nearly
150%.
Second, perikymata do not regularly
record exactly 7 days of growth. Crown formation times as derived above assume a 7
day periodicity (a circaseptan rhythm; Dean,
1987a). However, as was early noted by Asper (1916) and reported by Bromage and
Dean (19851, the number of cross striations
repeatedly found between striae of Retzius
range from 6 to 10 in different individuals.
Thus the derivation of crown formation
times from perikymata counts will vary in
accordance with the number of days (or cross
striations) between each striae. Correspondingly, significant changes in crown formation times would occur from calculations
based on 6 striaelperikymata vs. 10 striae1
perikymata, on the order of 150%.
When the fossil hominines are reassessed
by these cross-striation intervals, the crown
formation times range from 1.75 years (6
cross striations between striae) to 5.4 years
(10 cross striations between striae), for a
perikymata range of 57-180. This compares
with a modern human range of 1.7-6.1 years,
when the human perikymata range thus far
documented (75-202) is subjected to the
same recalculations. These ranges are notable for their close correspondence. However,
the lower range of the human crown formation time still underestimates the radiographic data and thus calls into question
these methods. A similar check on fossil
hominines is obviously unavailable, but the
human data should caution against precocious assumptions regarding these specimens.
Perikymata as predictors of human
Third, chronological age derived from
crown formation
perikymata counts cannot be as simple as
Based on this lack of correspondence be- outlined. It is highly probably (if not certain)
tween predicted and observed crown forma- that there is an error in the temporal estimation times, a reassessment of the proposed tion factors in the method as originally properikymatdcrown formation time method as posed, that is, with the 6 months of hidden
proposed is in order. A number of sources of increments andlor the time of onset of calcierror may account for this failure of periky- fication. For example, the proposed method
mata-derived crown formation times t o pre- oversimplifies dental growth and development in failing to account for variability in
dict known human crown formation times.
First, it may be that the range of human the time of onset of incisor calcification. The
perikymata is still too narrow. The data from data of Moorrees et al. (1963) cited above
the present study extend the previously pub- document the later develoment of the lateral
lished range, but still underestimate crown incisors in the children in their sample. Thus
186
A.E. MA” ET AL.
a distinction in these two teeth should be
made in the timing of their respective development.
Furthermore, based on the well-documented interindividual variability in maturational rate, it is not possible that all humans begin calcification of their central and
lateral incisors a t precisely 3 postnatal
months.of age, as this method assumes. This
faulty generalization would lead to an error
in the estimation of chronological age, which
would over- and, more importantly, underestimate both crown formation times and age
a t death by a substantial amount. This may
be an important source of crown formation
time underestimates calculated from perikymata counts.
Finally, perikymata, striae of Retzius, and
cross striations may not represent an accurate maturation clock as proposed. Enamel
matrix formation and calcification is an extremely complex biological process resulting
in a structure, dental enamel, that itself is
three-dimensionally complex. This threedimensional aspect of enamel is thus far
unappreciated in the “counting” approaches
presently employed in anthropology. Cross
striations may, or may not, be daily growth
increments. In contrast to previous publications, this is not yet an experimental fact.
While it is assumed to be the case (e.g.,
Osborn and Ten Cate, 19831, data for this
derive from inferential analysis of related
structures (e.g., Rimes, 1986) and from calculations of observable cross striations between tetracycline markers in developing
teeth, the tetracycline following calcification, not enamel rod growth (e.g., Okada,
1943; Bromage, 1989). In this context, it
should be noted that some recent histological
research has suggested that these observable
phenomena may be morphological structures, artifacts of specimen preparation, or
other epiphenomena not formed in anytimeordered predictable sequence (Weber and
Ashrafi, 1979; Weber et al., 1974; Warshawsky and Bai, 1983; Warshawsky, 1989).
A consideration of the data presented
here, and a review of the methods surrounding the use of perikymata, require a reassessment of the crown formation times and
ages at death of the fossil hominines published in recent years, as well as the conclusions drawn from these data. Whether
viewed from the perspective of perikymata
counts or the crown formation times derived
from them, there is no valid evidence to
distinguish total incisor crown formation
times in modern humans from those in the
fossil hominine sample (Table 3). Taken together with the recently published data from
Dean (1989) on the timing of ape incisor
crown calcification, previous conclusions regarding early hominines as “ape-like in
growth and development” must be reconsidered.
Moreover, the mismatch between incisor
crown formation times derived from radiographic data and the present data base on
perikymata raises questions on the validity
of the method described and on the accuracy
of chronological age assessment.
CONCLUSION
The data presented here document a range
of perikymata number in H . sapiens that
includes those observed in many australopithecines, including early Homo, and the
single neandertal previously reported. Employing these perikymata numbers as originally proposed (Bromage and Dean, 1985),
incisor crown formation times in early hominines and modern humans cannot be reliably distinguished. Taken together with recent reports of evidence on ape incisor crown
formation times, we conclude that previous
claims based on these data for “good evidence
of a great-ape-like developmental growth period (Dean et al., 1986:308) in early hominines are invalid. The data presented here
support a more critical evaluation of these
methods and suggest that it is premature to
base major biological reconstructions on
these approaches prior t o substantial further research.
Furthermore, we find that calculation of
incisor crown formation times from the
present modern human perikymata range
does not predict the known range of modern
human incisor calcification times, but consistently underestimates modern humans. It
is likely to do the same for the fossil specimens. Potential error in this method leads t o
the conclusion that it is inappropriate with
the present methodology to age fossil specimens using perikymata counts. With attention to the problems noted, the use of perikymata, striae of Retzius, and cross striation
data may, assuming they are shown to be
time-related structures, or may not be able to
offer us a range of ages forindividual teeth in
the future. The original precision of the
method that published ages at death to the
tenths of a year in the absence of any standard deviation or range is invalid.
PERIKYMATA AND CROIWN FORMATION TIMES
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