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Molar Wear Stages in Theropithecus gelada
W.E. Meikle
Questions of paleoecology are among the most chal-
lenging and difficult to deal with of those confronting
the student of the past. Once fossil remains have been
recovered, once basic taxonomic assignments have
been made and possible phylogenetic lines traced, it
remains to attempt a synthesis of all the available
geological and paleontological evidence, guided by
principles derived from contemporary studies of ecol-
ogy and environments. For such an analysis of the past
it becomes important to have data on factors such as
the number of individuals preserved per species, por-
tion of the body preserved per individual, and popula-
tion parameters such as the sex and age of preserved
specimens. These are features which are of relatively
little importance during the initial phase of taxonomic
description, but which become vital for more complex
analyses. Interest in taphonomy, the study of all the
processes which intervene between the living and fossil
assemblages, is currently increasing in paleoan-
thropology (Behrensmeyer 1975, 1976). As detailed
cases are studied and principles established,
taphonomic considerations will undoubtedly be more
explicitly involved in interpretations of all kinds of
fossil occurrences. It will probably never be possible to
eliminate the effects of all the factors which have acted
to distort our view of the past. The best we can hope for
is to minimize these. To do this we must extract as
much information as possible from the faunal and
floral remnants which have been preserved and from
their geological context.
In recent years there has been a noticeable increase
in interest in and publication on fossil Cer-
copithecidae (Delson 1975; Jolly 1972; Freedman
1976; Leakey and Leakey 1973a, 1973b). In African
Plio-Pleistocene fossil localities, monkeys are fre-
quently recovered. Probably the largest collection of
fossil cercopithecids is that from the Omo Group de-
posits of southwestern Ethiopia (Eck 1976). At this
writing the Omo fossil catalog maintained in Dr. F.C.
Howell's laboratory contains 6363 cercopithecid en-
tries. Of these, 2563 have been identified as
Theropithecus. With such a large sample, it seemed
reasonable to attempt a detailed study of the number
of individuals present and the nature of their remains
throughout a span of about two million years (Meikle,
in progress).
As a preliminary to this projected study of Plio-
Pleistocene fossil Theropithecus (Simopithecus) from the
Omo Valley, an attempt has been made to construct a
relative scale of molariform tooth wear for the living
gelada monkey, Theropithecus gelada. Criteria of age
determination in fossil mammals are limited to
symphyseal union and sutural closure and to dental
eruption, replacement, and wear. Since fossil collec-
tions rarely include associated body parts and so fre-
quently consist largely of jaws and teeth, attempts to
assign relative or absolute ages to fossils will almost
always be confined to the use of dental events.
Bramblett (1969) has established an age scale based
on tooth wear for Papio, following the method used for
Homo sapiens by Miles (1963; see also Zuhrt 1956;
Mann (1975) has assigned ages to South African aus-
tralopithecines in a similar way). This method consists
essentially of the following: for all applicable members
of a sample, observe the state of wear or dentine expo-
sure of each molar of a toothrow which has a more
posterior molar just erupting. If ages of eruption and
occlusion are known, one can correlate the "functional
age" of a tooth, the length of time it has been in use,
with the amount of wear which has occurred on its
occlusal surface. By reversing this procedure, indi-
viduals of unknown age can be assigned an age based
on the amount of wear on their teeth. For example,
assume that, in humans, the Ml comes into occlusion at
age six years, the M2 at twelve years, and the M3 at
eighteen years. At twelve,just as the M2 becomes func-
tional, Ml will have been in use for six years. At eigh-
teen, M3 will be erupting, M2 will have a functional age
of six years, and Ml's functional age is twelve. There-
fore, if rates of wear are constant, the M2 should now
have approximately the same wear pattern as M 1 did
when the per5on was twelve. Similarly, M3 should
reach this "six-year" pattern when the individual is
about twenty-four, at which time M2 (functional age of
twelve) will be as worn asMI at eighteen, and MI will
have a functional age of eighteen. By extrapolating it is
possible to extend such a relative system well into
adulthood, where such criteria as age of eruption or
replacement of teeth are no longer useful. See Miles
(1963) for more details and complications; he suggests
that, in Homo sapiens at least, the rate of wear is not
exactly the same for all three molars. However, Mann
(1975:51) did not encounter this problem in aus-
tralopithecine samples.
The scale devised and illustrated by Bramblett
(1969) for Papio cynocephalus works on exactly the same
principle. However, since the teeth, especially the
molariform teeth, of Theropithecus are quite distinct
from those of Papio (Jolly 1972), it is not possible to use
the same scale for this genus. The unique morphology
of Theropithecus molars has been described by Jolly
(1972) and Delson (1975), and does not need to be
repeated here. Because of such features as greater
relative crown height and columnar cusps, molars of
Theropithecus display a wear pattern which differs con-
siderably from that of Papio, although certain elements
are shared by the patterns of both genera.
As pointed out by Jolly (1970), there is a relatively
steep wear gradient in the molars of Theropithecus
compared, for instance, to Papio. This means that at
any given age, more anterior molars show heavier
wear relative to that of more posterior teeth than in
other cercopithecines. In addition, there are clear dif-
ferences in detail of molar wear pattern between Papio
and Theropithecus. In Papio, wear at first heavily affects
the buccal half of lower molars (lingual of uppers),
while only gradually affecting the opposite half. Only
after considerable wear is there dentine exposure a-
cross an entire loph(id); very shortly after this stage,
surface detail of the tooth is obliterated, leaving a
dentine "lake" surrounded by an enamel rim (see
drawing in Bramblett 1969:163). In Theropithecus, on
the other hand, attrition soon produces more even
dentine exposure across the width of the loph(id)s.
This might be caused by a greater horizontal compo-
nent in the Theropithecus chewing cycle, compared to
Papio. Because of the height of Theropithecus cusps and
the depth of the clefts and basins, the Theropithecus
molar retains occlusal surface features longer than
Papio molars. This is reflected in the amount of time
during which dentine "lakes" exist separately on the
loph(id)s and in the persistence of the "hourglass"-
figure once it develops (see below). Jolly (1972:20)
points out that T. (Simopithecus) is also distinguished by
the length of this stage. Wear scales based on Papio
cannot help but give inconsistent ages for Theropithecus
under these conditions. Therefore, a new scale for
Theropithecus has been constructed.
Since upper and lower molars of Theropithecus are in
some ways close to being mirror images of each other
(Jolly 1972) and since both members of an occluding
pair have been consistently found to display similar
amounts of wear, quite similar wear stages can be de-
fined to describe upper and lower dentitions. These
will be outlined separately below. The stages described
here were developed by study of the dentition of nine
Theropithecus gelada now in Dr. F.C. Howell's labora-
tory at the University of California, Berkeley. Six of
these animals had been wild-shot in Ethiopia; the other
three died in zoos. They range from juvenile (M2 just
erupting; dC, dP3-4 still present) to adult; five are
male and four are female. Although this sample is
small, the pattern of molar wear observed is so consis-
tent in all these animals that I have considerable confi-
dence in its generality for T. gelada. In addition, the
observed wear patterns have been found to be similar
to those in the much larger sample of fossil
Theropithecus from the Omo. Terminology follows Del-
son (1975).
Molar Wear Stages (see Figure 1)
Lower dentition: Stage 1. Tooth unworn. Used for
molars which are unerupted or otherwise not yet in
occlusion. Once full occlusion- is reached, the tooth
does not remain at this stage very long.
a           b          c           d          e
f           9           h          i          j
0           2cm
Figure 1. Occlusal views of T. gelada molars as examples of wear stages. (a)-(e) are
upper molars, (f)-(j) are lowers. Stage 2:(a), (f); Stage 3:(b), (g); Stage 4:(c), (h);
Stage 5:(d), (i); Stage 6:(e),(j). Enamel shown in white and and dentine in black. In
Stage 2, enamel facets are indicated by encircled white areas. (Stages 1, 7, and 8 not
illustrated)
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Stage 2. "Facets" start to appear on the enamel;
however, no part of the crown is worn enough to
expose the underlying dentine. The enamel facets are
best seen by holding the specimen under a good light
and slowly turning it in various directions. The facets
are shiny, reflecting surfaces, most of which appear at
or towards the top of the columnar molar cusps.
Further use and wear on these highest points on the
tooth crown result in the next stage.
Stage 3. Circles of dentine appear on the tooth
crown at the cusp apices. The buccal cusps are affected
before the lingual ones. The protoconid is the first
cusp with dentine exposure, and throughout this stage
it continues to have the largest area of dentine. Usually
the hypoconid is the second cusp affected and has the
second largest dentine area. The metaconid is the last
cusp affected. During this stage the size of all the
dentine exposures increases, but there are no connec-
tions between them.
Stage 4. Total dentine area exposed is increased.
The buccal and lingual areas of dentine start to merge
along the metalophid and hypolophid, although
dentine exposure is still noticeably greater on the buc-
cal half of each lophid. Because of the relative height
of the cusps and depth of the enamel-covered talonid
basin and median buccal notch, there is no dentine
contact between these mesial and distal dentine areas.
Stage 5. The dentine areas exposed form larger and
more symmetrical "lakes", with more even contribu-
tions from buccal and lingual halves of each lophid.
There is the beginning of mesio-distal dentine contact
between the metalophid and hypolophid "lakes".
Wear now starts to remove the enamel surface of the
deepest parts of the talonid basin and of the median
buccal cleft. Also during this stage, interstitial wear has
progressed enough to result in the loss of the enamel
margin at the mesial and distal borders of the tooth
(mesially only in M3). At first this dentine exposure
along the margin occurs over only a small area, but it
soon spreads buccally and lingually as interstitial wear
continues. When viewed from above, teeth in this stage
of wear display a very narrow-waisted "figure-eight"
or "hourglass" of dentine. The removal of enamel rims
by interstitial wear results in direct contact between
dentine and the mesial and/or distal margins of
neighboring teeth.
Stage 6. The hourglass-shaped dentine surface of
the tooth remains well-defined by the enamel floors of
the talonid basin and median buccal cleft. However,
the waist becomes steadily wider as these floors are
worn away. Interstitial wear also continues, widening
the area of dentine contact with neighboring teeth
initiated in the previous stage. Often teeth in this stage
will have broad dentine-dentine contact with their
neighbors because of this wear.
Stage 7. At this stage all or almost all crown detail of
the molar has been removed by wear. The tooth is
worn down almost to its neck. There is a simple,planar
dentine area filling the occlusal surface. This is
bounded by a narrow enamel rim lingually and buc-
cally; mesially and distally, interstitial wear has re-
sulted in very broad dentine exposure, and little
enamel remains along these rims. (This stage was ap-
proached, but not reached, in the T. gelada sample
studied. It has been observed in fossil Theropithecus
specimens.)
Stage 8. The entire crown is worn away. No enamel
remains. Wear is progressing onto the roots. (This
extreme stage has been found in only one fossil speci-
men so far. It was not found in the T. gelada sample.)
Upper dentition (Since upper and lower wear stages
are similar, these will be discussed in less detail):
Stage 1. Tooth unworn.
Stage 2. Enamel facets appear at or near cusp apices.
Stage 3. Dentine circles appear, first on the lingual
cusps and then on the buccal ones. The protocone is
affected first and at any time has the largest dentine
exposure. Paracone and hypocone exposures are
often about the same size. The metacone has the small-
est dentine area. No connections exist during this stage
between any of these dentine areas.
Stage 4. There is a dentine connection between pro-
tocone and paracone along the paraloph. Dentine ex-
posure on the hypocone is noticeably greater than that
on the metacone, and during this stage these areas
start to merge along the metaloph. Mesio-distal con-
nection between the dentine of these lophs is pre-
vented by the enamel of the trigon basin and median
lingual notch. Interstitial wear has reduced enamel
thickness mesially and distally, but there is still a com-
plete enamel rim around the tooth margins.
Stage 5. Both paraloph and metaloph are worn
through to dentine. There is the beginning of dentine
contact between the lophs, and a narrow-waisted
hourglass of dentine is established. Interstitial wear
breaks through the enamel of at least the mesial border
of the tooth, if not both mesial and distal borders.
Stage 6. The hourglass waist widens as wear removes
the enamel of the median lingual and buccal clefts.
Interstitial wear continues.
Stage 7. Wear nears the tooth neck, leaving little
surface detail. An enamel rim remains only along the
lingual and buccal tooth margins.
Stage 8. The crown is worn away entirely. (As in the
lower teeth, Stages 7 and 8 were not observed in the T.
gelada sample.)
Application of these stages to assign ages to indi-
vidual specimens depends on a kind of seriation by
amount of wear. Functional ages are deduced by corre-
lation with stage attained as more posterior molars
come into occlusion. Assignment of "absolute ages"
thus depends crucially on accurate knowledge of the
interval between tooth eruptions. Obviously this in-
formation will never be available for fossil species. In
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fact, I know of no published data on age of tooth
eruption or interval between molar eruptions in T.
gelada. The most useful substitute is probably data for
Papio. It seems likely that age of eruption is correlated
with body size and with length of developmental
periods. Comparisons of Papio and Macaca show that
the teeth, deciduous and permanent, of the former
consistently come into occlusion later than those of the
latter (Siegel and Sciulli 1973; compare also Reed 1967
and Freedman 1962 with Hurme 1960). Living
geladas are within the considerable range of variation
in size found in Papio. Many fossil Theropithecus speci-
mens are quite large, but as a group they overlap the
top of the Papio size range. Becuse of this lack of data,
ages assigned to Theropithecus by this method are not
truly "absolute." However, I am confident that they
can be of the right order of magnitude. In any case,
relative age attained at death can be accurately calcu-
lated.
There have been relatively few attempts to apply
demographic methods of analysis to fossil mammal
populations (Clark and Guensburg 1970; Kurte'n
1953, 1958; Van Valen 1964a). Even if detailed studies
are prevented by the incompleteness or scatter
(through time and space) of specimens from the Omo,
the attempt may still prove worthwhile. The nature of
the fossil assemblages may provide clues as to how they
accumulated, and thus to paleoenvironments. Taken
in conjunction with detiled geological studies and with
the evidence of other fossil remains, interpretations of
associations or communities of the past may be possi-
ble. This, in turn, contributes to our understanding of
the processes and contexts of human evolution. Previ-
ous attempts to reconstruct paleocommunities based
on abundance and preservation of fossils (Shotwell
1955, 1958, 1963) have been seriously criticized be-
cause of their implicit assumptions (Voorhies 1969,
1970; Clark, Beerbower, and Kietzke 1967). There do
not seem to have been very many efforts to derive
information related to populations from the mamma-
lian fossil record, or to establish limits on what can be
derived (Guthrie 1967, 1968; Van Valen 1964b; Wolff
1973).
Of course, data from the fossil monkeys alone will
clearly be insufficient to answer many questions. They
form, after all, only a portion of the total preserved
fauna from any site. However, it seems possible that a
detailed study of this portion, which can be conven-
iently compared with ecological studies of living
monkey species (Altmann and Altmann 1970), may
provide significant information when combined with
other available paleontological and geological evi-
dence. This is only one step towards a better under-
standing of the past, but it may clarify what is possible
and what kinds of inferences it is still impossible to
draw in reconstructing the puzzles of evolution.
Acknowledgment
I thank Dr. F.C. Howell for access to specimens. The
figure was drawn by J. Ogden.
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