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Arch form tooth size and occlusomandibular kinesis in the Ceboidea.

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Arch Form, Tooth Size, and Occlusomandibular
Kinesis in the Ceboidea
M. R. ZINGESER
Oregon Regional Primate Research Center, 505 N.W. 185th Avenue,
Beuverton, Oregon 97005
KEY WORDS Teeth . Dental occlusion . Mastication . Feeding
behavior Evolution . Ceboidea.
ABSTRACT
Correlations between dental morphology, arch configuration,
and jaw movement patterns were quantitatively investigated in 23 ceboid species
to elucidate integrative aspects of occlusal functional anatomy in a n adaptive
and evolutionary context. Differential maxillary-mandibular arch widths are
primary in guiding lateral jaw movements. These movements are characterized
according to their associated condylar shifts as either predominantly translatory
or rotational. Predominantly translatory movements result from peripheral contact relationships between maxillary arches which are considerably wider posteriorly than their opposing mandibular arches. The greatest degree of mandibular movement is i n the molar region in functional association with wide “primitive”
maxillary molars, narrow mandibular molars, constricted maxillary intercanine
widths, and narrow maxillary incisors. In contrast, predominantly rotational
masticatory jaw movements result from differential arch widths which are greatest in the maxillary canine and incisor regions. Here most jaw movement is in
the anterior segment and this is reflected in small maxillary-mandibular molar
width differences, a high degree of premolarization, wide-set maxillary canine
teeth, and wide maxillary incisors. Possible selectional factors i n the putative
evolution of rotational predominance in mastication from the more primitive
translatory pattern are discussed
Comparative studies of integrative aspects of the functional anatomy of teeth
and jaws have considerably enhanced our
understanding of the masticatory system.
In this regard, the Ceboidea constitute a
valuable adaptive array. These New World
primates of Central and South America are
a vaned group revealing many traits of
common descent. Diversity is very much i n
evidence i n tooth and jaw anatomy and
these variations on a common theme afford interesting insights into selectional
processes and evolutionary dynamics. In
addition, the remarkable parallelisms between the New World and Old World primates are potential sources for elucidating
general principles.
A brief review of the literature and associated terminology will clarify subsequent
points to be considered. The concept of
centric occlusion as applied i n this paper
is defined as that position of the articulating dentition which provides maximum
AM. J . PHYS. ANTHROP.,45: 317-330
intercuspation. In higher primates and
other mammals with fused mandibular
symphyses, centric occlusion corresponds
with centric relation, i.e. the mandible is
simultaneously positioned symmetrically
with reference to the midsaggital plane.
In these isognathic forms (Crompton and
Hiiemae, ’70), centric occlusion serves as
a reference position separating buccal and
lingual phase jaw movements. These functional jaw movements a s adduced from
tooth wear facets, were first described in a
series of papers by Butler (’52, ’73),Butler
and Mills (’59), and Mills (’63, ’67, ’73).
Additional insights into masticatory function have been obtained from cinematographic observations of living animals.
Most recently, Crompton and Hiiemae (’70)
used these techniques to study Didelphis,
and Hiiemae and Kay (’73) and Kay and
I Publication No. 869 of the Oregon Regional Primate
Research Center, supported in part by Grant RR00163-17
of the National Institutes of Health.
317
318
M. R . ZINGESER
IIiieniae ('74) used tooth functional analysis and cineradiography to analyze mastication in some primates.
According to Hiieniae arid Kay ('73), the
masticatory cycle is either puncture-crushing or chewing. In the former, food is reduced by pulping and crushing between
the teeth without tooth-to-tooth contact
after which the chewing cycle with its
tooth-to-tooth contact occurs in two scquential phases. During Phase I (the buccal
phasc o€ Butler and Mills, '59), the buccal
cusps of the lower teeth on the active
sidc are brought into initial contact with
the buccal cusps of the uppers (figs. l A ,
2A) and then move into centric occlusion
(figs. l B , 2B). The lower molars then move
across the upper molars duiing Phase I1
which is similar to but not identical with
the lingual phase of Butler and Mills ('59).
Lingual phase movements are not easily
measured and are often ill-defined and
quite variable. In this paper, I focus upon
determinants of and variations in buccal
phase movements and their canine complex counterparts because associated form
and functional relationships uniquely lend
themselves to quantification and analysis
in terms of whole arch configuration and
tooth size gradients. Shared characteristics
common to buccal phase and canjne shear
and honing function suggest a n encompassing descriptive term p e T i p h e r d occlzi.som a n dib ul or mou erne nts since peripher a1
elements of both arches are opposed through
mandibular function. These essentially lateral movements lack a propalinal component during (peripheral) incisor functioning. Transverse incisal contact shear in
these edge-to-edge or underbite teeth involves the same masticatory movements
that characterize the post-canine occlusion.
Peripheral mastication is the p r i m a l WLOprimitively associated with shear
and ingestion and considerably antedating
the appearance of lingual or Phase IT function with its associated distal and lingual
grinding and crushing talon and talonid
clcrnents (Patterson, '56: Crompton arid
Iliiemae. '70; Hiiemae and Kay, '73; hlills.
' 7 3 ; arid Butler, '73). In this connection,
the canine complex with its shearing irnd
honing functions is clearly a buccal phase
homolog, with primitive relationships between paracone, protoconid, and talonid
rim. therian dental components with built-
in reciprocal honing capabilities not confined to the canine region (Zingeser, '68c,
'69).
The buccal phase is commonly described
as associated with ipsilateral condylar rotation and contralateral distal movement
as the jaw shifts from buccal to centric
position (fig. 3A). Together with a slight
translatory component (Bennett movement),
i t describes the human condition. That this
pattern is riot ubiquitous among nonhuman
primates is now recognized (Mills, '63, '67;
Kay and Hiiemae, '74). Our studies of ceboid teeth and jaws confirm that the rotational modality, universally present in maxillary canine tooth shear and honing, is
wide-spread in mastication. In the Ceboidea,
however, condylar rotation is often combined with and frequently subsidiary to
translatory mandibular movements (fig. 4).
The morphological basis for these variations
in peripheral occlusomandibular movements, their functional arid adaptive significance, and the putative selectional factors in their evolution i n the Ceboidea are
examined.
MATERIALS. RATlONALE AND METHODS
Skulls of mature animals with relatively
unworn teeth and intact dental arches
were selected. The primary- source was the
U.S. National Museum of Natural History;
supplementary material was obtained from
the Museum of Comparative Zoology, The
British Museum of Natural History, the
Field Museum of Natural History, arid the
Oregon Regional Primate Research Center.
In all, 106 specimens representing 12 genera and 23 species were measured. These
include all of the Ceboid genera except
Callithrzx, Callimico, and Cricrijuo (Napier
and Napier, '67).
Whenever possible, I tricd to achieve a n
equal sex distribution. The procedures for
securing tooth measurements were those
previously described by Zingeser ('67). The
odontometric data are more extensive than
the arch width measurements because (1)
unilaterally missing teeth, which preclude
accurate arch width estimates, suffice for
tooth measurements, and (2) the odontometric information is supplemented by previously published data (Zingeser, '67, '73).
Because of the paucity of specimens i n
many genera, I used pooled samples which
were heterogenous for sex and species but
319
CEBOID OCCLUSOMANDIBULAR KINESIS
as far as possible balanced in numbers for
both, to offset metric bias (table 1).
Masticatory excursive movements are
guided by contact relationships between
opposing reciprocally functioning teeth. At
the initiation of buccal phase contact, both
maxillary and mandibular buccal cusps
occlude in almost parallel fashion throughout the buccal row (figs. l A , 2A) to maximize shear. Exceptions to uniformity are
most often seen in P2, a tooth which tends
toward caniniform morphology (fig. lB),
and the terminal molars which are morphometrically variable. In contrast with
these uniform buccal phase relationships,
considerable intergeneric variation is observed in the degree of movement at different sites along the tooth row which occurs as the mandibular teeth shift to centric
occlusion (figs. l B , 2B). Variations in degree and direction of mandibular and associated condylar shifts needed to accommodate this buccal phase movement from
initial contact to centric depend upon maxillary and mandibular arch peripheral
width differences. These factors guide the
mandible in varying degrees of condylar
rotational and/or transverse translatory
buccal phase movement (figs. 3, 4).
In documenting peripheral arch width
differences in a functionally meaningful
manner, I observed the following characteristics. At the beginning of buccal phase
mastication, the lingual tips of the maxillary premolar paracones (buccal cusps)
and molar paracones (mesiobuccal cusps)
rest against the buccal aspects of their respective mandibular articulating teeth at
the talonid rim ridges distal to the protoconids (mandibular premolar buccal cusps
and molar mesiobuccal cusps (figs. lA, 2A).
Since this region is the hypoflexid in mandibular molars, the homologous region of
the mandibular premolars and canine teeth
will also be called hypoflexid i n this paper
for the sake of conciseness (figs. 5B, 6B).
As the mandibular teeth shift into centric
occlusion, the amount of buccal phase
traverse equals the distance from the lingual paracone tip to corresponding hypo-
TABLE 1
Ceboid slriLll mnterinl
Key
1
2
3
4
5
6
7
8
9
10
11
12
Species
Crbuelln p y q m n r a
Sugurn.lcs 1
Marikina geoffroyi
Mystax nigrtcolis
Leontocebus geofli-oyi
Midus m i d n s
Suguinus s p p
Oedipomidas geoffroyi
Oedipomidas o e d i p u s
Saimiri sciureus
S a i m i r i orstedti
Saimiri s p p
Aotu s t n ue rgcc tic 5
Callicebus ornutiis
Callicebus torqiiatzts
Callicebus s p p
Pithrcin p i t h r c m
Pithrcici monachn
Chrropotea sutunus
Cebus capticinus
Cebus upella
Ateles punisciis
Ateles geoffroyi
Ateles b e l z e h u t h
Lagothrix lagotricha
Brachytrles arachnoides
Alouatta caraya
M
F
M/ F
5
1
5
2
1
1
3
1
2
1
1
1
1
2
1
5
1
6
2
2
1
1
1
1
1
5
2
2
10
2
2
1
5
1
4
2
2
5
Total
11
4
1
1
1
1
1
1
3
2
2
8
8
2(+4)
4
2
2
8(+16)
3
3 (+2)
4(+22)
2
5
5 (+2)
12 ( 38)
+
The numbers i n brackets ( + n) represent specimens which yield odontometric data i n excess of those
supplying both odontometric and arch width data.
1 Taxa subsumed under Saguinus after Napier and Napier ('67).The twelve listed genera are keyed
to table 2 and figure 7.
320
M. R. ZINGESER
Fig. 1 Aloutrttn rrcmyrr. male. A . Buccal phase. peripheral contact. B. Centric occlusion. P2 (maxillary first premolar) is caniiiiform, set out laterally and functions with
the canine tooth. Greatest shift from buccal phase to centric occurs i n the posterior
arch.
Fig. 2 Crbrcs crrptcciwits, female. A . Buccal phase peripheral contact. B. Centric
orclusion. Compare with figure 1 for relative posteroanterior shift at P3 a n d M1 i n
buccal phase to centric n ~ o v e m e n t sGreatest
.
shift occurs anteriorly in Cebrts.
flexid contact region (figs. l B , 2B). This
distance is difficult to measure, but i t can
he closely approximated by halving the
transparacone and transhypoflexid differences:
Buccal Shift
=
transparacone width minus
transhypoflexid width
2
When maxillary widths (1/2 transparacone, figs. 5A, 6 A ) are plotted against man-
dibular widths (1/2 transhypoflexid, figs.
5B, 6B), a diagrammatic representation of
buccal shifts at varying sites along the
tooth row is obtained together with a crude
but informative approximation of relative
buccal arch forms (figs. 9, 10, 11). Relative
arch widths arid hence mandibular shifts
can be usefully expressed by computing
the posterior shift as a percentage of the
anterior shift. The extreme ends of' the
CEBOTD O C C L U S O M A N D I B U L A R K I N E S I S
tooth row are avoided. P 3 is chosen in preference to the caniniform P2, and M (1-t)
in preference to the variable Mt. In the
latter designation, variations in molar number among the Ceboidea necessitate using
- Maxilla
__ Mandible - Centric
Mandible -Peripheral Shift
Fig. 3 Schematic diagram of extreme peripheral shift patterns which indicate differential arch
form determinants. A. Rotational. B . Translatory.
32 1
t for the terminal molar. M 0-t) is the penultimate molar. Thus Mt is M3 in the
Ceboidea except for most of the Callitrichidae which lack this tooth. Percentage buccal shift at posterior versus anterior arch
sites are ranked and listed i n table 2A.
The variations in buccal shift patterns
expressed by posteroanterior shift percentages can be characterized as predominantly
translatory when the molar shift equals or
exceeds the premolar shift, and predominantly rotational when the buccal shift at
the premolar site exceeds the molar shift.
These designations accord with the observed condylar movements which occur
concurrently with the variations in buccal
shift in the articulated skull specjmens
(fig. 4).
These two peripheral occlusom andibular
functional categories are schematized in
figure 3 in terms of total arch form determinants, including incisor and canine segments as well as the post-canine dentition.
Fig. 4 B a s a l view of male A . curnyu articulated skull in various functional positions. i Ipsilateral
and c contralateral sides. A . Extreme buccal phase occlusion with arrows indicating approximate shift
directions to and from centric. Largely translatory. B . Extreme canine honing position with arrows indicating approximate shift directions to and from centric. Mostly rotational but with a translatory component. Compare with figures 1, 3 , 5 , and 9 .
322
M . R. ZINGESER
Fig. 5 Occlusal view, A . c n r o y o , male. A. Maxilla, transparacone widths at three sites identified by roman numerals a n d keyed to functionally related widths in B. Mandible, transhypoflexid widths, arabic numerals.
Fig. 6 Occlusal view, C. c . u p i u . i n u s , female. A. Maxilla transparacone widths keyed to
functionally related widths in B . Mandible at transhypofiexids. Compare with figure 5 for arch
form a n d tooth size differences.
A comparjson of figures 3A,B suggests a
dichotomy of form-functional relationships.
In the rotational category (fig. 3A), one
would expect to find wide-set maxillary
canine teeth with large honing ranges, relatively broad maxillary incisors (mesiodistally), and premolars (Buccolingually) which
are compatible i n form and function with
the wide anterior maxillary arch segment
and associated large anterior range of jaw
movement. Because of the small range of
mandibular molar traverse, relatively slight
differences i n buccolingual size are to be
expected between opposing molars. In con-
trast, in the translatory modality (fig. 3R),
the greatest range of motion is in the molar
region; accordingly the maxillary molars
should be appreciably wider than the mandibular molars to accommodate to the large
posterior translatory traverse. The relatively narrow anterior maxillary arch form
and decreased anterior functional range
suggest a narrower intercanine width with
an associated small honing range and
smaller buccolingual maxillary premolar
and mesiodistal maxillary incisor sizes.
To test these premises, I measured the
canine honing shifts, which are computed
CEBOID OCCLUSOMANDIBULAR KINESIS
323
TABLE 2
Peripheral occlusomartdibular shijt charmteristics
Key
Species
A
B
C
125.0
109.0
100.0
100.0
100.0
157.1
234.8
163.6
236.4
118.1
76.7
83.6
73.9
69.4
70.8
44.4 Predominantly translatory
76.2 “Primitive” maxillary molars very wide
compared with mandibular molars.
69.8
67.6
Rami often expanded. C honing
54.2
range reduced. Small maxillary
D
Functional categories
12
10
5
2
1
Alotiatta ctirtiyrc
Lrryothrix s p p .
Callicehiis s p p .
Saguinirs s p p .
Cebuella pygmnecc
11
4
Brachyteles
a rac hnoides
Aotus trivirgcitiis
96.8
92.9
117.9
138.5
69.9
73.8
53.9
97.4
9
3
7
8
6
Atcles s p p .
Sainziri s p p .
Clriropotrs sntanas
Crbus s p p .
Pithecia s p p .
80.0
80.0
64.7
63.2
62.5
293.8
225.0
581.8
366.7
450.0
88.4
69.2
86.7
83.3
81.8
90.9 Predominantly rotational
77.1 Maxillary and mandibular molars approach each other in widths. Rami
95.3
“normative.” Premolarization
88.0
marked. C honing range large. Max75.6
incisors. Posterior arch form and
functional dominance.
illary incisor complex well-developed.
Anterior arch form and functional
dominance .
Species are numerically keyed to figure 7. Peripheral shift inetrics are based upon differential maxillary-mandibular arch widths. A. Shift at penultimate molar region M (1-t) as a percentage of shift at second premolar region
P3. These values are ranked and serve to differentiate translatory and rotational functional categories. B. Shift at
the canine region C as percentage of shift at penultimate molar region M (1-t). Large honing ranges are associated
with predominantly rotational masticatory patterns. C. Penultimate mandibular molar buccolingual width as a
Percentage Of Penultimate maxillary molar width, Relative maxillary-mandibular molar widths reflect differences
between the two functional categories. D. Maxillary central incisur width (mesiodistal) as a percentage of maxillary penultimate molar width (buccolingual). Maxillary incisor size differences in the two categories correlate with
occlusomandibular functional pattern differences.
like the buccal shifts (figs. 5, 6) against
their corresponding molar buccal shifts:
C/M (l-t) x 100. These values are listed
in table 2B. Relative molar buccolingual
widths were determined:
and are listed in table 2C. Finally, maxillary incisor widths are measured against
corresoondine molar widths:
Gesiodistal 11
Buccolingual M(1 1)
x 100
and these are listed in table 2D.
The buccolingual dimensions of M (1-t)
were plotted against the corresponding
measurements of P3 on logarithmic coordinates, and the results were analyzed for
features that were consistent with buccal
shift ratios which derive from arch width
differential characteristics (table 2A).
Trend lines were then extrapolated (fig. 7).
The exigencies of the sample size required
that the data be pooled and balanced with
regard to sex, but the need to investigate
possible sex-related dimorphism ir. occlusomandibular kinetics led to a limited supplementary investigation. Sexually segregated peripheral shift data representing
three genera are plotted in figure 8. Finally, relevant odontometrics are graphically
presented together with arch width buccal
shift diagrams for three representative
genera i n figures 9, 10, and 11.
FINDINGS
Column A of table 2 lists the molar buccal shifts at M (l-t) as percentages of their
corresponding premolar shifts at P3. These
ranked percentages fall into two groups:
predominantly translatory (posterior shift
2 anterior shift) and predominantly rotational (posterior shift < anterior shift).
These two functional categories are associated with distinctive tooth and jaw morphological differences which will be discussed
(table 2). The inclusion of Brachyteles and
Aotus in the translatory group is based
upon morphological concordance with this
group together with a consideration of the
vagaries of percentage data. The 96.8%
reading for Brachyteles and the 92.9%
value for Aotus both reflect a 0.1 m m difference between numerator and denominator (3.0/3.1 and 1.3/1.4), well within the
error of measurement.
Canine honing shift values expressed as
percentages of buccal phase shifts are
listed i n table 2B. As predicted, rotational
predominance is associated with the greatest canine honing shift ranges. However,
324
M. R. ZINGESER
the predominantly translatory Layothrix
and Suguinus show exceptionally large
honing ranges at 234.8 % and 236.4 % (see
DISCUSSION). In connection with canine
tooth excursive movements, rotation of the
condyle on the ipsilateral side and concomitant antero-posterior displacement of the
contralateral condyle appear to be universal
among the Ceboidea (figs. 3A, 4B). These
movements may entail considerable associated translatory mandibular shifts especially in animals characterized by predominantly trans1atory buccal phase functioning
(fig. 4).
Except for Lagotl.rrix and Snimiri (see
DISCUSSION), mandibular vs. maxillary buccolingual molar ratios expressed as percentages also accord with our theoretical
"model" (table 2C). They indicate that in
translatory predominance maxillary molars
tend to be much wider than mandibular
molars, whereas i n rotational predominance
the differences in opposing molar widths
are considerably smaller in order to conform to the mechanical requirements of
the two jaw movement patterns (cf. fig. 3).
The maxillary central incisors tend to be
wide relative to maxillary molars i n the
rotational group compared with the translatory forms, and they thus accord with the
predicted trend (table 2D). However, these
ratios reflect diverse effects which are not
confined to simple correlation with maxillary anterior arch widths, but include such
factors as the relative differences in maxillary molar size in the two functional modalities, and dietary adaptations. An analysis of' these factors is underway.
Premolar buccolingual size gradients are
consistent with my theoretical expectations
(fig. 7). In rotational predominance, there
is a trend toward premolarization i n the
maxilla, i.e., anincrease in premolar widths
posteroanteriorly. This, together with a relative reduction of maxillary molar widths,
is compatible with condylar rotational kinetics with its large anterior range of motion (figs. 3A, 6 A , 10). Premolarization is
not equally marked in the five predominantly rotational genera. It is most evident in SuimiTi (No. 3), Chiropotoes (No. 7),
and Cebus (No. 9), and less so in Pithecia
(No. 6) and Ateles (No. 9) all of which are
keyed to table 2 and figure 7 for cross reference. The slopes of the rotational and
translatory extrapolations in figure 7 are
:I
10
M!A
(B-L in mm )
2
3
I
I
2
3
4
4
5 6 78910
I
I I
, , , $ I
p3 3
2
g4'
3
2t
7
Mcl
,
,
I
l
l
,
5 678910
(B-L in mm)
Fig. 7 Buccolingual of P3 plotted against buccolingual of M(1-t), the penultimate molar, on
logarithmic coordinates for maxilla (upper) a n d
mandible (lower). Buccal phase trends extrapolated.
The genera are numerically keyed to tables 1 a n d 2.
informative. In the maxilla (upper graph)
the rotational slope is 53" vs. 45" for the
translatory slope, a clear indication of the
trend towards premolarization in the former. In the mandible (fig. 7, lower graph),
the rotational slope is 4 3 " vs. the 45" translatory slope. In the rotationally predominant mandibular arch form, the posteroanterior premolar reduction evident i n
figure 7 is consistent with the narrowing
of the anterior mandibular arch; this accentuates rotational movements in contact
relationships with the wide anterior maxillary arch (cf. figs. 5 B and 6B).
Sex-segregated peripheral functional
ranges are graphed for three genera i n
figure 8. Sex-related dimorphism in asso-
CEBOID OCCLUSOMANDIBULAR KINESIS
32 5
I20
11.0
8.0
Alouutto coraya
Suimiri snoreus
Cebus ssp
d=7
o o t t &k&Mlhbd3
U
l
l
d =5
J
.u
CPZP3WMIMZ)IK
-
CPZP3WMIMZM3
Fig. 8 Sex segregated peripheral shift patterns. Significant sexual dimorphism is confined
to canine honing ranges in Cebus and Saimiri. In Alouatta, premolar peripheral shift sexrelated differences correspond with caninization of male maxillary premolars.
ciation with canine honing range is predictably evident. In Cebus and Saimiri,
rotationally predominant forms, postcanine
functional dimorphism is slight. However,
in the predominantly translatory genus
Alouatta, the premolars are markedly dimorphic in functional range and appear to
reflect size sex-related dimorphism in the
premolars as well as in the canines (Zingeser, ’67, ’68a). The dichotomy of peripheral occlusomandibular functioningin Alouatta is typical of translatory forms, since
occlusomandibular rotational movements
are always present in varying degrees i n
association with canine shear and honing.
In Alouatta males, these rotational movements also involve the large caninized maxillary premolars.
Correlations between arch form, tooth
size gradients, and occlusomandibular kinesis are further clarified by reference to
graphs which detail relevant odontometrics
together with arch width (and hence approximation of form) characteristics and
associated peripheral shift patterns i n each
genus. Three examples are chosen to elucidate specific facets (figs. 9, 10, 11). The
Alouatta data plots (fig. 9) typically reflect
translatory predominance. In conformity
with the restricted honing range, the maxillary incisors are narrow and frequently
in underbite relationship (Zingeser, ’67,
’68a, ’73). Maxillary molars are “primitive”
with well-defined ectocingula and are very
wide in comparison with the mandibular
molars in adaptations for efficient transverse translatory functioning (fig. 5). The
maxillary premolars decrease markedly i n
width posterio-anteriorly ; the mandibular
premolars (except for P2 which hones
against Cl) are subequal in widths. These
tooth size trends correspond roughly with
arch forms as is evident in the arch widths
graphed in the lower section of figure 9.
They are adaptively compatible with the
posterior arch form and functional dominance which characterizes this occlusomandibular kinetic category (cf. figs. 5
and 7).
Cebus (fig. 10) presents quite a different
picture. Tooth and arch form traits associated with the anterior functional dominance of the rotational modality are clearly
evident. These include wide maxillary incisors, premolarization, and relatively slight
differences between maxillary and mandibular molar widths. The rough correlation between tooth size gradients and arch
forms is again evident. The role of the anterior constriction of the mandibular arch
in reinforcing rotational occlusomandibular kinetics is particularly evident (fig. 9,
lower section).
Saguinus (fig. 11) taken as representative of the Callitrichidae, has interesting
specializations, many of which are related
to small body size. For example, in the foreshortened jaws of this animal, M 3 is missing and M 1 is dominant. The “primitive”
maxillary first molar is very much wider
than the apposing mandibular first molar
in co-adaptation with the translatory modality. P4 represents a “nodal” point to
the mesial of which the caniniform P2=:$
teeth shear together with the canine i n
predominantly rotational movements. This
326
M. R. ZINGESER
Alouatta caraya
1-1 1-2 C P-2 P-3 P-4 M-1 M-2 M-3
1-1 1-2
c
1
-
9
Mcxillc
-
talonid
Mandible
meon
trigonid
u)
-
L
W
W
1
c
Pp
._
z
1
P'
Tronsporocone
Mi
P'
MZ
M"
I
19.0 r
18.0 17.016.0 -
/
p
3lmm
150 -
14.013.012.0 11.010.0 -
Lf
2.8
0
28
m
d
e-P,
9
'
'<
',
/O'
P,-e
e-M,
i
M,
M,
M,
Tronshypoflexld
Fig. 9 Aloziatta carayn odontometrics (above) and buccal phase shift diagram (below).
Differential widths of maxillary and mandibular arches determine the buccal shift pattern. The
approximation to arch shapes (lower diagram) should be compared with tooth size gradients
(upper diagram and figs. 5, 7). Tooth size and arch form characteristics are typical of translatory predominance.
pattern is common to both sexes and is also
evident i n Cebuella.
DISCUSSION
The translatory jaw excursive modality
is primitive, appearing early i n therian
evolution (Crompton and Hiiemae, '70;
Mills, '67, '73). It is associated with molar
(as opposed to both molar and premolar)
mastication and correlates with primitive
molar morphology in which buccal phase
features dominate (Kay and Hiiemae, '74).
Buccal phase translatory predominance is
typical of' most Ceboidea (table 2) and is
not correlated per se with dietary predilections. In common with most primates, the
Ceboidea exhibit much plasticity i n feeding
behavior. However, feeding adaptations
have considerable selectional advantage i n
competitive situations such as those that
accrue to Alouatta which can digest mature leaves when more desirable fruit is
lacking (Hladik and Hladik, '69).
The ramus expansion i n many predominantly translatory ceboids is a herbivorous-follivorous adaptation associated with
highly developed superficial masseter and
internal pterygoid muscles, which parallel
similar morphology in ungulates and other
herbivores (Radinsky, '66; Turnbull, '70).
It is functionally associated with the "universally specialized" temporomandibular
joint-occlusal region orientation pattern
(Biegert, '63) which allows simultaneous
CEBOID OCCLUSOMANDIB ULAR KINESIS
32 7
Cebus spp
1-1 1-2 c
tngonid
~~ _ _
~
_ _ _
u)
I
I
al
9.0
6
I
I
I
5-5 5-5
10
Transporocone
P,-M,
I
M,
L
M,
M,
Tronshyp:flexid
Fig. 10 Cebits s p p as in figure 9. Premolarization is marked. Tooth size and arch form
traits are typical of rotational predominance.
contact upon closure coupled with great
functional mobility (Zingeser, ’73). However, this form-functional configuration is
not confined to the predominantly translatory category but is also found i n the
Pithecinae. On the other hand, ramus expansion is not seen in such predominantly
translatory forms as the largely insectivorous Callitrichidae.
The inconsistencies in table 2 need to
be examined. Among the translatory group,
Lagothrix and Saguinus are exceptional
in having high values for the canine honing range whereas in the rotational group
Saimiri shows a relatively small honing
range (column B). It is to be noted that
Lagothrix is a “misfit” with regard to the
other values i n table 2. This genus is intermediate among the Atelinae in many masticatory traits, falling between the primitive Brachyteles (translatory) and Ateles
(rotational) (Zingeser, ’73). It appears to
be actively evolving variations in sex-related canine tooth dimorphism according
to the evidence put forth by Fooden (’63).
Large canine teeth in S a p i n u s (the Tamarins) together with the apparent anomolous retention of translatory chewing may
have evolved because of jaw foreshortening
in this genus, thus necessitating this combination for maximum efficiency. The status of Saimiri in this scheme clearly points
to translatory functional affinities. For example, the relative canine honing range
(column B) is the smallest of the rotational
group and the mandibular/maxillary molar
width ratio suggests translatory function.
This ratio reflects the primitive morphology
of Saimiri molars. Further evidence of translator): buccal phase tendencies is seen i n
figure 8 i n comparisons with the peripheral
occlusomandibular patterns of’ Alouattn
and Cebus. These characteristics suggest
that rotational buccal phase masticatory
functioning in Saimiri is a relatively recent
evolutionary development. Whereas the
maxillary incisor/molar ratios (table ZD)
generally come up to paradigm expecta-
32 8
M. R. ZINGESEH
Saguinus spp
-
Maxilla
tolonid
'I
4
10.0
4.0
11
P'
-
e-P,
P'
P4
M'
I
I
5-c
4
M2
I
MI
MI
M,
Transhypoflexid
Fig. 11 S n g t t i n u s s p p as i n figures 8 a n d 9. The size dominance of M1 is typical of the
Callitrichidae. MI is appreciably wider than M , i n adaptation to transverse translatory masticatory kinesis. P2=3 are caniniform and jaw- movements involving these teeth and the maxillarv
canines are largely rotational.
tions? they show much variation which can
be attributed to complex factors.
The data in table 2 appear to substantiate a direct correlation between the range
of maxillary canine tooth honing excursion
and the presence of the rotational modality
in mastication. As I have mentioned, rotational jaw movements are associated in all
genera with canine tooth honing and shear;
the relationship between the two seems
clear. To effectively hone the length of
long maxillary canine teeth against their
functionally complementary mandibular
honing notches at C, and P2 (Zingeser,
'69), the maxillary teeih m u s t splay laterally in proportion to their length. If they
did not, the degree of jaw opening and associated orthal honing movements required
would probably be inadaptive. The extension of rotational jaw movements to include
the entire premolar-molar row can be explained in terms of evolutionary dynamics.
With the evolution of larger, and hence
more widely splayed maxillary canine teeth
through whatever selectional pressures (see
Zingeser, '68b), the cervical bases of these
teeth also extend farther laterally. For
functional efficiency in the maintaining
arch continuity, p2 first and then others i n
the premolar row tend to align with the
canine teeth. The resulting single rotational modality is assumed to have functional
and hence selectional advantages.
Secondary molar and premolar changes
probably followed these primary arch form
and related mandibular excursive evolu-
CEBOID OCCLUSOMANDIBULAR KINESIS
tionary developments, and a somewhat different course is discerned among the various Ceboid lines. In the Cebinae, especially
Ccbus, premolarization is marked, and the
premolars clearly assume and even dominate masticatory function (figs. 6, 9). The
situation is more diverse i n the Atelinae.
Brachyteles, a facultative leaf eater with
small canines, is probably closest to the
atelinine-alouattine ancestral form (Zingeser, ’73). Jaw functioning is largely translatory, maxillary molars are very large and
replete with “prkitive” features, the rami
are expanded, and maxillary incisors are
small and in underbite relationship. At the
other end of the atelinine spectrum, Ateles
with its large, splayed-out maxillary cuspids, exclusively rotational jaw function,
moderate degree of premolarization, large
edge-to-edge incisors, and rim-ridged molars and premolars stands in marked and
informative contrast, with Lugothrix intermediate. When metric and morphological
comparisons are made between the dentitioris of these three Atelinae, important
trends emerge. In odoiitometric comparisons, differences in buccolingual size
among the molars are appreciably greater
in the maxilla than i n the mandible. Thus
in comparing the predominantly translatory
Brnchyfeles with the rotationally predominant Ateles, it is the maxillary molars that
are reduced in size as a consequence of the
loss of the ectocingula; these are prominent in Brtichyteles but absent i n Ateles.
That maxillary molars, primitively wide i n
comparison with mandibular molars i n order to be compatible with translatory function, should show reduction in assuming
rotational modality is completely i n accord
with “mechanical” considerations (figs.
3A,B).
Less diversity is seen among the Pithecinae. All show rotational predominance.
Chiropotes (fig. 7, No. 7) exhibits extreme
premolarization and concomitant reduction
i n maxillary molar widths, while these
trends are not as obvious in Pithecia (fig.
7, No. 6). Ctrcajao, appears to be intermediate, but the data are incomplete.
The interrelationships between arch
form, tooth morphology, and occlusomandibular kinesis shown in this study of the
Ceboidea provide promise of elucidating
form and functional correlations and evolutionary trends in other groups. In view of
329
the many striking parallelisms between
New and Old World primates, the extension of these investigations to the catarrhines seems eminently worthwhile.
ACKNOWLEDGMENTS
I gratefully acknowledge the generous
assistance of Dr. Richard W. Thorington,
Jr. of the United States National Museum
who made available the material upon
which this paper is largely based. I am indebted to Ms. Louise Zingeser for assisting
in data processing and to the staff of the
Oregon Regional Primate Research Center
who cooperated in the preparation of the
manuscript. I am especially grateful to Mr.
Joel Ito for his skilled rendition of the diagrams and graphs and to Mr. Harry Wohlscin for the photography.
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~
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