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Cranial equilibrium index.

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CRANIAL EQUILIBRIUhf INDEX
MVIUZAFI’ER SULEYMAN QENYtfREK
Department of Anthropobgy, Harvard University
INTRODUCTION
I n the preparation of this work, the writer utilized the
primate collections of the Museum of Comparative Zoology of
Harvard University and of the American Museum of Natural
History. I am extremely grateful to my teachers, Prof. E. A.
Hooton and Prof. G . Allen, for limitless advice and kind
permission t o study the specimens in their laboratories. I am
also grateful to the staff of the American Museum of Natural
History of New York for kind permission to work in their
laboratories. Also, I am extremely indebted to Miss. E. T.
West, of Boston University, who prepared the illustrations
for this paper and the coming publications.
CRANIAL EQUILIBRIUM INDEX
Every anthropologist knows that in the evolution of the
Hominidae from a common primate ancestor, the cranial
I n t h e literature, the writer waa able t o find only the following references
which would euggest the cranial equilibrium index, but which are different from
it:
Broca (Bulletin de la societh d’anthropologie de Parie, 1862) gives the relstivo
poeition of basion on the alveolo-condylean plane. Martiii in hie Lehrbuch der
Anthropologie mentions a n index which gives the relative position of baeion on
the inner Skull length. Aurel V. Toriik (Qrundztige einer systematiechen kraniometrie, 1890) mentione three indices which give the relative position of basion and
opisthion on a line parallel t o Frankfort plane.
These indices give the relative position of baaion and opisthion which (LLB
respectively in front and behind the cranial fulcrum, while the cranial equilibrium
index deals with the cranial fulcrum.
23
A I E R I C A N J O U R N A L 0. PHYSICAL ANTHROPOLOOY, VOL. X X I V , NO.
JUI.Y--8LPTIXBEO,
1938
1
24
XVZAFFER S ~ L E Y M A S ~ E S Y ~ R E K
fulcrum at the occipital condyles, in connection with the
upright posture, has moved forward with the result of a
better cranial equilibrium. Since, so far this observation has
not been the subject of satisfactory metric expression, the
writer has undertaken the task. To reduce this rather important morphological observation to measurements, the
writer devised the following technique :
The cranium is a lever of the first order, the fulcrum being
at the occipital condyles. However, if the middle points of the
lower surfaces of the occipital condyles are taken we have
a better approximation to the true fulcrum. I f these points
a r e connected by a Line and the middle point of this line is
marked we get the fulcrum on the sagittal plane of the skull.
Figure 1
For convenience we may call this new point the intercondylar point (point B, fig. 2). The distance of this point
from the prosthion (AB) was measured and then the distance
from prosthion to the most posterior point of the skull (AC),
projected on a plane passing through the prosthion and the
lower-most points of the two condyles, was measured, as
is shown in figure 1. Then in the calculation of the index this
(AB) distance was expressed as a percentage of (AC) distance.
The method of finding the intercondylar point and the distance from prosthion to the intercondylar point and to the
most posterior point of the skull was as follows: It is observed that generally the two occipital condyles, when their
anterior- and posterior-most points are connected, make an
CRANIAL EQUILIBRIUM INDEX
25
isoceles trapezium in norma basilaris. This trapezium is
represented in figure 2 by DEFK. If the middle points of
EK and D F are connected by a line this line divides the
sagittal condylar height (LM) into two equal parts or in
other words it bisects this height at the intercondylar point
(point B in fig. 2). Conversely, if a line parallel to DE and
FK is drawn through the intercondylar point it bisects EK
and DF, and this makes the measurement rather easy.
To find the distance of intercondylar point from the
prosthion, at first the most anterior and most posterior points
of the two occipital condyles were marked with a pencil.
The occipital condyles can be differentiated very easily by
their shine, smoothness and the contrast between them and
the surrounding parts. Always the points at which the convexity ended, sometimes sharply and sometimes slightly, were
marked for we must get only the functional base on which
the cranium is balanced. If the condyles were too low
and their anterior- and posterior-most points could not be
made out definitely such specimens were eliminated from
the series. After markinp: these points the inside of the
fixed arm of a sliding caliper was brought in line with the
posterior-most points of the two condyles and the movable
arm was applied to prosthion and this distance recorded,
viz., AM. Then the inside of the fixed arm of the caliper
was brought in line with the anterior-most points of the two
condyles and again the movable arm was applied to prosthion
and this distance recorded, viz., AL. These two distances
AL and AM were added up and then divided by two, with the
resulting figure giving the distance of the intercondylar point
from prosthion, viz., AB distance. I n measuring AL and AM
it must be noticed that the bar of the caliper is parallel to
the median sagittal axis of the cranium.
Here, necessarily, consideration must be given to the fact
that the measured intercondylar point, because of technical
difficulties, does not coincide with the true intercondylar
point. This is due to the fact that in norma lateralis the occipital condyles present a downward convexity and that the
26
MUZAPFER S ~ L E Y M A N ~ E N Y U R E K
measurements had to be made from the ends of this convex
surface. This difference is illustrated in figure 3. I n this
drawing the two occipital condyles are represented as one
and the true intercondylar point is represented by (b). Since
the measurements were made from E and K, it is evident that
the measured intercondylar point lies at (B), that is, a few
millimeters above the true intercondylar point. However, the
error introduced thus is negligible and further, since all the
Primates were measured by the same technique, it does not
affect the final conclusions. Hence, in figure 1, no attempt
has been made to mark the exact position of B and it has
been p u t at the place of (b) for convenience.
I n measuring both the AB and AC distances it was highly
desirable that these two lines coincided. But in the preceding
paragraph reasons have been given to show that this could
not be attained and consequently the AB line deviated by a
very small angle from the AC line. To measure AC distance
it was first necessary to have the lowermost points of the
condyles and the prosthion fall i n the same plane. To secure
this plane two methods had to be used in case of different
Primates. I n case of large Primates the skull was laid on
an osteometric board such as is used to measure the length
of the long bones. I n doing this care was taken to see that
the sagittal plane of the cranium formed a right angle with
the vertical plate of the measuring board. After that, wooden
plates of known height were placed under the condyles (both
condyles supported by the same plate). Then the height of
the prosthion, already marked with a pencil, from the surface
of the horizontal plate of the measuring board, was measured
by means of a vertically placed ruler. These processes were
continued until the height of prosthion from the surface
equaled the height of the materials placed under the condyles.
Then it was seen once more that the sagittal plane of the skull
was at a right angle to the vertical plate of the board and that
the posterior-most point of the cranium touched this board.
After this the distance of prosthion from the vertical plate was
read on thin paper rulers on the horizontal plate of the measur-
CRANIAL EQUILIBRIUM INDEX
27
ing board,2 I n case of some small Primates, like lemurs, a
different method was used. I n forms where there was a considerable gap between the upper central incisors the bar of
the caliper was passed between them touching the bone, and
the fixed arm of the caliper was brought against the most
Figures 2 and 3
posterior point of the skull. Then care was taken so that
the lower-most points of the condyles were on the same plane
as the top of the bar of the caliper and when this was assured
the movable arm was applied to prosthion. I n case of small
Primates this is an easier method.
* I would suggest that a more accurate and standardized measurement would
be obtained if zoologists measured the maxinium skull €ength by the technique
employed here.
28
MUZAFFER SULEYMAN ~ E N Y U R E K
CALCULATION O F THE CRANIAL EQUILIBRIUM I N D E X
I n the calculation of this index, which for convenience we
mag call the cranial equilibrium index, AB distance was expressed as a percentage of AC distance as is shown in the
following formula :
AB X .-100
Cranial equilibrium index = --AC
IXPLICATIO&?3 O F T H E I N D E X
First implication is that the index gives the relative position
of the cranial fulcrum on AC line. Also it tells us the relative
position of foramen magnum on AC line because the foramen
magnum is invariably bordered by the occipital condyles in
its anterior part, and thus it accompanies the condyles in
their evolutionary migration.
The second implication is that the higher the index the less
suited is that specimen for erect posture and that if the cranium
is going to be kept erect on the vertebral column it would
require stronger riuchal musculature and stronger occipital
attachments than one with a lower index. However, since
this conclusion is the result of a complicated process of correlation it is necessary to elucidate it. The formula expressing the cranial leverage in a n erect Primate is as follows :
W-Q'R = B H * W '
+ BC*F
Here W is the weight of the skull anterior to the fulcrum,
and including the mandible which has been omitted from the
drawing, GB
' is the distance from fulcrum to the center of
gravity of the prefulcral portion of the skull, W' is the weight
of the postfulcral portion of the skull, BH' the distance from
the fulcrum to the center of gravity of the postfulcral part,
BC the distance from fulcrum to the most posterior point of
the skull projected on AC, coinciding with the attachments
of the nuchal musculature in most Primates, and (F) is the
This equation i s an approxiniation to the leverage formula of IIomo sapiens.
F o r comparison, i t was also applied t o the erect primates. B y the terin 'erect
primate' is meant the relatively erect position of the hend and body in the
actions of sitticig, elimhing, brachiation and bipedal loconiotion. I t should he
mentioned here that in quadropedal locomotioii the whole skull is pulled clown by
gravity.
CRhNIAL EQUILIBRIUM INDEX
29
force applied by the nuchal musculature to keep the head
balanced on the vertebral column. These points are shown
in figure 4. From this formula it is clear that if the relations
of these dimensions and weights are changed, depending on
the nature of changes, F (muscle force) will have to change,
that is, it will have to be either decreased or increased.
The sagittal sections of a number of Primate crania were
projected carefully on thick cardboard and two or three
copies were made for each skull. On these cardboard copies
a BX line perpendicular to AB was drawn. In one copy the
Figure 4
cardboard figure was cut along BX line. Then the centers
of gravity of the two parts of the skull were determined by
hanging the cardboard figures from three corners. I n figure 4
the center of gravity of the prefulcral part is represented by
G and that of postfulcral part by H. G’ and H’ represent
the vertical projections of these points on AC line. The
G’B and BH’ distances were recorded. I n the second copy
of the same skull the point B was moved forward to B’ and
B’X’ vertical was drawn and the cardboard was cut along
this line. Then again the centers of gravity of the pre- and
30
MUZAFFER SULEYMAN EJENYUREK
postfulcral parts were found. These new centers of gravity
are represented by g and h, and their projections on AC by
g‘ and h’. As the fulcrum was moved forward it was found
that B’g‘ distance became shorter than G’B and R’h’ became
longer than H’B. By the anterior movement of the fulcrum
the weight of the prefulcral portion is decreased while that
of the postfulcral portion is increased and also BC distance
is increased. These findings, when applied to the leverage
formula of the erect cranium, show that as the fulcrum moves
forward, that is, as the cranial equilibrium index decreases,
the cranium can be balanced on the vertebral column with a
reduced (F). This would mean that as the fulcrum moves
forward this cranium can be kept in balance with weaker
nuchal musculaturc and weaker occipital attachments. In a
number of cases the process was reversed, that is, the fulcrum
was moved backward and it was found that the results also
were reversed and to keep this cranium well balanced (F)
had to be increased.
Besides this, BH’ distance was expressed as a percentage
of BG’ distance. The index thus obtained is 56.25 for a
Homo sapiens and 23.4 for a chimpanzee. This finding corroborates the results obtained by changing the position of
cranial fulcrum of a single skull.
It is from the correlation of the cranial equilibrium index
with these experiments that the second implication of the
index is concluded, that is, the higher the index, the less
suited is that cranium for erect posture and conversely, the
lower the index, the easier it is to keep a well-balanced
cranium.
CRANIAL EQUILIBRIUM INDEX OF THE PRIMATES
I n table 1, where the indices of the adult male members of
the leading genera of the order are given, it is seen that, in
general, the index decreases from suborder Lemuroidea to
suborder Anthropoidea. The suborder Lemuroidea ranges
from 93.53 to 80.4, one Tarsioid has an index of 79.94, which is
lower than that of any lemur, and the Anthropoidea run from
31
CRANIAL EQUILIBRIUM INDEX
94.39 in Alouatta to 64.18 in Saimiri sciureus, and to a minimum of 50.74 in Homo. However, it must be noted that this
difference is only general and that all the three suborders overlap. The only gap in the series is between Homo sapiens
TABLE 1
Cranial equilibrium inde5 of adult male Primates
OEANIAL EQUILXBEKIM INDEX
SUBOXDEB
FAMILY
GENUS
-
Lemuroidea
Lemuridae
Indriidae
Lorisidae
Lemur
Lepilemur
Microcebus
Propithecus
Perodictieus
Nycticebua
Loris
Galago
Euoticus
Tarsius
Tarsiidae
Cercopithecidae Erythrwbus
Colobus
Pygathrix
Papio
M8CaCuS
Cercopitheous
Cercaeebus
____.Hylobatidae
Symphahngus
Hylobates
Simiidae
Pongo
Anthropoidea
Pan
Gorilla
Hapalidae
Mico
Hapale
Mystax
Oedipomidas
-~
Alouatta
Cebidae
Pithecia
Callicebus
Ateles
Aotus
&bus
- Saimiria
Hominidae
Homo sapiena
Tarsioidea
lpecimens
Range
8
1
2
5
5
3
2
8
5
1
2
14
4
5
7
23
5
1
5.2
3
7
8
2
8
3
14
5
5
11
4
15
10
27
86.98-90.23
--
-
I
..........
83.27-84.16
_____
85.43-88.93
91.92-93.53
88.59-91.22
82.60-84.14
80.40-87.46
81.10-83.26
..........
78.76-87.61
75.25-83.33
77.50-80.31
74.87-80.23
72.90-78.81
66.66-81.08
71.42-76.25
..........
79.24-83.64
77.14-83.95
78.42-82.08
69.84-75.47
74.78-78.93
75.68-77.14
71.95-77.45
71.30-75.56
84.68-94.39
76.31-82.53
72.50-81.88
74.78-80.08
74.13-77.50
68.10-78.78
64.18-69.26
50.74-59.57
Average
88.67
88.44
83.71
86.86
92.71
90.18
83.37
83.02
82.24
--79.94 83.18
79.41
78.93
77.02
76.09
76.01
75.00
81.10
80.99
80.54
79.67
_ 72.44
_ 77.72
76.41
74.19
73.98
89.65
78.42
78.05
76.72
75.82
75.68
66.91
54.34
32
MUZAFFER SULEYMAN ~ E N Y U R E K
and the rest of the Primates. Most advanced species of the
infrahuman Anthropoidea in this respect are Cercopithecus
talapoin, Saimiri sciureus and Saimiri oerstedi, whose respective averages are 67.01, 66.94 and 66.88 (table 2). The
lowest individual value of the infrahuman Primates is 64.18,
and it is that of a Saimiri sciureus; the highest individual
value of Homo sapiens, 59.57, that is, there is a gap of approximately 5 index units, which is a considerable difference.
The Eocene Lemuriformes undoubtedly had high indices
and the foramen magnum looked backward.' These primitive features have been retained by the existing Lemuriformes. Also the early Eocene tarsioids, before and at the
Necrolemur stage, had high indices and their foramen
magnum also looked backward. However, by the end of the
Eocene period the modern condition found in the living
Tarsius, that is, an anteriorly placed and downward looking
foramen magnum, was almost attained. We have no direct
evidence about the position of the foramen magnum of the
Anthropoidea in the early part of the Tertiary era, but from
the fact that the living members of this suborder have a
more anteriorly placed and downward looking foramen
magnum we can infer that their Eocene ancestors, after they
were separated from the tarsioids, were possibly more advanced than the Lemuriformes. Thus it is clear that the
evolution in this index was from a high t o a low one.
With the exception of Alouatta the Anthropoidea have
values below 83 and from this and from the condition in the
Tarsioidea, which undoubtedly have a community of origin
with the Anthropoidea, it may be conjectured that the earliest
Anthropoidea probably had an index not lower than 83 to 82.
It has been thought (Clark) that in Alouatta the foramen
magnum has secondarily moved backward and that the condition found in this genus, where the foramen magnum looks
rather backward, is by no means a primitive feature. The
highest individual value in the order is 94.39 and is found in
the genus Alouatta, but I suspect that if there were more
' W. K. Gregory. On t h e structure and relations of Notharctus, an American
eoceiie primate, 1920.
CRAXIAL EQUILIBRIUM INDEX
33
specimens of Lemuroidea we would have still higher indices,
as would be expected from the fact that Perodicticus and
Nycticebus have the highest generic averages.
It is also seen that the Hylobatidae and, with the exception of gorilla, the Simiidae have relatively higher indices
than some of the lower monkeys. Among the anthropoid apes,
gorilla has the lowest index, and though this is consequent, in
a great measure, upon the enormous development of the occipital crests, nevertheless, as is shown by the female gorilla
whose cranium is devoid of such ridges, it is somewhat
nearer to Homo sapiens than the other anthropoid apes.
Living Homo sapiens have the lowest indices, explaining the
fact that modern man has a relatively round and smooth
occiput, but, geologically speaking, this is undoubtedly a recent
development for such archaic Hominidae as the Pithecanthropus erectus, Sinanthropus pekinensis, Homo neanderthalensis
and Homo soloensis appear t o me to have high indices. This,
together with the fact that the great anthropoid apes, which
are man’s nearest relatives, have high indices would indicate
that the ancestral Hominidae also had high indices. Eanthropus dawsoni would seem to have a relatively low index and
from this it may be conjectured that the modern condition of
Homo sapiens was attained at least in early Pleistocene times.
Thus, I infer that while there is a gap between the living
Homo sapiens and the rest of the living Primates in this
respect, this hiatus is fully filled in by the fossil Hominidae.
Since the anterior portion of the head is heavier than the
posterior, the most advantageous and ideal condition for the
Homo sapiens would be an index lower than 50, but so far as
the thirty-four specimens studied are concerned this desirable
condition has not yet been attained.
From an examination of tables 1 and 2, it appears that
certain monkeys such as Cercopithecus talapoin, Saimiri,
Oedipomidas and Mystax are more advanced in this respect
than the great anthropoid apes. These monkeys are lowly
pronograde Primates and this feature is certainly not of much
use when they pursue a quadrupedal locomotion, but when
they are climbing they can keep their heads erect more easily
34
MUZAFFER S ~ L E Y M A NS E N Y ~ R E K
TABLE 2
Cranial equilibrium indsz of adult males and adult female8
-
ADULT MA130
Lemur fulvus
Propithecua verreauxi
Perodicticus potto
Qalago demidovii
Euoticus elegantulua
Tnrsius spectrum
Colobus polykomoe
Colobus badiue
Pygathrix aurata
Papio doguerr
Macacue nimistrinus
Cercopithecus mitis
Cercopithecus nictitam
Cercopithecus aethiops
Cercopithecus mona
Cercopithecus cephua
Cercopithecus talapoin
Cercocebue torquatus
Cercocebus agilis
Hylobates lar
P a n satyrus
C)orillagorilla
Hapale santaremensis
Mico argentata
Mystax ruflmanus
Oedipomidaa geoff royi
Alouatta seniculus
Alouatta beelzebul
Alouatta palliata
Pithecia monachus
Callicebus remulue
Ateles vanegatus
Ateles geoffroyi
Ateles paniecus
Aotus zonalis
Aotua trivirgatus
Cebus flatuellus
Cebua capucinus
Saimiri sciureue
Saimiri oeretedi
Homo eapiens
ADULT R M A U J I
-
--
Bpeeimenr
Range
ireraga
Spscimans
Range
4
87.71-89.83
85.43-86.8 7
91.92-93.53
80.40-81.62
81.10-83.26
88.90
86.35
92.71
81.01
82.24
79.94
81.61
77.70
79.33
76.08
77.41
79.05
77.95
76.58
74.26
74.62
67.01
75.77
73.83
81.48
80.30
71.94
76.41
76.79
74.04
73.98
92.03
89.98
87.79
70.42
77.10
77.58
76.62
76.12
76.66
75.83
77.13
74.28
66.94
66.88
54.34
2
3
3
2
82.97-87.95
85.59-86.53
92.18-93.65
80.00-81.59
4
5
3
5
1
7
6
2
3
1
5
5
3
2
4
2
3
2
3
2
6
2
7
7
3
4
4
5
2
4
1
1
1
2
1
6
6
6
4
27
-
..........
79.91-83.33
76.19-79.36
78.35-80.31
74.87-77.01
..........
77.27-81.08
75.25-80.80
75.24-78.57
73.78-74.74
72.39-77.04
66.6667.36
75.00-76.25
71.42-76.25
79.24-83.64
78.53-82.08
69.84-74.53
75.68-77.14
74.78-78.93
71.95-77.45
71.30-75.56
89.61-94.39
89.25-90.97
84.68-92.98
76.31-82.53
72.50-78.75
..........
..........
..........
75.83-77.50
..........
75.25-78.78
68.10-77.55
64.1848.79
65.48-69.26
50.74-59.57
--
-
1
1
5
3
3
2
1
5
3
5
2
1
1
1
1
3
5
4
3
2
4
1
4
2
5
1
4
1
LTerage
.
..........
..........
78.71-82.03
75.75-78.65
73.53-77.65
70.27-73.20
..........
75.28-77.89
75.96-77.88
71.51-72.57
72.15-73.62
..........
..........
..........
..........
75.70-79.89
74.46-81.73
71.32-78.89
74,5677.87
77.14-77.96
70.95-76.51
..........
87.50-89.21
86.13-88.78
81.30-85.57
..........
75.16-79.08
85.46
86.08
92.88
80.79
84.10
75.64
80.87
76.74
75.57
71.73
72.54
76.79
76.69
72.07
72.88
72.04
67.09
76.15
72.47
77.95
78.74
75.63
75.86
77.55
73.89
73.88
88.40
87.45
84.01
76.42
i 77.02
..........
4
74.77-75.90
i,
2
2
1
5
2
3
1
6
76.98-81.81
76.27-76.72
I 79.39
..........
!
60.34-66.66
iI
- 50.55-55.05
1
70.17-73.09
70.22-75.00
..........
I
75.34
77.96
76.49
74.17
72.39
73.61
64.45
62.58
52.73
CRANIAL EQUILIBRIUM INDEX
35
than the great anthropoid apes. It is a zoological fact that
if the size of a species is increased, as compared with the size
of its near relatives the face grows more than the neurocranium, and conversely, if it is reduced, the face is decreased
more than the brain case that is the result is a more infantile
form. All these low monkeys with a low index are small and
infantile and, as will be shown in the discussion of age differences, since the young Primates usually have lower indices
than the adults of the same species it becomes clear that in
this respect also they have been infantilized. This low index
in these monkeys has thus been attained independently of the
forward migration of the condyles in Homo sapiens for I
infer from the fossil Hominidae and the living great apes that
the ancestral Hominidae had a rather high index which is a
relatively more primitive condition than the low indices of
these monkeys.
SEX DIFFERENCES
From an examination of table 2 it will be seen that in
females also, the index, in general, decreases from suborder
Lemuroidea to suborder Anthropoidea. The suborder
Lemuroidea range from 93.65 to 80. One Tarsius has an
index of 75.64. The Anthropoidea run from 89.21 in Alouatta
to 60.34 in a Saimiri sciureus and to a minimum of 50.55 in
Homo. The gap between the most advanced of the infrahuman Primates and the least advanced of Homo is about
5 index units. From this table it is seen that females of most
Primates have lower averages than the males of the same
species, that is, in this respect the females are more advanced
than the males. This is due to the fact that female Primates
are relatively more infantile than the males, that is, they have
a less snonty face and this is responsible for the decrease in
the index. This would mean that the female Primates need
less muscular power and weaker nuchal attachments than
the males, explaining the fact that usually the female Primates
have rounder and smoother occiputs than the males of the
same kind.
36
MUZAFFER SULEYMAN ~ E N Y U R E K
However, there appear to be a few exceptions to this rule.
Most of these exceptions can be ruled out for the time being
by the fact that these species are represented by insufficient
specimens. But in the case of the gorilla this condition is
due to an entirely different factor. I n the gorilla the females
on the average are about 4 index units higher than the males.
This is because of the fact that the male gorilla has developed
large occipital tori which makes an addition to the posterior
part of the skull, and thus decreases the index. This is
further confirmed by the fact that a sub-adult male gorilla
(table 3) has an index higher than any female. This exception
in the case of the gorilla shows that when the front of the
head is extremely heavy it requires strong occipital attachments. This observation also explains the low index of the
baboons which have large snouts. However, in the genus
Alouatta there has been a secondary backward displacement
of the foramen magnum and hence the development of large
occipital tori does not decrease the index.
AGE DIFFERENCES
Age criteria. Any individual in which the basal suture was
completely synostosed and the number of permanent teeth
peculiar to that species was completed was called adult. However, the age was not determined by the complete number
only, but also by the complete eruption of the individual
permanent teeth which, in turn, was decided upon by a comparison of dental morphology in each species. Individuals in
which the basal suture was open and which had not completed
their permanent dentition, that is, if some of the deciduous
teeth were not replaced or some of the permanent teeth had
only slightly erupted were called sub-adults. Those that
had completed their permanent dentition while only a trace
of the basal suture was left open were considered young adults
and were included in tables 1and 2. Individuals in which the
basal suture was open and only milk teeth were present were
called young.
Range
ADULT Y A L E
3peci.
mens
- -
Average
__
Range
ADULT FEMALE
Average
3peci.
mens
78.35-80.31
72.90-78.81
75.00-76.25
74.87-77.01
..........
78.53-82.08
79.24-83.64
3
3
5
5
6
1
2
3
1
3
7
5
4
1
6
1
1
27
Cerccopithecusaethiops
Cercopithecusmitie
Cercopithecusnictitans
Gorilla gorilla
Gorilla beringei
Pan satyrus
Hylobates lar
Sympbalangus syndaetylus
Oedipomidaa geoffroyi
Mystax rufimanus
Alouatta palliata
Alouatta seniculus
Alouatta caraya
Cebus flatnellus
Atelea geoffroyi
Ateles paniscus
Homo sapiens
81.01
81.61
75.25
79.33
75.05
75.71
50.74-59.57
..........
..........
75.25-78.78
..........
71.30-75.56
71.95-77.45
84.68-92.98
89.61-94.39
..........
69.84-74.53
77.27-81.08
75.25-80.80
75.24-78.57
76.62
76.12
54.34
77.12
73.98
74.04
87.79
92.03
88.18
81.10
75.47
80.30
81.48
71.94
79.05
77.95
76.58
76.08
.. .......... ....
..........
80.40-81.62
79.91-83.33
3
7
1
2
4
3
80.00-81.59
78.71-82.03
80.79
80.87
1
1
5
78.74
77.95
70.95-76.51
81.30-85.57
87.50-89.21
..........
73.88
73.89
84.01
88.40
4
2
6
5
74.77-75.90
76.98-81.81
50.55-55.05
70.17-73.09
75.34
79.39
52.73
72.39
.. .......... ....
1
4
5
4
.. .......... ....
3
74.46-81.73
75.70-79.89
.......... ....
5
..
75.63
76.79
76.69
71.32-78.89
75.28-77.89
75.96-77.88
72.07
76.15
76.45
71.73
4
3
71.51-72.57
74.16-78.75
70.27-73.20
2
2
5
..........
1
a .
Average
3peci.
men8
-
..........
..........
..........
68.07-71.05
72.52
69.56
70.95
67.39
72.07
71.71
1
f
0
9
P
d
0
6
d
1
0
1
T
0
d
80.55
80.35
74.45
81.25
I
.......... 73.94
.......... 72.50
.......... 82.18
.......... 82.77
.......... 84.53
.......... 71.66
67.07
.......... 69.60
.......... 74.51
.......... ....
..........
..........
..........
71.51-77.40
.......... 73.45
.......... 71.48
6 .......... 80.02
.. .......... ....
P
0 ..........
d ...........
d
d
6
79.43
79.17
70.31
68.83
?
d
~~
63.15-66.66
I
.
.
i9.20
i4.90
i7.10
52.38
i1.26
50.94
i4.58
.
Average
-
__
i6.60
.. .. .......... ....
.. .. .......... ....
.. .. .......... ....
.. .. .......... ....
.. .. .......... ....
.. .. .......... ....
.. .. .......... ....
.. .. .......... ....
.. ..
....
.. ..
....
1
1
1
1
..........
..........
..........
..........
..........
0 ..........
0
?
?
2
Range
YOUNQ
.. .......... ....
.. .......... ....
.. .......... ....
.. .......... ....
.. .......... ....
?
.......... 62.66
.. .......... ....
.. ..........
d .......... 71.35
.. .......... ....
-
Sex
-
1
1
1
..
1
..
.... ..
..
..
d ..........
?
..
..........
..
P ..........
1
.. .......... .... ..
Range
.. ..........
d ..........
Jex
- SUBADULT
.. ..
1
1
1
1
1
1
1
1
1
1
1
1
2
..1
1
1
1
1
1
1
2
.. .......... .... 1
..
73.53-77.65
75.57
3
.......... .... 1
5
2
2
82.60-84.14
83.37
.. .......... .... ..
.. .......... .... .. .......... .... 1
Ipeci.
mens
-.
Loris tardigratus
Loris gracilis
Galago demidovii
Colobus polykomos
Colobus kirki
Pygathrix aurata
Maeseus rhesus
Cbrcocebus torquatue
Cercocebus albigena
Papia doguera
SPECIES
TABLE 3
A g e differences in cranial equilibrium indez
38
MUZAFFER SULEYMAN ~ E X Y U R E E
An examination of table 3 would indicate that the young
and sub-adults of most of the Primate species have lower
indices than the adults, that is, a low index is a neo-genetic
feature that appeared in the young of the infra-human
Primates. I n the ontogeny of the infra-human Primates the
index increases from young to sub-adult and then to adult.
Homo sapiens has evolved by retaining the infantile character of the infra-human Primates i n this respect, as also
in other features, but in human evolution the index has further
been decreased, for the lowest index observed in a young
infra-human Anthropoidea is 60.94, whereas the highest index
found i n an adult male Homo sapiens is 59.57. A 9-month-old
human infant has an index of 56.6, but whether this means
that man has become fixed in this respect or that this specimen
is exceptional is a question that cannot be answered without
the examination of larger series of specimens.
Whatever we can deduce from this meager series of young
Primates is sufficient to caution us not to compare the position
of the foramen magnum of a young Primate with that of the
adult of another species, as has been done by Dart of South
Af r i ~ a . ~
PROBLEM OF TARSIUS
Clark,6 in his illuminating book on the evolution of Primates,
expresses suspicion that the pithecoid appearance of the
tarsioid cranium may be a secondary development consequent
upon the distortion caused by the enlarged orbits. Therefore, since it involves the position of foramen magnum, it
is not superfluous to consider this problem in some detail.
I n the Advanced Insectivores and Lemuriformes a line,
perpendicular to the alveolar process, dropped from the most
anterior point of the orbits intersects the upper dental border
between the last pre-molar and first molar. In Tarsius this
line passes between the first and second pre-molars, while in
Nycticebus it passes between the second and third pre-molars.
Raymond A. Dart. Australopithecus afrieanus : The Man-Ape of South
Africa. Nature, 1925.
Sir Arthur Keith. New discoveries reiating t o the antiquity of man.
a W. Le Gros Clark. Early Forerunners of Man, pp. 56-60.
CRANIAL EQUILIBRIUM INDEX
39
So we have evidence that in Tarsius and to a slightly less
extent in Nycticebus the orbits, besides their enlargement,
have moved forward and, consequently, have shortened the
face. In Old World hthropoidea this line passes, except in
gibbons, on or behind the gap between the last pre-molar and
first molar. I n New World monkeys this line is either on the
gap between the last pre-molar and first molar or before it.
I n Anthropoidea where this line is rather anterior it is undoubtedly due to the bending of the face on the basi-cranial
axis.
The skull of Tarsius is characterized by a short face and
large orbits, and also by the partial closure of the back wall
of the orbits, by a rounded neurocranium and by an anteriorly
situated and downward facing foramen magnum. Professor
Clark is inclined to think that these are all secondary features
consequent upon the anteroposterior pressure caused by the
enlarged orbits. In Lemurs the orbits look lateralward, but
in Nycticebus they are not only larger but also look more
forward. The neurocranium of Nycticebus is also relatively
rounder than that of Lemur. Thus, it is clear that some of
the changes that characterize the tarsioid cranium have been
attained by Nycticebus, though to a lesser degree.
Let us connect the anterior and posterior borders of the
two external auditory meatuses with two lines and obtain a
horizontal band as is shown in figure 5. Now, if we observe
the relation of occipital condyles to this band we observe that
in Tupaiidae and all the Lemuroidea the condyles are way
behind this band, while in Tarsius and Anthropoidea, excepting a few cases, the anterior part of the condyles fall invariably into this band. In man they reach to a more anterior
position in this band than in the other Primates. This condition is undoubtedly an advanced and specialized feature
peculiar to higher Primates. However, in Alouatta, some
gibbons and a few other Anthropoidea the condyles are behind
this band. I n Nycticebus not only the index is very high but
also the condyles are way behind this band, probably even more
posterior than in the Lemurs. From this we may infer that if
Clark’s suspicion that the enlargement of orbits was responsi-
40
MUZAFFER SULEYMAN ~ E N Y U R E K
ble for the anterior position of the foramen magnum in Tarsius
was right we should expect a more anteriorly placed foramen
magnum and thus a lower index in Nycticebus.
Let us suppose that the most primitive Primate neurocranium was oval, or, for the sake of discussion say, rectangular, and that the foramen magnum was at the very back of
the skull; then, if the orbits were enlarged as in Tarsius, and
if, on account of enlargement, the breadth of the neurocranium was widened at the expense of the length, the distance between the condyles and the band between the two
auditory meatuses would be gradually decreased, but always
the foramen magnum would be a t the very back of the skull.
Figure 5
It would just be pulled forward with the posterior wall of the
cranium and would always be at the back. If we suppose that
this process was continued for a long time, then the occipital
condyles would fall in this band, but in this case the auditory
meatuses would be at the very back of the skull rather than
both the condyles and auditory meatuses being nearer the
center of the neurocranium as in Tarsius. The pull of the
enlarged orbits would be exerted along the zygomatic arches
and along the occipital crests and it is plausible that this would
tilt the foramen magnum and make it look even more backward than it did before. I n view of these theoretical objections, and the evidence of Nycticebus, Clark’s suspicion is
unjustified. We are justified in concluding that the foramen
CRANIAL EQUILIBRIUM INDEX
41
magnum of Tarsius has moved forward irrespective of the
enlargement of the orbits.
However, it appears from the fossil record that the relatively low index of the Tarsius has been acquired independently of the low index in the Anthropoidea; for even at
the Necrolemur stage, where the index was still high, the
Tarsioids were already specialized and thus could not give
rise to the living Anthropoidea. The Anthropoidea and
Tarsioidea could only have separated before the Necrolemur
stage when the index was relatively high. Thus it seems that
this relatively anteriorly placed foramen magnum of Tarsius
was attained independently, and also that it was not a secondary development consequent upon the distortion caused by
the enlarged orbits as Clark is inclined to believe. I believe
it was brought about by latent potentialities in this direction,
which were developed by the common ancestors of the
Tarsioidea and Anthropoidea, and thus, in the course of time,
this advanced feature was independently acquired in the two
suborders.
SUMMARY AND CONCLUSIONS
The evolution in the order Primates, except in Alouatta,
has been from a high to a low index, or, in other words, the
cranial fulcrum has moved forward. Homo sapiens has the
most anteriorly placed cranial fulcrum of all the Primates.
This provides a mechanical advantage in the erect posture.
But it is a relatively recent feature, for the ancestral Hominidae undoubtedly had a much more posteriorly placed cranial
fulcrum. The female Primates usually have a more anteriorly
located cranial fulcrum than the males of the same species.
The anteriorly placed cranial fulcrum appeared as a neogenetic
feature in the young of the infra-human Primates. Homo
sapiens in his evolution has retained this infantile feature
of the infra-human Primates ;but in his phylogeny the cranial
fulc.rnm has moved more forward than it has in any of these.
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