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CRANIAL AND DENTAL VARIABILITY IN COLOBUS MONKEYS

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SUMMARY

A total of 309 skulls of wild guereza moakeys from one small area in Liberia is described in detail for the chief purpose of contributing new data on the exact degrees of variability in non-human primates for ultimate comparisons with the variability in past and recent populations of man. Three different forms of Golobus are represented in this material, namely polykomos polykomos (Zimmermann), badius badius (Kerr) and badius verus (Van Beneden). It seems quite likely that the latter two are different species, rather than subspecies. Polykomos is the largest and verus the smallest of the three. Sex differences in size are generally most marked in polykomos and least in badius, the former also being more variable than the other two. On a general average the measurements of the facepart of the skull are much more variable than those of the brainpart. Preliminary comparisons regarding the variability of cranial dimensions show little difference between Colobus and most other primates for which corresponding information has become available.

Among the distinguishing characters of these Colobtis the degree of prognathism is to be specially mentioned, since it is very marked in polykomos in contrast to verus which is orthognath. Verua also differs from the other two forms by having the proportionately largest orbits and narrowest inter-orbital region. The orbital roof is much thinner in polykomos than in the smaller species. The relative position of the cranio-vertebral joint differs somewhat according to species and to sex, it being most orally situated in verus, most aborally in polykomos and regularly more orally in females than in males.

The temporal lines do not approach each other as closely, nor meet nearly as often in polykomos as in the other two species. Sagittal crests develop by far most frequently in male verus in which they often extend forward over a large part of the frontal bone. These crests increase with advancing age particularly in specimens with comparatively small braincases. Occipital crests are best developed in polykomos and least in verus, which lacks this structure in numerous specimens.

As the absolute dimensions, so are the relative ones, or indices, more variable for the splanchnocranium than the neurocranium. The degrees of variability of cranial indices are roughly alike in Colobus and in other wild primates from limited regions. The number of infraorbital foramina varies intensively in all these Colobus series, but the number of mental foramina is fairly stable. The formation of the pterion region varies in all Colobus in a high degree, often even on the two sides of the same skulls. A parieto-sphenoid contact exists in three-fourths of the polykomos, but in less than half of the badius.

Rare and abnormal cranial variations are not nearly as frequent in Colobus as in the man-like apes. Some of these variations are evidently of a hereditary nature. Bregmatic bones, e.g., are fairly common in badius whereas totally absent in polykomos. The ascending arms of the intermaxillary bones are highly variable in all Colobus and in badius they frequently reach the frontal bone and can occasionally even meet above the nasal bones. In the lower lateral corners of the intermaxillaries there are symmetrical accessory sutures in eight Colobus. These sutures run diagonally to the rim of the alveoli for the lateral incisors and have never been found so far in any other primates except in gorillas, which possess them with remarkable regularity.

Suture closure is recorded in detail. Most facial sutures remain open much longer than those of the braincase, but even the latter persist in Colobus to relatively much later stages of life than in the man-like apes. The exact sequence of suture closure can vary considerably, but no more than in most other simian primates. The interfrontal suture, which normally closes early in postnatal life, was found open in several adult Colobus. Such cases of metopism, extremely rare in other monkeys and apes, are apparently not uncommon among Colobus according to the literature and these new instances.

Among the few juveniles Colobus in the present material there are several indicating that the permanent molars tend to erupt relatively earlier in this genus than in the great majority of monkeys and apes. In Colobus, as in the other genera of the Colobinae, underbite is much more common than in the remaining catarrhines and overbite is extremely rare.

The present series of skulls and dentitions of wild-shot Colobus contains in general comparatively few cases with malformed or diseased conditions. These are described with particular reference to their frequency. Healed fractures have been found in three skulls and ten others show signs of infections, mostly in the orbital region. By far the most frequent pathological conditions are represented by alveolar abscesses which exist in a fairly high percentage of the oldest specimens. Not a single case of caries could be found and only one instance of a supernumerary tooth, but congenitally lacking teeth exist in seven skulls.

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