ANTH 310 Human Origins
LABORATORY #1
PRIMATE SKULLS AND TEETH
Reading:
In preparation for this lab please be sure to read background material in A Photographic
Atlas for Physical Anthropology Chapters 1,3,4 paying particular attention to the teeth and skulls of the specimens used in this lab (e.g., slow loris, ring-tail lemur, capuchin, baboon, gorilla, etc.)
Introduction:
The objectives of this lab are to introduce you to some of the terminology used in discussing teeth and skulls and to review the basic morphological characteristics of both. This information will enable you to compare the dentition and skulls of existing species (fossil humans and fossil ape, for example). The lab is divided into two sections, dentition and skulls.
I.
Dentition.
All eutherian mammals (primates are eutherian mammals as are dogs, deer, elephants, etc.) have heterodont teeth. That is, heterodont teeth have different shapes and functions according to their positions in the jaws. Heterodont mammalian teeth have evolved from the homodont
teeth of reptiles.
Homodont
teeth are basically the same in morphology and function regardless of their positions in the jaws.
The study of teeth is useful for several reasons. Tooth form shows little plasticity
, changes slowly through time, and tends to reflect specialized adaptations for grooming or for particular diets.
Teeth are also decay-resistant after death and are among the most common and useful paleontological remains. Presented below are some dental terms and concepts you should know.
A.
Dental formula
In mammals there are four types of teeth in each quadrant of the mouth, from front to back: incisors (I), canines (C), premolars (P), and molars (M).
I = Incisors (bladelike or peglike in form, anterior in position)
C =
Canines
(conical or dagger-shaped teeth, at forward corners of jaws in most species)
PM =
Premolars
(shorter, heavier teeth, behind canines)
M =
Molars
(large-crowned teeth, at rear of tooth row)
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Premolars and molars together are commonly called "cheek" teeth and in some mammals they are separated from the incisors or canines by a gap called a diastema
. The dental formula expresses the number of each kind of teeth (counting from the front to the back) for one half of the mouth (upper and lower quadrants).
Example
: The basic permanent dentition of eutherian mammals is believed to be represented by the formula:
3.1.4.3 = 11 X 2 = 44
3.1.4.3 = 11
This formula represents 3 incisors, 1 canine, 4 premolars and 3 molars in each half jaw; i.e., 11 in one quadrant, 22 in one half of the mouth and 44 teeth for the entire mouth. In using this notation, it is important to remember that if the first molar (designated M
1
) is missing, the next tooth in line,
M
2
, is still called M
2
, even though it is the first of the molars which are actually present. This is also true where teeth have been "lost" in evolution. For example, humans have only two premolars rather than the ancestral mammalian four premolars. Since the "lost" teeth are P
1
and P
2
, the premolars present in humans are designated P
3
and P
4
, at least when comparing humans with "lower"
(prosimian) primates.
In humans the dental formula is 2.1.2.3
X 2 = 32
2.1.2.3
In primates the greatest dichotomy in dental formula and in teeth in general, probably occurs between prosimians and higher primates.
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B.
Dental function
. Teeth vary in size and shape, partly as a result of adaptations to different functions within the mouth.
The characteristics of the teeth of any species of animal reveal a great deal about the species.
The teeth of herbivores are very different from the teeth of carnivores, for example. Similarly, the patterns of wear and breakage of the teeth tell us a lot about the kinds of foodstuffs which the individual ate and, in the case of humans, other purposes for which are individual used his teeth.
Since the teeth of closely related species are usually more similar to each other than the teeth of more distantly related species, teeth are useful in assessing the relationships of existing species.
Because teeth are very durable, they are also very useful in determining the relationships among extinct species.
In humans , teeth serve a limited function--they are used mainly for eating and occasionally as accessory tools. Indeed, the importance of manufactured tools as a part of man's biology is largely responsible for this decreased role. Below are some features of the dentition of nonhuman primates to which specialized roles have been ascribed.
1.
Dental comb
. In many prosimian species, the lower incisors and sometimes the lower canines have become thin and bladelike and lie in a nearly horizontal position
(called procumbency
). They form the so-called comb, which is used in grooming the fur as well as for eating. Be sure you recognize this specialization either in casts of teeth, on skeletons on exhibit, or in the drawing of the tree shrew.
2.
Hypertrophied incisors
. You will notice, in the examples of gorilla, chimpanzee, and orangutan jaws available, that the incisors are quite broad and heavy compared with those of yourself and most other primates. This is an apparent adaptation to a diet of coarse plants, which require very strong ripping and tearing on the part of the animal.
Notice also that the canines not only project, but are massively built. The upper canines shear against the sharp (sectorial) lower first premolars (P
3
) and act as accessories to the heavy incisors. Be sure you recognize these anatomical features.
3.
Projecting canines
. Many primates have large, dagger like canine teeth (especially upper canines). These are well developed in males of various monkey species. Aside from the obvious grasping functions that they serve in nearly all primates, they also have social functions. Males may use them in offense or defense, but they are more commonly used in threatening or offensive behavior. Such threats have a major role in maintaining order within the group, and in defending the group against incursions by outsiders of their own or other species. By contrast, small canines are one of the most distinguishing features of human teeth. Throughout human evolution, the canine have become more incisors-like, and extend only very little beyond the tooth row.
C. Additional terminology and tooth anatomy.
1.
Occlusal plane
- the line on which the teeth meet when the jaws are closed.
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2.
Symphyseal region
- the thickened area at the point where the mandible is most curved
(the midline between the central incisors). The mandibles of higher primates (other than humans) are strengthened in this area by the development of internal buttressing
. If the thickening occurs at the lower margin, it is called a simian shelf . If it occurs halfway up the body of the mandible, it is a mandibular torus
. In the evolution of humans both of these kinds of internal buttressing have been lost, but have been replaced by an external buttress
, the mental protuberance
(chin).
3. - very primitive (early) primates have tooth rows that are divergent toward the back. Throughout evolution the shape of the tooth row has changed from those like the modern gorilla (rectangular, long tooth row), to a parabolic, or arc, shape as in modern humans. One coincident result of this is that the entire face in humans has become less prognathic. The snout has become generally smaller and the cranial vault has expanded out over the face. In hominids evolution, the parabola of the tooth row has become wider.
4. Premolar morphology - elongation of lower first premolar (P
3
) crown for shearing against the upper canine is termed sectorial condition
. This single-cusped premolar forms a cutting slicing complex with the cutting edge of the upper canine. The cuspid pattern of premolars is either bicuspid or unicuspid. The vast majority of premolars in humans are bicuspid. Another trend in human evolution is the molarization of the premolars, that is, they become wider and provide a more effective grinding surface.
5.
Molar tooth morphology
Many of the major differences in molar morphology in primates reflect differences in diet. Primates that primarily eat insects, for example, have high, pointed cusps enabling them to puncture and digest the insect=s body).Folivore primates have high shearing crests or blades on their molars that enable them to break up leaves and other plant material more easily. Fruit eating primates, in contrast, have low, pointed cusps for crushing fruit. Primates the eat a lot of tough, hard foods such as seeds and nuts have low, rounded sups on their molars for eating these materials. Further, primates that eat tough good objects also have thick enamel (the hard outer layer of a tooth) on their molars to help them withstand the strong chewing forces.
A clear distinction is made between the morphology of the molars of Old World monkeys and apes. The Old World monkeys (cercopithecoids) have bilophodont
molars, which usually have four cusps situated in two parallel rows. That is, a marked division occurs between the mesial and distal sets of cusps. Hominoids, on the other hand, have two or more 5-Y (or Y-5) molars in their mandibles. The 5-Y molars have five cusps in a patterns that resembles a AY@.
Other molar cusp patterns include +4, Y-4, +5 etc.
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Exercise 1
. Record observations on the dental morphology of the specimens provided.
Feature/Primate Loris Capuchin
(N.W. Monkey) or
Macaque (O.W.
Monkey)
Name (Print):____________________ or
Chimp
Human
Dental formula
Tooth row shape
Incisors (tooth comb, etc.)
Canines
Premolars
(sectorial, bicuspid)
Molar morphology
(bilophodont, cusp pattern)
Simian shelf/chin
Other
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II.
Skull Morphology
A.
Prognathism
. Some higher primates possess a muzzle, but one which is a dental muzzle rather than an olfactory muzzle as commonly seen in non-primate mammals. Prognathism is the angle which the face makes with the rest of the skull; both alveolar (maxillary) and facial (upper and lower jaws) prognathism are recognizable. One measure of alveolar prognathism is the so-called
Alveolar index = Basion-prosthion length Nasion-basion length x 100
; whose values may be classified into the following categories:
Orthognathic = 97.9 & under
Mesognathic = 98 - 103
Prognathic = 103+
(straight faced)
(medium faced)
(jutting face)
B. Muscle attachment . The surface of the skull serves as a place for the attachment of muscles. When the muscles are large and the surface area of the skull is small, special crests develop. In an animal with a heavy jaw but a small brain case, a special crest, the sagittal crest
, develops along the top for the attachment of the temporalis muscle
. The nuchal crest
, in the back, is for the attachment of nuchal muscle
which serve to hold the skull upright on the spine.
The brow ridge is also a manifestation of a heavy jaw. Compare these features on the skulls of the apes.
C.
Evolutionary trends
. W.E. LeGros Clark
*
lists the following trends in primate evolution which can be seen in the skull:
1. The progressive enlargement of the brain case, which becomes more globular in shape and often lacks the prominent bony crests of flanges found in many other mammals.
2. The enclosure of the orbit by a complete bony ring and, in higher primates, the formation of a bony wall shutting it off behind.
3. The gradual rotation forward of the orbits until their openings look directly forward.
4. The attenuation of the bony arch of the cheek (zygomatic arch).
5. The recession of the snout, and the retraction of the face to a position below rather than in front of the cranium.
6. The shrinkage of the bony apparatus of the nose.
7. The progressive shifting of the foramen magnum from the back end of the skull to its basal position, so that it comes face downward instead of more or less directly backward.
*
W.E. LeGros Clark, History of the Primates , fifth ed. (Chicago: University of Chicago Press,
1965), p. 23.
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Exercise 2
: Record the following cranial observations in the following primates.
Feature/Primate Prosimian (Lemur) Capuchin monkey)
(
N.W.Monkey)
Prognathism & alveolar index
Muscle attachments
(sagittal & nuchal crest, temporal line)
Size, location & appearance of eye sockets (e.g., postorbital closure)
Facial skeleton
(zygomatic arch etc.)
Foramen magnum
position
Brow ridge development/pos t-orbital constriction
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Name (Print):_________________
Gorilla Human