Geology 143

Lecture #22

November 15, 2000

Introduction to Primates

Primates (Eocene - Recent) are mostly small, arboreal placental mammals, living in moist, tropical environments that are not conducive to fossilization. As a result the primate fossil record is diffuse. Fortunately, a great diversity of primates survive, and the evolutionary relationships and taxonomy of the living forms is beginning to stabilize. With the exception of humans, all primates have an opposable hallux (big toe) bearing a flat nail instead of a claw. The human hallux is aligned with the other digits of the foot, as adaptation for habitual bipedal locomotion. Many (though not all) primates possess an opposable pollex (thumb) as well, and all the anthropoids (see below) possess flat nails on all digits. The digits bear soft tactile pads for improved grip and dexterity. Primate orbits are directed forward, producing a broad range of stereoscopic vision. No primate has more than two incisors per quadrant.

The primary taxonomic division within primates distinguishes strepsirhines (which, from a human perspective, retain a number of primitive traits reminiscent of generalized mammals) from haplorhines (to which we and our closer relations belong). Strepsirhines primitively possess a postorbital bar, with the orbit confluent with the temporal fenestra; haplorhines have a closed postorbital region. Haplorhines also possess a short snout and a dry nose. Strepsirhines primitively possess a wet nose, like other mammals. See Primate Taxonomy.

Strepsirhini (lemurs, lorises, and galagos) are characterized by a specialized grooming claw on digit II of the foot and the presence of a "tooth comb" comprised of closely-packed, thin, cylindrical lower incisors and canines. Lemurs are indigenous to Madagascar, where there are no monkeys, and include both nocturnal and diurnal species. Lorises and galagos ("bush babies") inhabit Africa and Southern Asia and are exclusively nocturnal, avoiding competition with monkeys.

Members of the Tarsioidea are haplorhines, but like strepsirhines (and other mammals in general) they have a pair of dentary bones, each comprising a single side of the lower jaw (a primitive trait). Tarsiers comprise a genus of tiny, nocturnal, arboreal insectivores. They leap through the trees with the aid of extremely long tarsal bones, for which they are named. Much of the skull is occupied by the two large orbits.

(Note: the tarsiers and strepsirhines are occasionally lumped together under the heading of "prosimians," an informal term for non-anthropoid primates.)

Members of the Anthropoidea all have a larger braincase as compared to strepsirhines and tarsiers, the dentaries are fused to form a solid mandible (lower jaw), and all digits bear flat nails. There are two living subgroups of anthropoids:

Platyrrhini (New World monkeys) have a specialized prehensile tail, but commonly retain the primitive anthropoid dental formula of 2-1-3-3 / 2-1-3-3. These originally South American primates now range northward into Mexico. (If you like, you may download the "howl" of a howler monkey.) The term playrrhine refers to a broad nose; catarrhines typically have a more elongate nose.

Catarrhini (Old World monkeys, apes, humans) do not possess a prehensile tail (and some species lack a visible external tail). However, all catarrhines have a reduced dental formula of 2-1-2-3 / 2-1-2-3. Most species live in Africa or Southern Asia; there are two living catarrhine subgroups.

Cercopithecoidea (Old World monkeys) include some primarily terrestrial forms such as baboons as well as many arboreal species, which typically are specialists for leaping among branches with their long hindlimbs. The tail is retained in arboreal groups for balance and steering while airborne, but is lost in some terrestrial species. Molar teeth of cercopithecoids are bilophodont (having two prominent enamel crests) as an adaptation for herbivory. Leaf-eating specialists have a complex stomach (seen as the "paunch" of this male proboscis monkey). More generalized feeders among cercopithecoids (baboons) will occasionally prey on small mammals, with the aid of large, sharp canines, and a blade-like lower premolar.

Hominoidea (apes and humans) lack a (visible) tail. Their molar teeth on lower jaw have five prominent cusps. Apes are specialists for knuckle-walking and brachiating, with forelimbs longer than hindlimbs, and a shorter trunk in comparison to monkeys. Apes are intelligent mammals with large brains.

Three living families are recognized:

Hylobatidae: Gibbons are relatively small, Southeast Asian apes that are excellent brachiators (specialists at swinging among branches using their hands and arms; brachiation is highlighted in this movie, which you can download). The siamang, featured in the pictures and media clips here, is the largest gibbon species and a famous vocal inhabitant of many zoos.

Pongidae (Asian great apes: orangutan and its fossil relatives)

Hominidae (chimpanzee, bonobo, gorilla, and humans).

As compared to the chimpanzee and gorilla, which have a rectangular jaw and dental arcade (array of teeth), humans have a shorter jaw with a more curved, parabolic dental arcade.