Living primates cover a very large range of body sizes, extending from 1 oz (30 g) for the pygmy mouse lemur (Micro-cebus berthae) to about 375 lb (170 kg) for a full-grown adult male gorilla. There is accordingly a more than 5,000-fold difference between the smallest and largest living primates. As a rule, fossil primates fall at the lower end of this size range, although some of the recently extinct subfossil lemurs of
Madagascar were comparable in size to an adult female gorilla (175 lb [80 kg]). The earliest known fossil primates from the Eocene and Oligocene were generally quite small. Some of them were apparently even smaller than the pygmy mouse lemur, while the biggest probably did not exceed 22 lb (10 kg). Among living primates, it is notable that nocturnal species are generally markedly smaller than diurnal species. The average body weight for nocturnal primates is about 1 lb (500 g), whereas the average body weight for diurnal primates is approximately 11 lb (5 kg), representing a ten-fold difference.
The hands and feet of primates are typically adapted for grasping rather than grappling while moving around. A widely divergent big toe (hallux) provides the basis for a powerful grasping action of the foot in all primates except humans, while the hand usually exhibits at least some grasping capacity. In most primates, the digits (fingers and toes) typically bear flat nails rather than narrow claws, and in all cases the hallux bears a nail. On the ventral surfaces of the hands and feet there are tactile pads with skin ridges (dermatoglyphs) that serve an anti-slip function on twigs and branches. These skin ridges, in combination with special tactile sense organs (Meissner's corpuscles), also permit enhanced tactile sensitivity. Patterns of movement (locomotor sequences) are typically hindlimb-dominated. The location of the body's center of gravity is typically closer to the hindlimbs, with the result that the typical walking gait shows a diagonal sequence (forefoot precedes hindfoot on each side). In the foot, there is usually at least some degree of relative elongation of the distal segment of the heel bone (calcaneus). Primates also tend to have longer limbs, in relation to overall body size, than other mammals, and this results in increased stride length. The visual sense is greatly emphasized in primates. The eyes are relatively large and in the eye sockets (orbits) there is at least a bony strut (postorbital bar) on the outer margin. A large degree of binocular overlap is ensured by pronounced forward rotation of the eyes and orbits. The proportions of nerve fibers passing from the retina of each eye to the two sides of the brain are approximately balanced and they are organized in a very unusual way such that the opposite half of the visual field is represented in each half of the brain. The ventral floor of the bony capsule protecting the middle ear (auditory bulla) is formed predominantly by the petrosal bone, which is unusual among mammals. Partly because of the increased emphasis on vision, the primate brain is typically enlarged at least to some extent, relative to body size, in comparison to other living mammals. The brain of living primates always possesses between the frontal and the parietal lobes a true Syl-vian sulcus (joining the rhinal sulcus) and a complex calcarine sulcus on the inside of the occipital lobe. Primates are unique among living mammals in that the brain constitutes a significantly larger proportion of body weight at all stages of fetal development. The dental formula exhibits a maximum of two incisors, one canine, three premolars and three molars on each side of upper and lower jaws, differing from ancestral mammals in the loss of one incisor and one premolar from each toothrow. In association with the reduction in the number of incisors, the premaxilla bone at the front of the upper jaw is very short, and the incisors are arranged more transversely than longitudinally. The cheek teeth are typically relatively unspecialized, although the cusps are generally low and rounded, while in the lower molars the heels (talonids) are raised and enlarged.
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