Primate Morphology and Evolution

Primate Evolutionary Trends

• Relatively large complex brain, especially cerebral cortex.
• Decreased reliance on olfaction and corresponding reduction of snout and brain area associated with smell.
• Increased reliance on vision and corresponding enlarged visual cortex and adaptations that increase visual acuity and color and depth perception.
• Generalized diet and dentition.
• Tendency toward upright posture.
• Prehensile (grasping) hands and feet with opposabil ity(big toe and thumb can touch other digits of foot and hand, respectively).
• Nails instead of claws.
• Increase in efficiency of placental nourishment to developing fetus.
• Long pre- and post-natal periods with greater reliance on learning.
• Tendency toward diurnality.

Prosimians vs. Anthropoid Primates

Prosimians exhibit skeletal characteristics that distinguish them from anthropoid primates. Two joints of the craniofacial region differ between prosimian and anthropoid primates. When growth ceases in anthropoids, the mandibular symphysis (midline lower jaw joint) and the frontal suture (midline forehead joint) fuse. In prosimians, these joints remain open throughout life. The prosimian dental formula, that is, the numbers of the various types of teeth (for example, incisors, canines, premolars, molars) in each half of each of the jaws differs from all but some of the New World monkeys, in that they have an extra premolar. Thus, they have 36 rather than 32 teeth. They exhibit procumbent, or forward-oriented, lower incisors. They use this “dental comb” to comb through and groom their fur. The retention of a claw on the second digit of the feet of most species also serves a grooming function. Anthropoid primates are distinguished by the presence of extra bone in the back of the eye sockets/orbits so that their eyes are encased in bone. Prosimians lack this postorbital closure.

Prosimian Characteristics

• Unfused metopic suture and mandibular symphysis.
• Dental formula 2–1–3–3.
• Dental comb.
• Grooming claw.
• Lack of postorbital closure.
• Nocturnality with corresponding visual adaptations:enlarged eyes with rite mirabile.
• Greater dependence on hearing and mobile external ear flaps.
• Greater reliance on olfaction: relatively long muzzle with moist rhinarium.
• Upper lip is split and fused to the upper jaw.
• Lack of facial musculature result in inexpressive face.
• Digits act together (powerful grip).
• Tendency toward vertical clinging and leaping as locō motor pattern.
• Multiple births and corresponding pairs of teats.

Most of the extant prosimians are nocturnal and, with the exception of the tarsier, which is intermediate in many physical characteristics between the prosimians and anthropoids, they possess an adaptation for increased visual acuity in low light conditions, termed a tapetum lucidum.They have relatively more acute auditory abilities as well, and have mobile external ear flaps, as seen in other mammals. They also rely on olfaction more than most anthropoid primates and, thus, retain the primitive mammalian characteristic of a moist fleshy pad (rhinarium) at the tip of their relatively elongated muzzle. They practice scent marking and possess scent glands on various parts of their body. Their upper lip is split and fused and they lack the facial musculature seen in the higher primates, resulting in a lack of facial expression capabilities.

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