LOWER PRIMATES The Tree-Shrews.—The tree-shrews (Tupaiidae) are generally classed as Insectivora (q.v.) but they are related to Primates in many anatomical details. The pen-tailed tree-shrew (Ptilocercus) of Borneo is a mouse-like arboreal animal. Its hands and feet are five-toed and provided with claws, the thumb and great toe some what divergent. The food includes insects and fruit; the upper molar teeth are tritubercular, the lower molars tuberculo-sectorial (see MAMMALIA). The skull is lemur-like, especially in the ring shaped orbit and inflated auditory bulla, which completely en closes the ring-like tympanic bone and ear-drum. The brain is primitive in having relatively unreduced olfactory parts and very feebly-developed neopallium or higher part of the brain. Tupaia and related genera of India and the Malayan region are in many ways more advanced toward the lemur type.
Imperfect fossil jaws and teeth, apparently of tree-shrews not greatly different from modern forms, have been found in Lower Eocene deposits in Wyoming.
equal antiquity are the oldest known fore runners of the true primates, represented by fossil jaws and teeth from the Lower Eocene of Wyoming and New Mexico and ap parently belonging respectively to the tarsioid and lemuroid di vision of the order. Even at this early date a breaking up of the groups into different genera was in progress. In some of the tarsioid genera the incisors were already enlarging and the molars acquiring round, blunt cusps instead of the sharp-edged V-shaped cusps of their Cretaceous insectivorous ancestors. Probably from some of the less specialized of these Lower Eocene tarsioids sprang the line leading to Pseudoloris of the European Eocene. This genus in turn had already progressed far toward the modern Tarsius. The most famous Lower Eocene tarsioid was the little skull Anaptomorphus (Tetonius) homunculus, at one time, though erroneously, supposed to be in the direct line of human descent. That strange-looking animal, the spectral tarsier, sur vives today in the forests of the larger Malay islands. About as large as a good-sized rat, it has long jerboa-like hind legs, with which it can make extraordinary leaps, presumably to capture insects. Both the hands and feet have long spreading digits tipped with flattened discs for grasping the branches of the trees in which it lives. The lower part of the heel-bone and navicular are length ened into long narrow rods to increase the leaping power. The
huge eyes are brought forward so that their inner borders almost meet across the nose, which is small and resembles the platyrrhine type. The ears are very large. The brain, although large, is of remarkably low type for a primate. Accordingly the skull, which largely reflects the character of the brain and sense organs, re veals enormous circular orbits, a swollen braincase, expanded au ditory bullae and a reduced, greatly constricted nasal-chamber. The jaws are slender and the teeth small, the upper molars being transversely widened with low, rounded cusps. Tarsius appears to be the rather specialized survivor of a very old primate stock structurally intermediate between the tree-shrews and lemurs be low and the monkeys, apes and man above. Most of the known Eocene tarsioids are too specialized in their teeth to be the ancestors of the monkeys (with possible exceptions noted below). The foot structure of at least two of these forms already showed more or less of the characteristic elongation of the heel-bone and navicular.
Along with the tarsioids in the Lower and Middle Eocene of North America occur the fossil teeth and jaws of another group of Primates, constituting the family Notharc tidae, the members of which ranged in size from a chipmunk to a cat. During the Eocene, the first and second upper molar teeth gradually evolved from a three-cusped to a four-cusped stage. Apparently the entire family then became extinct, but some of them may possibly have given rise to the ancestral South Ameri can monkeys. In Notharctus osborni from the Middle Eocene of Wyoming the hands and feet were of the grasping, climbing type, much like those of existing lemurs. The skull was far less specialized than that of contemporary tarsioids, both skull and dentition retaining the primitive features of lemurs on the one hand and monkeys on the other. Thus the dental formula of Notharctus (Ii P1 1\13) X2=40 is the primitive one for all
primates. The braincase is primitive with no great expansion of the brain and with an unreduced nasal-chamber. The orbits were protected behind by a rim of bone, not by a fully developed par tition. The auditory region and base of the cranium, backbone, forearm, pelvis and hind-feet were like those of lemurs.