The development of the dentition of Macaca mulatta; its difference from the human pattern

Abstract A metric and roentgenographic study was made on the development of the dentition of 41 specimens and 6 skulls of Rhesus macaques. They covered four consecutive developmental periods. An outstanding finding consisted in the uniformity of the morphogenetic pattern of all the specimens of each period studied. The ontogenetic pattern of the Macaca dentition then was compared with that of man in order to gain some information about the direction of certain evolutionary trends in human phylogeny. 1. 1. In the deciduous dentition of Macaca all the teeth were in proximal contact with the exception of the canines. “Ape spaces” existed mesially and distally to the lower canines and mesially to the upper canines. This arrangement can be found in man among deciduous dentitions of the spaced type. The fully closed arch form, however, is an exclusive human type. 2. 2. A terminal plane forming a mesial step consistently allowed the first permanent molars of Macaca to erupt directly into proper cuspal interdigitation. This mode is rarely found in man. His proper molar occlusion requires a physiologic movement of tooth groups over a variable period of time. 3. 3. In Macaca there existed a distinct interval between the eruption of the permanent first molars and that of the permanent first incisors, during which an extension of the arches and a spacing of the deciduous incisors occurred. In man this phase is only rudimentary when, upon the eruption of the lower permanent first incisor, the still complete upper deciduous arch is somewhat spread in the anterior teeth. 4. 4. No enlargement of the arches was observed in Macaca during the replacement of the incisors. In man the expansion of the arches for the larger permanent incisors takes place during their eruption. Accommodating changes for the larger permanent incisors, therefore, are comparatively delayed and often reduced in man; they could be anticipated, however, in those deciduous human dentures which erupted with fully spaced anterior teeth. 5. 5. In the eruption order of the permanent teeth of Macaca the incisors were followed by the second permanent molars; after a pause, an almost simultaneous eruption of the posterior successional teeth occurred. In comparison to the whole eruption sequence of Macaca, that of man reveals a relative retardation of the accessional teeth and a relative acceleration of the successional teeth. Both phenomena were shown to be related to reduced longitudinal growth of the human jaws. The mandible behaves more progressively in this trend; this is responsible for increased incidence of crowded anterior teeth and deep overbite in modern man. 6. 6. The consistent end-to-end bite of the incisors in Macaca was found to be due to an equal forward development of both jaws in the absence of vertical growth of the alveolar process and mandibular ramus. The determining factor in the degree of incisal overbite in man is likely to be also the alveolar forward development and only secondarily the vertical growth. 7. 7. The dentofacial proportions in Macaca were seen to be subjected to considerable changes during development, whereas Brodie revealed in this respect a high individual consistency in man.