Function and fusion at the mandibular symphysis

Abstract

The purpose of this investigation is to determine the functional significance of a fused mandibular symphysis, characteristic of all Anthropoidea. The trait may date to the origins of the suborder. A histological study of 11 prosimian species determined the anatomy of the symphysis. A cinefluorographic film ofGalago crassicaudatus mastication was studied to identify movements at the symphysis. A similar pattern of fibrocartilage and ligaments characterizes all prosimians studied. These tissues are arranged to resist (a) movements seen during mastication inG. crassicaudatus, i.e., antero‐posterior shear and spreading of the inferior borders of the symphysis and (b) hypothesized dorso‐ventral shear resulting from the transfer of force from the balancing side muscles to the bite point. Partial fusion of the symphysis was found in the folivorous speciesLemur fulvus, L. macaco, Propithecus verreauxi, andHapalemur griseus. Only those tissues associated with resisting occlusally or dorso‐ventrally directed forces were calcifying or ossifying. This research suggests that the added occlusal force necessary for leaf‐eating has resulted in the evolution of varying degrees of symphyseal fusion in the above species. It is suggested that the protoanthropoids also ate tough foods that required relatively large bite forces. Copyright © 1977 Wiley‐Liss, Inc., A Wiley Company