MANDIBULAR FORM AND FUNCTION IN NORTH-AMERICAN AND EUROPEAN ADAPIDAE AND OMOMYIDAE

Previous experimental and comparative studies among a wide variety of primate and nonprimate mammals provide a unique source of information for investigating the functional and phylogenetic significance of vari ation in the masticatory apparatus of Eocene primates. To provide a qu antitative study of mandibular form and function in Eocene primates, t he scaling of jaw dimensions and the development of symphyseal fusion was considered in a broad sample of North American and European Adapid ae and Omomyidae. Statistical analyses indicate a significant size-rel ated pattern of symphyseal fusion across Eocene primates, with larger taxa often having a greater degree of fusion than smaller species; thi s trend is also evident at the family level. As adapids are mostly lar ger than omomyids and these taxa show allometry of symphyseal fusion, this may explain why no omomyids evince complete fusion. Controlling f or jaw size, species with greater symphyseal fusion tend to have more robust jaws than those with a lesser amount of fusion. Upon further ex amination, a primary reason why adapids have more robust mandibles tha n omomyids is associated with the presence of taxa with fused. symphys es, and thus more robust jaws, in the adapid sample, whereas no omomyi ds have fused symphyses. In addition, there is little indication of a dietary effect, as measured by molar shear-crest development, on symph yseal fusion. Moreover, as there is no correlation between molar shear -crest development and skull size, this also points to the absence of a size-related pattern of dietary preference underlying the allometry of symphyseal fusion. Based on the interspecific and ontogenetic allom etry of symphyseal ossification in Eocene primates, jaw-scaling patter ns are used to further examine the functional determinants of fusion i n this group. This study indicates that greater dorsoventral shear dur ing mastication is a more likely factor than lateral transverse bendin g (''wishboning'') in the evolution of symphyseal fusion among ''late- fusing'' mammals like adapids and omomyids. Given that wishboning is a n important functional determinant of symphyseal form in recent anthro poids, apparently the evolutionary development of marked wishboning oc curs only in taxa that shift the timing of fusion to a growth stage pr eceding the onset of weaning (before adult masticatory patterns are fu lly developed) and perhaps first ossified the symphysis to counter ele vated dorsoventral shear stress. As early anthropoids probably consist ed of members varying interspecifically and ontogenetically in the deg ree of ossification, it is especially informative to analyze the adapt ive setting in which anthropoid symphyseal fusion evolved from a simil ar primitive ''prosimian'' perspective. Finally, since taxa with fused symphyses are widely distributed across mammals, a similar analytical framework could be directed profitably at unraveling the functional a nd evolutionary significance of symphyseal fusion in other mammalian c lades. (C) 1996 Wiley-Liss, Inc.