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.