Biology Reference
In-Depth Information
FIGURE 2.12
The major anatomical features of
the rodent mandible and the landmarks used in
studies of mandibular developmental, modularity
and evolution.
assess the within-modular covariances. That obviously requires having more than one or
two landmarks per module. A classic hypothesis of mandibular modularity posits six
such modules (
Atchley and Hall, 1991
), but there are only 15 landmarks in the entire
data set, and four of them are within one hypothesized module (the molar alveolus) and
three are within another (the condyloid process). By simple arithmetic, it is obvious that
the remaining eight landmarks cannot be enough to sample the other four hypothesized
modules.
The landmark scheme shown in
Figure 2.12
is designed to analyze modularity, both
developmental and evolutionary, for dissecting the developmental origins of morphological
disparity and for exploring the evolution of jaw function in relation to ecology. Because
these are interrelated issues, a single scheme is needed that can be applied across disparate
morphologies without sacrificing the ability to detect subtleties of intraspecific variation.
This scheme has evolved; two prior studies used different landmarking schemes. The initial
one was designed for a myomorph, the prairie deer mouse Peromyscus maniculatus bairdii
(Zelditch et al., 2008) and then the analysis was extended to a sciuromorph, the eastern fox
squirrel, Sciurus niger (Zelditch et al., 2009). Myormorphs and sciurumorphs differ in the
number of cheek teeth so the scheme was modified to include the premolar, but the more
consequential difference is in the position of the landmark on the ventral curve of the ramus
(see
Figure 2.12
, landmark 14). In deer mice, there are two points where the landmark could
have been located; one is located where the curve of the mandible ceases to follow the curve
of the incisor and instead follows the curve of the angular process; this was the boundary
between incisor and ramal modules. No landmark was placed more proximally, at the most
anterior point on the angular process. The scheme devised for the sciuromorph included
two ventral landmarks, one that marked that transition in curvature from incisor to
