Biology Reference
In-Depth Information
ramus (which is located considerably more anteriorly in the fox squirrel than deer
mouse) and the other at the most anterior angular point. Efforts to reconcile these two
schemes and to apply it to other squirrels revealed the difficulty of designing one
general-purpose scheme for rodent jaws. That difficulty arises from the combination
of sparse landmarks and morphological disparity. That disparity makes the mandible
interesting but it also makes it difficult to rely on Type 2 landmarks that are defined by
inflection points along curves. Those inflection points, when visible at all, are not neces-
sarily in corresponding anatomical regions. Thus, the scheme that we devised has only
15 landmarks and relies heavily on semilandmarks.
The first two landmarks are the points at which the incisor emerges from the alveolus,
marking the upper and lower edges of the alveolar opening. Landmark 3 is the mental
foramen. Landmark 4 is a point on the masseteric fossa where it changes direction
the
most anterior point of the masseter muscle attachment. This point is often difficult to
locate reliably because the masseteric fossa is often effaced, the change in direction is not
always abrupt, and seeing it clearly depends on the depth of field and lighting, but photo-
graphing the mandible to make this point most visible would make other points difficult
to visualize. Landmark 5 is on the molar alveolus between the fourth premolar (p4) and
the first molar (m1), and landmarks 6 and 7 are also on the molar alveolus, 6 is between
m1 and m2, and 7 is at the midpoint of m2. Landmark 8 is the “tip” of the coronoid pro-
cess although, in many species, this process does not taper to a point but rather ends
bluntly. The landmark is then placed at the midpoint of the blunt end. The next two land-
marks are on the condyle, the part that articulates with the upper skull to form the jaw
joint. Landmark 9 is on the anterior of the condyle and landmark 10 is at the posterior end
of the articulating surface. Landmark 11 is at the posterior “corner” of the angular process
which is the most posterior point of the superficial masseter muscle attachment.
Landmark 12 is another point on the condyle, the tip of the zygomaticomaseteric fossa.
This point, like others that are out of anatomical order were added after finding that the
original scheme did not suffice. Landmark 13 is on the molar alveolus at the midpoint of
p4. The reason for using the midpoint of the tooth rather than its most anterior point is
that the alveolar bone is sometimes so heavily eroded that there is no bone at the anterior
point of the tooth. Using that point would mean having to find a point on the tooth where
the alveolar bone would be had it not eroded. Landmark 14, the most difficult one to
locate reliably, is on the ventral margin of the angular process, where the ramus curves
towards the angular process (this marks the boundary between ramus and angular pro-
cess). The landmark is the most anterior point that is definitively on the angular process.
Landmark 15 is the most anterior point on the base of the coronoid process, where it
departs from the plane of the molar tooth row (it forms a “V” with alveolus).
Given such a sparse and uneven coverage of the jaw, and the lack of landmarks where
we need them, we sampled the six complex curves as semilandmarks. In Figure 2.12 , one
curve may seem too densely sampled
the one between landmarks 8 and 9. It hardly
takes 14 semilandmarks to describe the shape of that curve. But that is because the land-
mark scheme is shown on a tree squirrel's mandible. In other squirrels, such as the ground
squirrels and chipmunk, shown in Figure 2.13 , that coronoid process is far longer and the
curve is much deeper. The measurement scheme is designed to capture that kind of infor-
mation about jaw shape regardless of where it occurs.
Search WWH ::




Custom Search