Biology Reference
In-Depth Information
Concern for homology extended to the corners as well as the ends of the ridges.
Landmarks 2, 3 and 4 are at the only corners that are not associated with the ends
of ridges. Other anatomical information was used to infer their homology. Landmarks 2
and 3 are corners where the acromion terminates in a flat surface that articulates with
the clavicle. The corner labeled as landmark 4 appears to mark the boundary between
the acromion and metacromion. This interpretation is reinforced by the point's proximity
to the line of the scapular spine, which separates anterior and posterior components of
both the scapula and the attached muscles.
The grounds for inferring homology are weakest for landmark 12. This is the only point
on the articulating structure, the “bell”, that could be seen in lateral view in all taxa. If more
points on this structure were visible, landmark 12 might not have been used. This point is
identified only as the cranial edge of the neck, which is the narrowest region between the
blade and the bell of the articulating structure. This criterion for recognizing a landmark is
harder to apply than the criteria for recognizing the other 11 landmarks because the bound-
ary between bell and blade is not marked by a corner or other distinctive feature. In this
regard, the neck of the scapula may seem similar to the least interorbital width of the skull,
as being poorly defined and of doubtful homology. However, unlike least interorbital
width, the neck of the scapula marks the boundary of two functionally distinct components
of the scapula. In addition, analysis of digitizing error indicated that this point was not sub-
stantially harder to locate than other landmarks. Therefore, doubts about the homology of
this point were set aside in favor of having at least one landmark on this structure.
Landmarks on the External Body of Piranhas
Figure 2.9
illustrates the landmarks used in several studies of shape change in piranhas.
These points were originally intended for analyses of shape by trusses (see
Strauss and
Bookstein, 1982
), so they were chosen to allow for constructing a series of boxes and
diagonals over the form. In addition, because the truss analysis was to be compared
FIGURE 2.9
Landmarks on the external
body of a piranha, used in several studies
of ontogeny and disparity of piranhas (e.g.
Fink and Zelditch, 1995, 1996; Zelditch et al.,
2000, 2003a
).
