Landmarks and Semilandmarks - Geometric Morphometrics for Biologists - page 25

major distinction between landmarks and semilandmarks for landmarks, their position

(in all directions) contains information about shape. In contrast, for semilandmarks, their
position in the direction of the curve contains no information about shape. Once that non-
shape information is removed, semilandmarks are treated just like landmarks, meaning
that they are also matched up one-to-one when computing the difference between shapes.
For that reason, semilandmarks are treated as if they are homologous, and one criticism
of semilandmarks is that decisions about their homology are not based on comparative
anatomy, as in the case of landmarks, but rather are delegated to algorithms ( Klingenberg,
2008 ). However, that contrast between landmarks and semilandmarks may be framed too
starkly because comparative anatomy is often not enough to establish the homology of
landmarks. For example, two points often used in studies of mandibular shape are the
superior and inferior “angular notches” ( Figure 2.3A ). These notches are inflection points
along curves whose curvature can vary considerably, even within species ( Figure 2.3B,C )
and more so across disparate species ( Figure 2.3B,D,E ). The notches correspond as inflec-
tion points on the curves but it is not clear where the anatomically corresponding points
are along those curves. Comparative anatomy may suffice to establish the homology of the
notches, but it does not specify where, precisely, a homologous point can be found within
the notch.

FIGURE 2.3 Angular notches. (A) Angular notches shown on the mouse mandible; (B and C) mandibles
of two conspecific ground squirrels, Spermophilus beecheyi; (D) mandible of another ground squirrel,
Ammospermophilus leucurus; (E) mandible of a tree squirrel, Sciurus nayaritensis.