Landmarks and Semilandmarks - Geometric Morphometrics for Biologists - page 33

shows the latter part of postnatal ontogeny of a ground squirrel mandible. The few Type 1
landmarks are the mental foramen and juxtapositions between teeth and bone ( Figure 2.7A ).
These are nearly invariant over ontogeny (relative to each other and to the centroid of the
form). The changes in proportion are evident in the regions sampled by Type 2 landmarks
( Figure 2.7B ) and a far richer description is obtained when that sparse set of landmarks is sup-
plemented by semilandmarks; there are dramatic changes in the proportions and orientations
of the angular and coronoid process ( Figure 2.7C ).
EX AMPLES: APPLYING IDEALS TO ACTUAL CAS ES
Having discussed some general principles and theory, we now turn to specific and
concrete examples of data. They display an obvious vertebrate bias, especially a bias
towards mammal skeletons. That is because these examples are taken from our own
work rather than from a review of the literature. We focus on our own examples for
tworeasons.First,wecanexplainourownreasoning.Second,becausewehavethe
data for these examples, we can use that data to demonstrate methods throughout this
book.
Landmarks on the Lateral Surface of the Squirrel Scapula
Figure 2.8 shows the major anatomical features of a tree squirrel scapula. Also shown
are 12 landmarks that were digitized in a study of changes in scapula shape associated
with the evolution of burrowing in chipmunks and ground squirrels ( Swiderski, 1993 ).
Studies of scapulae of other mammals have found important changes in the blade,
acromion and metacromion associated with functional shifts ( Oxnard, 1968; Taylor, 1974;
Stein, 1981 ). These same studies found little or no change in the coracoid process and the
bell-shaped structure that articulates with the humerus (hidden behind the metacromion
FIGURE 2.8 The major anatomical features of a tree squirrel scapula,
shown are the 12 landmarks analyzed in a study of the evolution of bur-
rowing in chipmunks and ground squirrels ( Swiderski, 1993 ).