Biology Reference
In-Depth Information
FIGURE 3.14
Ontogenetic change in body shape of
S. gouldingi
depicted by vectors of relative landmark dis-
placement computed from three superimpositions. (A) BC coordinates from the 1
7 baseline; (B) GPA; (C)
RFTRA.
ratio, but by decreasing the denominator. Because we come to the pictures informed by
our knowledge that body length increases over ontogeny, it may be difficult to grasp that
it decreases relative to depth. We would probably avoid saying that body length decreases
relative to depth, simply because that phrasing is disconcerting to biological intuition;
instead, we would say that depth increases relative to length. When pictures show a
relative
decrease in a feature that is increasing in
absolute
length, readers may need some
explanation of the unexpected contrast. In particular, it is important to explain that the
decrease is in
relative
(not absolute) length.
Other apparent inconsistencies between pictures can also be reconciled, usually by
concentrating on the changes in
relative position
s of landmarks rather than on the vectors at
individual landmarks. It may take a lot of practice before this is easy. For example, look
at the circled landmark in
Figure 3.15
. If you look only at this landmark, the results
from the different superimpositions appear to be inconsistent. That landmark appears to
“move” quite far anterodorsally in the BC superimposition (
Figure 3.15A
), but much less
and in two different directions in the other superimpositions: anteroventrally in GPA
(
Figure 3.15B
), and almost entirely ventrally in RFTRA (
Figure 3.15C
). However, none of
these statements actually reflect what the pictures show. None of the pictures shows the
independent movement of any one point in isolation; rather, what they show is the relative
displacements of all points.
We get a better indication of the displacement of the circled landmark relative to neigh-
boring landmarks by “connecting the dots”
drawing line segments between landmark
locations to approximate the profile of
S. gouldingi
's head. In
Figure 3.16
, we show the
same superimpositions and ontogenetic displacements of landmarks as in
Figure 3.15
, just
