Biology Reference
In-Depth Information
Until some experience in finding this landmark is gained, it may be difficult to be consis-
tent in reproducing this point. An inexperienced person usually has error in the anteropos-
terior axis. This is a landmark for which some argument regarding homology must be
made. This is because the internal skeleton may not be consistent with the point used
externally. However, consistently measured as the posterior termination of the body at the
lateral midline, the point may be considered homologous.
Landmark 12 represents the ventral side of the articulation between the quadrate bone
and the mandible. It thus lies lateral to the midline, although it usually lies on a vertical
from the ventral midline. This point is located by placing a fingernail in the joint between
the two bones, and then a pin is inserted in the joint. Landmark 13 lies at the intersection
of the maxillary bone and the infraorbital bone that defines the “cheek” area of the face.
The point lies well lateral to the midline, but marks an important area of the skull, approx-
imating the length of the upper jaw. This point is marked by slipping the pin under the
infraorbital bone adjacent to the posterior border of the maxillary. This landmark is com-
posed of an extreme point (the posterior maxillary border) as well as an intersection of two
structures. The homology of this landmark may be questioned. Landmarks 14 and 15 capture
the width of the bony orbit. Each point lies at the extreme of the orbit along the anteropos-
terior body axis. Both of these landmarks are of questionable homology, but are taken
because the eye has been shown to be highly allometric and has been used in traditional
measurement schemes. With practice, these landmarks can be taken with little error.
Landmark 16 is perhaps the most difficult to justify in this analysis. It occupies the
most posterior point of the bony opercle, the bone that forms the bulk of the gill cover,
and its original purpose was to duplicate the landmark used in traditional ichthyology
measurements of head length. This landmark was expected to be an articulation point
between the opercle and subopercle bones. However, comparisons among several species
showed that the position of the articulation varied excessively, and inaccurately repre-
sented the posterior of the head. The landmark now taken is simply the extreme along the
bone border as measured from the tip of the snout. No reasonable homology argument
can be made for this landmark; it may be that it is partially redundant with landmark 11.
However, our analyses have shown that this landmark can be consistently digitized and is
often informative about alterations in head shape.
Landmarks 4
11 represent points where the fins insert on the body, at the
anterior or posterior of the fin base. In most cases, these points are measured where the
bony fin ray intersects the body. Together, these landmarks provide a great deal of infor-
mation on post-cranial body shape. Landmarks 4 and 5 lie at the anterior and posterior of
the dorsal fin base, respectively. Ontogenetic variation in anterior fin ray morphology can
reduce the repeatability of landmark 4, as discussed in
Fink (1993)
.
Landmarks 8 and 9 represent the posterior and anterior of the anal fin base. Often the
fin is collapsed, and a pin must the inserted to make landmark 8 visible. In some piranhas,
there are accessory spines at the anterior of the fin base, and they are not included.
Landmarks 10 and 11 represent the insertion onto the underlying skeletal girdles of the
pelvic and pectoral fins, respectively. Both lie dorsolateral to the ventral midline.
Landmark 10 is easily visible in larger specimens but, in some smaller specimens, the
transparency of the fin makes it difficult to find; in this case it can be located by raising
the fin laterally and placing a pin at the anterior fin-ray's base.
6 and 8
