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ontogenetic changes in skull form are spatially integrated or localized (
Zelditch et al.,
1992
) and to study developmental constraints on variability in that species (
Zelditch et al.,
1993
). Because the studies were designed to analyze the ontogeny of skull shape and
its variation, and the data come from photographs, the only landmarks that could be
included are the ones that are visible in (approximately) the same plane at all ontogenetic
stages. Mammalian skulls are highly three-dimensional structures, and the cranial base
rotates during ontogeny, so landmarks that are parallel to the camera at one stage may
rotate out of that plane later. This produces what appears to be a change in shape (within
the plane). However, omitting all the landmarks that might be affected by such a rotation
would mean losing vital information about cranial length and width. The landmarks most
strongly affected by the extension of the cranial base are the ones marking the juncture
between the anterior and posterior cranial base, as well as those located on the posterolat-
eral braincase. Consequently, landmarks were placed on those locations even though that
complicates distinguishing between changes in shape caused by differential growth and
apparent changes in shape due to rotation in the third dimension.
A subsequent study was undertaken to compare skull shape ontogeny of S. fulviventer
to that of another rodent, the house mouse Mus musculus domesticus. A major objective
of that study was to examine the relationship between life-history strategy and timing of
skull morphogenesis (
Zelditch et al., 2003b
). Ideally, we would have sampled both skulls
densely, selecting homologous landmarks that provide a richly detailed description of the
ontogeny of both species. However, some landmarks could be seen in only one species or
another. For example, in S. fulviventer we can locate a landmark on the posterior of the gle-
noid fossa, but the curve of the glenoid is so smooth in M. m. domesticus that we cannot
find a distinct point anywhere comparable to the glenoid landmark of S. fulviventer.To
capture information about skull width in the region of the zygomatic arch of M. m. domes-
ticus, a different point had to be chosen, complicating the comparative analysis. Several
other points that are readily visible in S. fulviventer also cannot be found in M. m. domesti-
cus. However, the problem posed by the inability to find landmarks in M. m. domesticus
that are homologous with those already measured in S. fulviventer is partly mitigated
because there are landmarks in S. fulviventer that had not been previously sampled,
but which can be recognized in both species. Thus, in the comparative study, additional
landmarks were sampled on S. fulviventer. Even so, the set of landmarks common to both
species comprises a rather sparse sample of each skull. Therefore, analyses were done
separately for each species, using the landmarks providing the densest coverage possible
for each species, and the comparative analyses exploited the subset of landmarks common
to both.
The original analyses of S. fulviventer (
Zelditch et al., 1992, 1993
; see
Figure 2.10A
)
include 16 landmarks. Landmark 1 is the lateral margin of the incisive alveolus where
it intersects the outline of the skull in the photographic plane (IN). Landmark 2 is the ante-
riormost point on the zygomatic spine (ZS). Landmark 3 is the premaxilla
maxilla suture
where it intersects the outline of the skull in the photographic plane (PML). Landmark 4
is the premaxilla
maxilla suture lateral to the incisive foramen (PMM). Landmark 5 is
the posteriormost point of the incisive foramen (IF). Landmark 6 is the median mure of
the first molar (M1). Landmark 7 is the posterolateral palatine pit (PP). Landmark 8 is the
junction between squamosal, alisphenoid and frontal on the squamosal
alisphenoid side
