Page 155 - Introduction to Paleobiology and The Fossil Record

142 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

positive allometry is in the antlers of the Irish
100
deer (Box 6.2), and indeed in many other
sexually selected features that are minute or
absent in the juvenile, but very large in the
Orbit length (cm) 10 adult.
Allometry is commonly considered only in
the context of ontogeny, the growth from egg
or embryo through juvenile to adult. But
studies of form may compare species, and
shape variation can be accounted for in an
evolutionary context too. For example, a
1 10 100
comparison of species of antelope would
Skull length (cm)
(a) show positive allometry in leg width: scaled
against body length, the sturdiness or width
50 of the leg increases positively allometrically.
This is because of the well-known biological
scaling principle: some organs and functions
Skull length (cm) sional measure), whereas others relate to body
relate to the mass of an animal (a three-dimen-
length or body outline (one- and two-dimen-
10
sional measures). As body mass (three-dimen-
sional) increases, the diameter of the legs
(two-dimensional) increases in proportion to
support the added weight. So, in body outline,
10 100
small antelope have extremely slender legs,
Backbone length (cm) and larger ones have relatively more massive
(b)
legs.
Figure 6.4 Tests of allometry in the ichthyosaur These aspects of allometry may be under-
Ichthyosaurus. (a) Plot of orbit length against stood in terms of the allometric equation,
skull length, and (b) plot of skull length against
backbone length. The Somerset embryo (Fig. y = kx a
6.3b) is indicated by a solid circle. Both graphs
show negative allometry (orbit diameter = 0.355 where y is the measurement of interest (e.g.
(skull length) 0.987 ; skull length = 1.162 (backbone head length, eye diameter), x is the standard
length) 0.933 ), confirming that embryos and of comparison (e.g. body length), k is a con-

juveniles had relatively large heads and eyes. stant and a is the allometric coeffi cient. The
(Courtesy of Makoto Manabe.) constant k is calculated using the allometric
equation for each particular case. The allome-

tric coeffi cient a defines the nature of the
ratio of eye diameter to body length dimin- slope: if a = 1, the slope is at 45˚ and this
ishes as the animal approaches adulthood. defines a case of isometric growth; if a > 1,

This is an example of allometric (“different we have positive allometry, and if a < 1, we
measure”) growth. If there is no change in have negative allometry (see Fig. 6.4).
proportions during growth, the feature is said After the nature of any allometric change
to show isometric (“same measure”) growth. of parts or organs has been established quan-
Allometric growth is commoner than iso- titatively, it is possible to investigate why such
metric. Positive allometry is when the organ changes might occur. The large eyes and small
or feature of interest increases faster than the noses of babies are said to make them look
isometric expectation, and negative allometry cute so their parents will look after them, and
is when growth of the structure of interest is feed them. But the fundamental reason is pre-
slower than isometry. Head and eye size sumably because the eye is complex and is at
usually show negative allometry, starting rela- nearly adult size in the baby for functional
tively large in the juvenile, and becoming rela- reasons, and the relatively large head of a
tively smaller in the adult. An example of human baby is to accommodate the large