Page 153 - Introduction to Paleobiology and The Fossil Record
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140 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
the male and female look very different from 5 Ecophenotypic effects, where local eco-
each other? logical conditions affect the form of an
Actually paleontologists need not be too organism during its lifetime (see p. 123).
downhearted. Where the biological and mor-
phological species concepts have been applied Geographic variation may be substantial
to particular groups, both generally give the among members of modern species, particu-
same answers. Further, it would be wrong for larly those distributed over wide ranges.
a systematist of modern organisms to be too Sexual dimorphism is seen in living animals,
smug. Most decisions on the species bounds particularly in those where males engage in
of living plants and animals are based on ritualized displays, or where females have
assessments of the morphologies of dead spec- special reproductive activities. Sexual dimor-
imens in museums: it is impractical to carry phism is also common in fossils, and it has
out extensive crossbreeding tests even with often caused serious problems of identifi ca-
living organisms. tion where males and females look very dif-
Problems with fossil species usually arise ferent. For example, many ammonites show
from the added dimension of time. If a pale- sexual dimorphism, where the postulated
ontologist finds a long evolving lineage, where females are much larger than the males, and
should the dividing line be drawn between the males possess unusual lappets on either
one species and the next? Decisions are often side of the aperture (Fig. 6.2).
made easier by gaps in the fossil record that Most organisms change substantially in
create artificial divisions within evolving lin- form as they grow from egg to adult, and
eages. Where gaps are not present, splitt- these growth stages will be explored next.
ing events clearly mark off new species. If Ecophenotypic variation was introduced in
there are few of these, an evolving lineage is Chapter 5 (see p. 123) and this includes all
divided somewhat arbitrarily into chronospe- the changes in form that may occur through
cies (“time species”), each being defi ned by an individual’s lifetime, but that are not coded
particular morphological features. genetically. Ecophenotypic variation in a
human might include minor features such as
the acquisition of powerful arm muscles
Variations in form within species
through work or exercise or the loss of liver
Within a species, there may be a range of function through alcohol abuse. Major eco-
morphologies; think of the variation among phenotypic changes might include the loss of
humans, or more dramatically, among domes- a limb in an accident or a carefully main-
tic dogs. In naturally occurring species, mor- tained Mohican haircut. None of these
phology may vary as a result of several changes can be passed on genetically to a son
factors: or daughter by the legless, muscular or unusu-
ally coiffed individual.
1 Individual variation, the normal differ-
ences between any pair of individuals of a
species that are not identical twins; this Allometry
base level variation is the stuff of natural Changes in form during growth are common.
selection, as Darwin stressed. Think of human growth: babies have rela-
2 Geographic variation and physical differ- tively large heads and eyes, and small limbs.
ences between populations or subspecies Similar features are found in fossil examples
in different parts of the overall species too. Juvenile vertebrates, not just humans,
range. usually have large eyes and heads in propor-
3 Sexual dimorphism, in which males and tion to overall body size. A tiny embryo of an
females may show different sizes, and dif- ichthyosaur (Fig. 6.3) shows just these fea-
ferent specialized features (horns, antlers, tures. If measurements of the variable parts
tail feathers) often related to sexual (eye diameter, head length) are scaled against
selection. a standard measure of the animal (total body
4 Growth stages, where there may be quite length, for example), it is evident that the
different larval and adult stages, or where proportions change as the animal grows older
body form alters during growth. (Fig. 6.4). In the case of the ichthyosaur, the
