Page 157 - Introduction to Paleobiology and The Fossil Record
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144 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
brain, which again is rather well developed generally high early in the history of a clade
at birth. as the species “try out” all the possibilities of
Ichthyosaurs (see Figs 6.3, 6.4) were born their new body form, and then the disparity
live underwater, as shown by remarkable of the group remains rather constant for the
fossils (see p. 462), and did not hatch from rest of its history. Changes in disparity through
eggs laid onshore, as is the case with most time may roughly mimic changes in diversity
other marine reptiles. Their large head at birth (as diversity increases, so too does disparity),
would have allowed them to feed on fi shes but the correlation is usually not perfect, and
and ammonites as soon as they were born. shape change often goes ahead of diversity
The large eyes were perhaps necessary also for increase.
hunting in murky water, and had to be near-
adult size from the start. Or, perhaps, it made
them look cute and encouraged parental EVOLUTION AND DEVELOPMENT
care!
Ontogeny and phylogeny
Biologists have long sought a link between
Shape variation between species
ontogeny (development) and phylogeny (evo-
Within any clade there are many forms. lutionary history). In 1866, Ernst Haeckel, a
Related plants and animals usually show some German evolutionist, announced his Biogene-
common aspects of form, and species and tic Law, that “ontogeny recapitulates phylog-
genera vary around a theme. For example, eny”. His idea was that the sequence of
gastropods all have coiled shells and the three- embryonic stages mimicked the past evolu-
dimensional shape can be thought of as a tionary history of an animal. So, in humans,
result of variation in four parameters (see p. he argued, the earliest embryonic stages were
333). When form can be reduced to a small rather fish-like, with gill pouches in the neck
number of parameters like this, then the whole region. Next, he argued was an “amphibian”
range of possible forms governed by those stage and a “reptile” stage, when the human
parameters may be defined – the theoretical embryo retained a tail and had a small head,
morphospace for the clade. Studies of the and finally came the “mammal” stage, with
theoretical morphospace for gastropods, growth of a large brain and a pelt of fi ne
ammonoids and early vascular plants show hair.
that known species have only exploited a Haeckel’s view was attractive at the time,
selection of possible morphologies. Some but too simple. Haeckel had drawn on earlier
zones of morphospace may represent impos- work, including Von Baer’s Law, presented in
sible forms – such as gastropods or ammo- 1828, and this law can be matched with
noids with a minute aperture, with no room current cladistic models. Von Baer interpreted
for the living animal – but others have simply the embryology of vertebrates as showing that
not been exploited by chance, or they cannot “general characters appear fi rst in ontogeny,
be reached by normal evolutionary change special characters later”. Early embryos are
because of the impossibility of intervening virtually indistinguishable: they all have a
stages. backbone, a head and a tail (vertebrate char-
The range of forms within a clade may also acters). A little later, fins appear in the fi sh
be described as disparity, the sum of morpho- embryo, legs in the tetrapods. More special-
logical variation. Disparity may be quantifi ed ized characters appear later: fin rays in the
as the range of values for all possible shape fish, beak and feather buds in the chick, snout
parameters seen in species in a clade. All the and hooves in the calf, and large brain and
measures of shape may be combined in a mul- tail loss in the human embryo.
tivariate analysis that can simplify dozens of “General characters appearing before
shape measures to a smaller number of prin- special characters” has taken on a new
cipal coordinates or eigenvectors (see p. 139) meaning with the establishment of a cladistic
so that size and other general principles may view of phylogeny (see p. 129). Von Baer’s
be separated. It is possible to compare the Law draws a parallel between the sequence of
disparity of different clades, or to look at how development, and the structure of a clado-
disparity varies through time. Disparity is gram. In human development, the embryo
