Page 163 - Introduction to Paleobiology and The Fossil Record
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150 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
In early studies of the Hox genes of Dro- a storm? How well could pterosaurs fl y? Why
sophila, experimenters were amazed to dis- did sabertoothed cats have such massive
cover that mutations in particular Hox genes fangs? The most fascinating questions concern
might cause the insect to develop a walking those fossil organisms that are most different
leg on its head in place of an antenna. The from living plants and animals. This is because
mutations were not simple changes of the it is easy to work out that a fossil bat proba-
base-pair sequence, but knockouts or dele- bly flew and behaved like a modern bat. But
tions of entire functional portions and replace- what about a pterosaur: so different, and yet
ment of their expression domains by more similar in certain ways?
posterior Hox genes. Study of such knockouts There are three approaches to interpreting
showed how each Hox gene worked; in this the function of fossils: comparison with
case the Hox gene acted on the limb bud, the modern analogs, biomechanical modeling and
small group of cells on the side of the body circumstantial evidence. Let us look at some
that appears early in development and eventu- general assumptions first, and then each of
ally becomes a limb. A particular Hox gene those approaches in turn.
determines how many limb buds there are and The main assumption behind functional
where they are located, and other Hox genes morphology is that biological structures are
determine whether the limb bud becomes a adapted in some way and that they have
walking leg, a mouthpart or an antenna. If evolved to be reasonably effi cient at doing
experimenters induce a knockout within a something. So, an elephant’s trunk has evolved
Hox gene, it works its magic in the wrong to act as a grasping and sucking organ to
place, giving the fly extra legs or legs in the allow the huge animal to reach the ground,
wrong place. Mutations of Hox genes in ver- and to gather food and drink. The fl ower of
tebrates normally do not produce these spec- an angiosperm is colorful to attract pollinat-
tacular effects; the embryo often fails and is ing insects, and the nectar is located deep in
aborted. the flower so the insect has to pick up pollen
Such mutations need not always result in as it enters. The siphons of a burrowing
damage. Duplication of homeobox genes can mollusk are the right length so it can circulate
produce new body segments, and such dupli- water and nutrients when it is buried at its
cations may have been important in the evolu- favored depth.
tion of arthropods and other segmented Fossils can provide a great deal of funda-
animals. The new evo-devo perspective allows mental evidence of value in interpreting func-
us to understand that an arthropod with tion. For example, the hard skeleton of a
numerous body segments and 10 or 100 legs fossil arthropod reveals the number and shape
may have evolved by a single evolutionary of the limbs, the nature of each joint in each
event, perhaps a relatively straightforward limb, perhaps also the mouthparts and other
mutation of homeobox genes, rather than an structures relating to locomotion and feeding
elaborate multistep process of gradual addi- (see p. 362). Even a fossil bivalve shell gives
tion of segments and legs through many sepa- some functional information in the hinge
rate evolutionary events. The evo-devo mechanism, the pallial line (which marks the
revolution is beginning to explain some of the extent of the fleshy mantle) and the muscle
most mysterious aspects of evolution. scars (see p. 334). Exceptionally preserved
fossils may reveal additional structures such
as the outline of the tentacles of a belemnite
INTERPRETING THE or ammonite (see p. 344), muscle tissue (see
FUNCTION OF FOSSILS p. 64) or sensory organs. The fi rst step in
interpreting function then is to consider the
Functional morphology
morphology, or anatomy, of the fossil.
Inferring the function of ancient organisms is The vertebrate skeleton can provide a great
hard, and yet it is the main reason many deal of information about function. The
people are interested in paleobiology. Just maximum amount of rotation and hinging at
how fast could a trilobite crawl? Why did each joint can be assessed because this depends
some brachiopods and bivalves mimic corals? on the shapes of the ends of the limb bones.
How did that huge seed fern support itself in There may be muscle scars on the surface of
