Evolution__3rd_Edition

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Geological maps show the locations
of rocks of various ages
CHAPTER 18 / The History of Life 525
contain can then be picked up, or dug up, on land (a fossil is, etymologically, anything
that is dug up). Sediments may also be lost by tectonic subduction and geological
metamorphosis.
Any particular sedimentary rock will be made up of sediments that were deposited at
a certain time, or through a certain range of times, in the geological past. Any fossils in it
will be from organisms that lived at the time the sediments were deposited. It is possible
to draw a geological map of an area showing the ages of the rocks that are either exposed
at the surface or are near the surface but concealed beneath the topsoil. Plate 10
(between pp. 68 and 69) is a geological map of North America. Maps of this kind, at
varying levels of detail, have been produced for many areas of the globe. A geological
map is a first guide to where it may be possible to find fossils of particular ages.
Dinosaurs, for example, lived in the Mesozoic. We can read directly from the geological
map of the USA that the orange and red regions, for instance in Texas and New Mexico,
and up through Colorado, Wyoming, and Montana, are appropriate regions to hunt
for fossil dinosaurs. Abundant dinosaur remains can indeed be found at some sites in
those regions. The pattern of rock types on a geological map can be understood in
terms of the theory of plate tectonics.
Over geological time, the original hard parts of an organism will be transformed
while lying in the sedimentary rock. Minerals from the surrounding rock slowly
impregnate the bones or shell of the fossil, changing its chemical composition.
Calcareous skeletons also change chemically. There are two forms of carbonate:
aragonite and calcite. Aragonite is less stable and becomes rare in older fossils, and
calcite may be replaced in some fossils by silica or pyrite. In extreme cases, the calcite
may be dissolved away completely, and the space filled in by other material: the fossil
then acts as a mold, or cast, for the new material. The remains still reveal the shape of
the organism’s hard parts.
Fossilization is an improbable eventuality. It is more likely for some kinds of species
than others, and for some parts of an organism than others. After burial in sediment,
the fossils slowly transform through time, but the transformed remains, if they are preserved,
can still tell us (after expert interpretation) much about the original living form.
18.2 Geological time is conventionally divided into a series
of eras, periods, and epochs
18.2.1 Successive geological ages were first recognized by characteristic
fossil faunas
Figure 18.1 shows the main time divisions of the geological history of Earth during the
past 550 million years. Earlier divisions are recognized too, but the past 550 million
years are paleontologically the most important because fossils are much less common
before this time.
The time divisions in Figure 18.1 were recognized by nineteenth century geologists
on the basis of characteristic fossil faunas. The times of transition between two eras
are times of transition between different characteristic fossil faunas: the fossils of the