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Evolution__3rd_Edition

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Geological maps show the locations<br />

of rocks of various ages<br />

CHAPTER 18 / The History of Life 525<br />

contain can then be picked up, or dug up, on land (a fossil is, etymologically, anything<br />

that is dug up). Sediments may also be lost by tectonic subduction and geological<br />

metamorphosis.<br />

Any particular sedimentary rock will be made up of sediments that were deposited at<br />

a certain time, or through a certain range of times, in the geological past. Any fossils in it<br />

will be from organisms that lived at the time the sediments were deposited. It is possible<br />

to draw a geological map of an area showing the ages of the rocks that are either exposed<br />

at the surface or are near the surface but concealed beneath the topsoil. Plate 10<br />

(between pp. 68 and 69) is a geological map of North America. Maps of this kind, at<br />

varying levels of detail, have been produced for many areas of the globe. A geological<br />

map is a first guide to where it may be possible to find fossils of particular ages.<br />

Dinosaurs, for example, lived in the Mesozoic. We can read directly from the geological<br />

map of the USA that the orange and red regions, for instance in Texas and New Mexico,<br />

and up through Colorado, Wyoming, and Montana, are appropriate regions to hunt<br />

for fossil dinosaurs. Abundant dinosaur remains can indeed be found at some sites in<br />

those regions. The pattern of rock types on a geological map can be understood in<br />

terms of the theory of plate tectonics.<br />

Over geological time, the original hard parts of an organism will be transformed<br />

while lying in the sedimentary rock. Minerals from the surrounding rock slowly<br />

impregnate the bones or shell of the fossil, changing its chemical composition.<br />

Calcareous skeletons also change chemically. There are two forms of carbonate:<br />

aragonite and calcite. Aragonite is less stable and becomes rare in older fossils, and<br />

calcite may be replaced in some fossils by silica or pyrite. In extreme cases, the calcite<br />

may be dissolved away completely, and the space filled in by other material: the fossil<br />

then acts as a mold, or cast, for the new material. The remains still reveal the shape of<br />

the organism’s hard parts.<br />

Fossilization is an improbable eventuality. It is more likely for some kinds of species<br />

than others, and for some parts of an organism than others. After burial in sediment,<br />

the fossils slowly transform through time, but the transformed remains, if they are preserved,<br />

can still tell us (after expert interpretation) much about the original living form.<br />

18.2 Geological time is conventionally divided into a series<br />

of eras, periods, and epochs<br />

18.2.1 Successive geological ages were first recognized by characteristic<br />

fossil faunas<br />

Figure 18.1 shows the main time divisions of the geological history of Earth during the<br />

past 550 million years. Earlier divisions are recognized too, but the past 550 million<br />

years are paleontologically the most important because fossils are much less common<br />

before this time.<br />

The time divisions in Figure 18.1 were recognized by nineteenth century geologists<br />

on the basis of characteristic fossil faunas. The times of transition between two eras<br />

are times of transition between different characteristic fossil faunas: the fossils of the

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