Only a small fraction of the organisms that live ever become fossilized, and almost all of those that do have hard parts such as bones or shells. The many with only soft parts are not preserved, although now and again we find an imprint of one of their bodies. Bony and shelled creatures therefore dominate the discovered fossil record. We find only a fraction, and an unknown fraction at that, of the complete record.
A different problem arises from the way in which rocks are exposed at the earth's surface. As the Grand Canyon shows so beautifully, sedimentary rock formations generally are horizontal or not far from it. They may extend in area for hundreds or thousands of miles. But where are such rocks exposed? Not along a horizontal surface. With few exceptions, there they are covered either with soil or by other rock layers. To observe bedrock, we usually have to find a spot where some human or natural agent, like the Colorado River, has made a vertical cut down through the rocks, exposing a cross section. Although rock formations extend for vast horizontal distances, they can be seen and sampled only here and there, in road cuts, quarries, river banks, sea cliffs, and so forth, and therefore we have access to but a tiny fraction of their true volumetric extent. The lack of rock exposure causes us to find fewer organisms than actually lived and therefore to underestimate the true ranges of species.
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