Geoscience Reference
In-Depth Information
estimate the true ranges of species in the fossil record and to think
they became extinct before they actually did. (In Chapter 9 we see a
good example in the ammonites.)
Fossil material is usually composed of carbonate or phosphate,
chemicals that can dissolve in groundwater. Thus even though an
organism becomes fossilized, its remains may later be dissolved away
and disappear. This causes us to underestimate the true range of a fos-
sil species, or in the extreme, to miss it entirely.
POOR PRESERVATION AND EXPOSURE
Only a small fraction of the organisms that live ever become fos-
silized, 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 beau-
tifully, 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 sur-
face. 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 sec-
tion. Although rock formations extend for vast horizontal distances,
they can be seen and sampled only here and there, in road cuts, quar-
ries, 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.
BIOTURBATION AND REWORKING
As discussed in an earlier chapter, many marine animals—clams, for
example—burrow downward into the sediment beneath, dragging
down younger material from the surface and bringing older mater-
ial back up, an effect known as bioturbation. When these disturbed
sediments eventually harden into rock, the fossils that they contain,
as well as any iridium and tektite layers, are stretched out over a
broader range than the one in which they were deposited. Studies of
bioturbation in modern sediments have shown that material can be
