Evolution__3rd_Edition

524 PART 5 / Macroevolution
Fossilization of soft parts, ...
. . . and even hard parts, is rare
Fossils are preserved in sediments
18.1 Fossils are remains of organisms from the past and are
preserved in sedimentary rocks
A fossil is any trace of past life. The most obvious fossils are body parts, such as shells,
bones, and teeth; but fossils also include remains of the activity of living things, such as
burrows or footprints (called trace fossils), and of the organic chemicals they form
(chemical fossils). For any organism to leave a fossil requires a series of events, each
of which is unlikely. We can consider these events for the case of hard parts, though
analogous points also apply for trace and chemical fossils.
When an organism dies, its soft parts are usually either eaten by scavengers or decay
by microbial action. For this reason, organisms that consist mainly of soft parts (such as
worms and plants) are less likely to leave fossils than are organisms that have hard parts.
Some fossils of soft parts do exist, but they were either deposited in exceptional circumstances
or preserve exceptionally abundant life forms. Fossil plants often take the form
of “compression fossils” in which the soft parts of a plant have been squeezed flat. Coal,
for instance, contains huge numbers of compressed fossil ferns. However, even in
plants, the majority of fossils are of hard parts, such as resistant spores or seeds.
Although an organism’s hard parts stand the best chance of fossilization, even they
are usually destroyed rather than fossilized. Hard parts may be crushed by rocks,
stones, or wave action, or broken up by scavengers. If the hard parts survive, the next
stage in fossilization is for them to be buried in sediment at the bottom of a water
column a only sedimentary rocks contain fossils. (Geologists distinguish three main
rock types: igneous rocks, often formed by volcanic action; sedimentary rocks, formed
from sediments; and metamorphic rocks, formed deep in the Earth’s crust by the metamorphosis
of other rock types a when sedimentary rocks undergo metamorphosis,
any fossils are lost.)
Animals that normally live within sediments are more likely to be buried in sediment
before being destroyed. These animals are therefore more likely to leave fossils than are
species that live elsewhere. Likewise, species that live on the surface of the sediment
(i.e., on the sea bottom) are more likely to be fossilized than are species that swim in the
water column. Terrestrial species are least likely of all to be fossilized. The further a
species lives from sediments, the less likely it is to be fossilized. For most of the delicate
kinds of animals that live on the sea bottom, such as feather stars and worms, practically
the only way they may come to leave fossils is by “catastrophic” burial, such as a
slide of sediment from shallower water into the depths that carries with it and buries
some soft-skeletoned animals. Feather stars, for instance, are known to decay into
nothing within 48 hours of death on the sea bottom; they therefore have to be buried
rapidly to have any chance of fossilization.
Once an organism’s remains have been buried in the sediment, they can potentially
remain there for an indefinitely long period of time. As new sediment piles on top of
older sediment, the lower sediments are compacted a the water is squeezed out and the
sedimentary particles are forced closer together. The fossil hard parts may be destroyed
or deformed in the process. As the sediments compact, they are gradually turned into
sedimentary rock. They may subsequently be moved up, down, or around the globe
by tectonic movement, and can be re-exposed in a terrestrial area. Any fossils they
..