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Fossil Introduction
Fossils are any evidence of past plant or animal life contained in sediment or a
sedimentary rock, from bones, shells or teeth to imprints of a dinosaur’s foot. For
example, the ridges, bumps, and curves of a fossilized clam are the same ridges,
bumps, and curves that existed as it filtered water from the sea long ago. Studying
fossils gives clues about the Earth’s past: its climate, natural disasters, and changing
landforms and oceans. Fossils can also help scientists figure out the actual age of the
rock, since they were there when it originally formed from sediments.
Through fossils, scientists have learned that the Earth has been inhabited by
living things since very early in its four-and-a-half-billion-year history. For most of that
time, however, up until about six hundred million years ago, organisms were small
primitive cyanobacteria (blue-green algae) that lived in the oceans. Around that time,
evolution began to speed up. By about 500 million years ago the oceans were
teeming* with larger marine invertebrates (lacking a backbone or spinal column) such
as mollusks, brachiopods, bryozoans, and echinoderms. Marine is a term used to
describe things that lived in, come from, or deal with the sea.
The fossil record of early life is very scanty*. Fossils become much easier to
find, in the right kinds of sedimentary rocks, after the great, explosive development of
shelled invertebrates. Almost all fossils are in sedimentary rocks. A rock that
contains fossils is said to be fossiliferous (“fossil-bearing”). Fossils are pretty much
nonexistent in igneous rocks, and very uncommon in metamorphic rocks. Eventually,
both plants and animals invaded the land, which is not as hospitable to life as the
oceans. Life both on land and in the sea has had its ups and downs in the past half
billion years. There have been several major episodes during which large percentages
of all life forms on Earth became extinct. The cause (or causes) of these mass
extinctions is not yet entirely clear. After each, however, the surviving life forms
evolved into new living things that populated both the oceans and land.
Fossils give us an amazing picture of past life on Earth, but the picture is hardly
complete. Only a tiny proportion of organisms that have ever lived are preserved as
fossils. The ones that have been preserved tend to be mostly small, shelled
invertebrates (organisms without backbones) that lived on the beds of shallow
oceans. This is because hard skeletal materials, like bones and shells, have a far
higher probability of preservation than soft tissues, which tend to decompose
quickly. Fossils of soft-bodied organisms, such as mammals, are very rare. Because
fossils are limited to hard parts, we have little idea, or have to guess, about other
features of an organism, such as what color it was, whether or not it had fur, or even
what the size of its nose was.
For most organisms, fossilization is a very rare event. Most places where
organisms live and die, whether in air or underwater, have a good deal of
oxygen. Oxygen and water lead to decay. In this process the organic material that
makes up the tissues of the dead organism is changed back mostly to carbon dioxide
and water. Only when an organism is quickly buried by sediment and sealed off from
any oxygen can it escape decay and become fossilized. This happens as the sediment
is slowly transformed into sedimentary rock. The picture is better, of course, with
animals with hard skeletal materials.
Fossils are rarely the original unchanged remains of plants or animals. Fossil
formation begins when an organism or part of an organism falls into soft sediment,
such as mud. The organism or part then gets quickly buried by more sediment. As
more and more sediment collects on top, the layer with the organism or part becomes
compacted. Minerals from water then move into the pore spaces between sediment
particles. The sediment cements together and becomes rock with the organism or part
inside.
Sometimes, open pores in the rock let water and air reach the organism or part,
causing it to decay or dissolve. What is left behind is a cavity in the rock where the
organism or part was. This empty space is called a mold. A mold shows the original
shape and surface of the organism or part. Sometimes, sand or mud fills a mold and
hardens, forming a cast of the original organism or part. A cast is a replica of the
original organism. Petrification happens when mineral solutions remove the original
organism or part and replace it with new minerals. The replacement of the original
materials is generally a very slow process. The result is a nearly perfect mineral
replica of the original organism or part.
Bones, teeth, shells, and other hard body parts can be fairly easily preserved as
fossils. However, they might become broken, worn, or even dissolved before they are
buried by sediment. The soft bodies of organisms, on the other hand, are relatively
hard to preserve. There need to be special conditions to preserve organisms like
jellyfish. Sometimes these organisms fall to the muddy sea bottom in quiet water and
are buried rapidly by more mud. For that reason, the fossil record of soft-bodied
organisms is much less well known than the record of hard-bodied organisms. Other
organisms or their hard body parts have a better chance of becoming part of the fossil
record. These are called body fossils. Body fossils are the remains of the actual
organisms. Normally, only the hard skeleton is preserved (shell or bone), and the soft
tissue (skin, muscle, organs, etc.) rots away after death.
Trace fossils are physical evidence of the life activities of now vanished
organisms. Trace fossils include tracks, trails, burrows, feeding marks, and resting
marks. For example, a trace fossil is the trail left behind by an ancient reptile that
dragged its tail in mud. Another example is the footprints left by dinosaurs along an
ancient river or the hollow tubes created by worms burrowing in soft mud in an
ancient ocean. There are more trace fossils than body fossils because one organism
can leave behind many traces (e.g. footprints), but only one set of hard parts (e.g.
bones) to become a fossil.
Most trace fossils were formed in soft mud or sand near a pond, lake, river, or
beach. The imprints left by the organisms were quickly covered by sediment. The
sediment dried and hardened before the imprints could be erased by water or
wind. The sediment was then buried under more sediment and became compacted
and cemented together to form rock. This process is much the same as the formation
of body fossils.
Fossils may be preserved in a number of different ways, but they can be placed
into two groups, Direct Evidence, and Indirect Evidence. With direct evidence, ancient
organisms are preserved without any alteration. The typical type of direct evidence is
when an organism's bones, shell, or teeth, their hard body parts, are fossilized. In the
rare case, soft body parts may actually be preserved, giving us a rare look at the
organisms’ organ system. In both cases, the only changes in the original body
composition is that the less stable organic material has been removed. Hard body
parts can also be completely changed or replaced. Where direct evidence gives tells us
about the organism's anatomy, indirect evidence gives us information about how the
organism lived. Indirect evidence includes mold and casts, tracks and trails, burrows
and borings, and coprolites
From the Fossil Unit of the American Geosciences Institute http://www.k5geosource.org/2activities/1invest/fossils/index.html 7/31/14
Fossil Types by Stars and Sea http://www.starsandseas.com/SAS%20Evolution/SAS%20fossils/fossil_types.htm 4/15/16
FYI
Some very well-defined fossils can survive a certain degree of metamorphism
(change in a rock through heat and pressure). Finding a fossil in a metamorphic rock
is an exciting event for a geologist because metamorphism “resets” a rock’s age to
the date of the metamorphic event. Not all sedimentary rocks, however, have
fossils. If they parachuted you out of an airplane to land on an outcrop of
sedimentary rock, the chance of your finding a fossil would be rather small – nowhere
close to one hundred percent. Some kinds of sedimentary rocks tend to have more
fossils than other kinds of sedimentary rocks. Limestones generally contain the
most. That is not surprising, because most limestones consist in part, or even
entirely, of the parts of shelly marine organisms. Most coarse-grained limestones, and
many fine-grained limestones as well, consist mostly of whole shells or pieces of
shells.
Many shales, very fine-grained layered sedimentary rocks originating from
freshly deposited mud, contain large numbers of fossils, because certain organisms
like to live on muddy sea floors. Shales are often rich in trace fossils, but less so in
body fossils except when the chemical conditions during deposition were better for
preservation rather than decomposition. The best representatives of soft-bodied
organisms are from shales, although examples of such preservation are very
uncommon. Many sandstones are fossiliferous as well, although the body fossils in
sandstones are usually relatively tough shelly materials, which are not highly
susceptible to chemical decomposition. Conglomerates are the least fossiliferous of
sedimentary rocks.