Evidence of the Distant Past:
Have you ever looked through an old family photo album? Each photo shows a little of your family’s history. You
might guess the age of the photographs based on the clothes people are wearing, the cars they are driving, or even the
paper the photographs are printed on.
Just as old photos can provide clues to your family’s past, rocks can provide clues to Earth’s past. Some of the
most obvious clues found in rocks are the remains or traces of ancient living things. Fossils are the preserved remains or
evidence of ancient living things.
In your own words, explain how the study of relative dating and geologic time are supported by fossil remains.
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Formation of Fossils:
Not all dead organisms become fossils. Fossils form only under certain conditions. Most plants and animals are
eaten or decay when they die, leaving no evidence that they ever lived. Think about the chances of an apple becoming a
fossil. If it is on the ground for many months, it will decay into a soft, rotting lump. Eventually, insects and bacteria
consume it.
However, some conditions increase the chances of fossil formation. An organism is more likely to become a fossil if
it has hard parts, such as shells, teeth, or bones. Unlike a soft apple, hard parts do not decay easily. Also, an organism is
more likely to form a fossil if it is buried quickly after it dies. If layers of sand or mud bury an organism quickly, decay is
slowed or stopped.
What conditions increase the chances of fossil formation?
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Fossils Come in All Sizes:
You might have seen pictures of dinosaur fossils. Many dinosaurs were large animals, and large bones were left
behind when they died. Not all fossils are large enough for you to see. Sometimes it is necessary to use a microscope to
see fossils. Tiny fossils are called microfossils. Some microfossils can be the size of a speck of dust.
Types of Preservation:
Fossils are preserved in different ways. Listed below are many ways that a fossil can form.
Preserved Remains – Sometimes the actual remains of organisms are preserved as fossils. For this to happen, an
organism must be completely enclosed in some material over a long period of time. This would prevent it from being
exposed to air or bacteria. Generally, preserved remains are 10,000 or fewer years in age. However, insects preserved in
amber can be million of years old.
Carbon Films – Sometimes when an organism is buried, exposure to heat and pressure forces gases and liquids out of the
organism’s tissues. This leaves only the carbon behind. A carbon film is the fossilized carbon outline of an organism or part
of an organism.
Mineral Replacement – Replicas, or copies, of organisms can form from minerals in groundwater. They fill in the pore
spaces or replace the tissues of dead organisms. Petrified wood is an example.
Molds – Sometimes all that remains of an organism is its fossilized imprint or impression. A mold is the impression in a
rock left by an ancient organism. A mold can form when sediment hardens around a buried organism. As the organism
decays over time, an impression of its shape remains in the sediment. The sediment eventually turns to rock.
Casts – Sometimes, after a mold forms, it is filled with more sediment. A cast is a fossil copy of an organism made when a
mold of the organism is filled sediment or mineral deposits. The process is similar to making a gelatin dessert using a
molded pan.
Trace Fossils – Some animals leave fossilized traces of their movement or activity. A trace fossil is the preserved
evidence of the activity of an organism. Trace fossils include tracks, footprints, and nests. These fossils help scientists
learn about characteristics and behaviors of the animals. Dinosaur tracks reveal clues about the dinosaurs size, its speed,
and whether it was traveling alone or in a group.
Find real examples of each of the fossils listed below. Create an illustration to show how that fossil formed.
Preserved Remains
Carbon Films
Mineral Replacement
Molds
Casts
Trace Fossils
What can we learn from fossils?
Ancient Environments:
Scientist who study fossils are called paleontologists. Paleontologists can use the principles of relative dating to
learn about ancient organisms and the environments that ancient organisms lived. For example, they can compare fossils
of ancient organisms with organisms living today. The trilobite fossil and the horseshoe crab look alike. Horseshoe crabs
today live in shallow water on the ocean floor. Partly because trilobite fossils look like horseshoe crabs, paleontologists
infer that trilobites also lived in shallow water.
Shallow Seas:
Today, Earth’s continents are mostly above sea level. But sea level has risen, flooding the Earth’s continents,
many times in the past. For example, a shallow ocean covered much of North America 450 million years ago. Fossils of
organisms that lived in that shallow ocean help scientists reconstruct what the sea-floor looked like at that time.
Past Climate:
You might have heard people talking about global climate change, or maybe you’ve read about climate change.
Evidence indicates that Earth’s present day climate is warming. Fossils show that Earth’s climate has warmed and cooled
many times in the past.
Plant fossils are especially good indicators of climate change. For example, fossils of ferns and other tropical plants
dating to the time of the dinosaurs reveal that Earth was very warm 100 million years ago. Tropical forests and swamps
covered much of the land.
Millions of years later, the swamps and forests were gone, but coarse grasses grew in their place. Huge sheets of
ice called glaciers spread over parts of North America, Europe, and Asia. Fossils suggest that some species that lived
during this time, such as the woolly mammoth, were able to survive in the colder climate.
Fossils of organisms such as ferns and mammoths help scientists learn about ancient organisms and past environments.
What can fossils tell us about ancient environments?
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