Unit C
Chapter 2
Lesson 1
Pages C 36-39
Fossil Formation
How Fossils Form
Some rocks hold clues to life on Earth long ago. These clues, called fossils, are
found most often in sedimentary rocks such as sandstone, shale, and limestone. This is
because most fossils formed when a dead plant or animal was buried quickly by
sediments, such as sand or mud. As with your model in the investigation, the soft parts of
the plants or animal decayed, or rotted away. The hard parts took longer to decay. They
were preserved, or saved, by being buried. Unburied parts would have been eaten or
destroyed by weather. As the sediments hardened, the remains became trapped in rock
and formed a fossil.
Fossils also can form in other ways. Not all fossils are parts of once-living things.
Some fossils are traces, such as footprints, left by living things. Sometimes the shape of a
plant or animal is left as a hollow in a rock. The hollow left behind is a fossil. If minerals
fill the hollow, the new rock they from is also a fossil.
Captions:
 The bones of this dinosaur skull were buried in a layer of sedimentary rock.
 This fossil shell, buried deep beneath the desert, gives scientists clues about
Earth’s past. What might scientists be able to infer about this particular place of
Earth?
Trace Fossils
Wood, bones, teeth, and shells aren’t the only things that become fossils. Trace
fossils show changes long-dead animals made in their surroundings. Tracks, burrows,
droppings, and worm holes are some examples of trace fossils. They give scientists clues
about animals. They tell how an animal might have moved. They show how big or small
it might have been. They also show what it might have eaten. The photographs on this
page show different types of trace fossils.
Captions:
 Fossilized tracks give clues about an animal’s size and how it moved. The
dinosaur track above is found in limestone near Glen Rose, Texas. It is many
years old. The tracks above left were made recently in sand by a living bird. How
might they become a fossil?
 Scientists can learn a lot about what an animal ate by looking at coprolites.
Coprolites are animal droppings that have become fossils.
 A worm made this borrow, or tunnel, as it hunted for food in soft sediment. When
the sediment was buried and hardened into rock, the burrow was preserved.
Other Types of Fossils
Usually, the parts of living things that make up fossils are changed as rock forms.
For example, fossil molds and casts form after the parts are destroyed. A mold is an
imprint made by the outside of a dead plant or animal. A mold forms when water slowly
washes animal or plant remains out of rock. The space left in the rock layer matches the
shape of the once-living thing. Sometimes sediments or minerals fill a mold and form a
cast. The cast has the same outside shape as the original living thing.
Sometimes chemical and physical changes take place as remains become fossils
Heat and pressure may destroy most of the buried remains. Only a thin film, or sheet, of
black carbon is left. This type of fossil is called a carbon film.
Some fossils form when minerals slowly take the place of original, once-living
material. Fossils that form in this way are called petrified fossils. Petri- means “stone.”
The trees in Petrified Forest National Park became fossils in this way. Their wood was
replaced by the mineral quartz.
A whole plant or animal, including soft parts, can become a fossil. Sometimes
insects were trapped in the sticky sap of trees. When the sap hardened, the whole insect
was preserved. Large animals such as wooly mammoths have also been trapped and
preserved in glacier ice.
Tar pits have also preserved once-living things. Animals got stuck when they tried
to walk through the sticky tar. Plants and plant parts were blown or fell into the tar and
got stuck. Many animal fossils have been found in the La Brea tar pits in Los Angeles,
California. They include deer, elephants, horses, birds, and saber-toothed tigers.
Captions
 A carbon film forms when heat and pressure force out most of the materials that
made up a once-living thing. Only a thin sheet of carbon is left.
 Millions of years ago, this dragonfly got stuck in sap from a pine tree. Later the
sap hardened to form a clear yellow material called amber.
 Fossil molds and casts give clues about the outside of the once-living thing.
 Petrified Forest National Park in Arizona has many petrified logs like this one.
The fossils began forming when water containing minerals soaked into fallen
trees. As the wood slowly rotted away, minerals took its place.
Summary
Fossils are the remains of living things that lived on Earth long ago. Fossils are
usually made of the hard parts of plants and animals. But they can form in several other
ways. For example, trace fossils, such as tracks, burrows, and droppings, give clues about
what animals did.