1. Looking at Fossils Chapter 3 Section 4 p. 74 - 79 Vocabulary:
2. trace fossil
3. mold
4. cast
5. index fossil

2. Fossilized Organisms p. 74
The remains or physical evidence of an organism preserved by
geologic processes is called a fossil. Fossils are most often
preserved in sedimentary rock.
Hard parts of organisms, such as shells and bones, are more
resistant to decay than soft tissues are. So when sediments
become rock, the hard parts of animals are much more
commonly preserved than are soft tissues.
Hardened tree sap is called amber.
Some of our best insect fossils are
found in amber, as shown in this
picture.

3. Fossils can be found in other ways as well. Some organisms
are preserved by petrification. This is process in which
minerals replace an organism’s tissues. Permineralization
is a process in which the pore space in an organism’s hard
tissue (bone or wood) is filled up with mineral. Replacement
is a process in which the organism’s tissues are completely
replaced by minerals.
In some places on the Earth’s surface, asphalt wells up and
forms thick, sticky pools. The LaBrea asphalt deposits in Los
Angeles, California, for example, are 38,000 years old and
have preserved organisms for all of those years.
Sometimes, cold temperatures slow down decay and fossils
are preserved from the last ice age.
Here is an example of a fish found
in ice from Argentina.

4. Other Types of Fossils p. 76
Any naturally preserved evidence of animal activity is called
a trace fossil. Animal footprints fill with sediment and become
preserved in rock. Burrows are another trace fossil. Burrows
are shelters made by animals such as clams, that bury in the
sediment. A third type of trace fossil, called a coprolite, is
preserved animal dung (yuck!)

5. Molds and Casts p. 76
Molds and casts are two more examples of fossils. A cavity in rock where a plant or animal was buried is called a mold.
A cast is an object created when sediment fills a mold and
becomes rock. A cast shows what the outside of the
organism looked like.
This photograph shows two molds from
an ammonite. The image on the left is
the internal mold of the ammonite, which
formed when sediment filled the ammonite’s
shell, which later dissolved away. The
image on the right is the external mold of
the ammonite, which preserves the external
features of the shell.

6. Using Fossils to Interpret the Past p. 77
The fossil record only offers a rough sketch of the history of
life on Earth. Scientists know more about organisms that had
hard body parts than about organisms that had soft body parts.
They also know more about organisms that lived in environments
that favored fossilization. The fossil record is incomplete
because most organisms never became fossils.
The fossil record also reveals a history of environmental change.
Marine fossils help scientists reconstruct ancient coastlines
and the deepening and shallowing of ancient seas. Fossils of
plants and land animals help scientists reconstruct past climates
by telling whether the climate was cooler or wetter than it
currently is.

7. Sometimes, scientists find fossils where they don’t expect to
find them. For example, would you expect to find marine
fossils on the top of a mountain? The presence of marine
fossils means that the rocks of mountaintops formed in a
totally different environment – the bottom of the ocean!
This scientist found marine
fossils on mountaintops
in Canada. The fossil of
Marrella, shown in the
picture tells the scientist
that these rocks were
pushed up from below sea
level millions of years ago.

8. History of Changing Organisms p. 78
Scientists know that older rock layers contain organisms that
are often different from the organisms found in younger rock
layers. The fossil record is incomplete and doesn’t provide
paleontologists with a continuous record of change. They look
for similarities between fossils and their closest living relatives
and try to fill in the blanks on the fossil record.

9. Using Fossils to Date Rocks p. 78
Scientists have found that particular types of fossils appear only
in certain layers of rock. By dating the rock layers above and
below these fossils, scientists can determine the time span in
which the organisms that formed the fossils lived. These types
of fossils are called index fossils. Index fossils are fossils of
organisms that lived during a relatively short, well-defined
geologic time span.
To be considered an index fossil, a
fossil must be found in rock layers
throughout the world. One example
is an index fossil is the fossil of a
group of ammonites called tropites.
They were similar to a modern
squid. They lived in a coiled shell
about 230 – 208 million years ago.

10. Trilobites p. 79
Fossils of a group of trilobites called Phacops are another
example of an index fossil. Trilobites are extinct. Their
closest living relative is the horseshoe crab.
Phacops lived approximately 400 million years ago.
Scientists assume that any rock
layer containing a fossil of the trilobite
Phacops is about 400 million years
old.

11. How Fossils Are Formed

12. Quiz Time! 1. How are a cast and a mold different?
A mold is a cavity in rock where a plant or an animal was
buried. A cast is an object created when sediment fills a
mold and becomes rock.
2. Why do the frigid temperatures of Siberia and the sticky
asphalt of the La Brea tar pits preserve fossils well?
Both environments slow down the decay of an organism
and help preserve it.
3. _________ can be used to establish the age of rock layers.
Index fossils

13. 4. Fossils are most often preserved in
A. ice
B. amber
C. asphalt
D. rock D
5. Name three types of trace fossils.
tracks, burrows, and coprolites
Science Humor…
What do you call a fossil that never wants to work?
Lazy bones!!