The History of the Earth: Supplemental Guide 7A | Fossils 137 Show image 7A-4: Fossil hunter
As you dig down into the earth, the soil and rocks are divided
into layers. These layers represent different geologic periods, or
times during which the crust and surface of the earth changed.
For instance, if you fi nd a layer of sandstone on dry land, then
you know that there may have been an ocean or river over land at
some point in the distant past.^6 We can estimate how old certain
fossils may be thanks to our understanding of geology and rock
layers.
Fossils are usually found in layers of sedimentary rocks, though
they can be found in other rock formations as well. It looks like the
paleontologist in this picture has found a good place for fossil hunting.
He has to dig very carefully to make sure he keeps the fossils in good
condition.
Every fossil is part of the earth’s fossil record. The fossil record
includes everything we have learned about the history of life from
studying fossils. The fossil record is what paleontologists study
in order to fi gure out what life on earth was like many years ago.
Paleontologists can determine when the animals and plants imprinted
in the fossils lived based on the rock layers in which they were found.
They use information from all fossils to create a timeline of life on
earth.^7 Today, I would like to show you several different fossils from
different time periods during the history of the earth.
Show image 7A-5: Trilobite
This is a fossil of a trilobite, an animal that some scientists
believe lived about 550 million years ago. Trilobites may look like
insects, but they are more closely related to lobsters and crabs.
Trilobites came in many varieties, from a half-inch up to twenty-
eight inches in length.^8 They had antennas, lots of legs, and a hard
outer shell called an exoskeleton. That exoskeleton is important
because it meant that dead trilobites were easily fossilized when
they became buried in the sand.6 That means that by examining
rocks in our area, scientists can tell
if there used to be an ocean where
our city is now and how long ago
that was.
7 A timeline shows the order in
which plants and animals existed,
from long ago to today.
8 [Show these lengths with a ruler or
yardstick.]