FOSSILS NOTES
Scientists use fossils and other dating techniques to determine the sequence of
events in the Earth's past. Fossils are preserved evidence of ancient living things.
A scientist who studies fossils is called a paleontologist.
Some fossils are the actual remains of living things. This type of fossil is
extremely rare. When an animal or plant is frozen in ice, caught in tar, or trapped
in tree sap (amber), they can be preserved. The actual remains of mammoths,
saber-toothed tigers, plants, and insects have been found. Egyptian mummies are
another form of actual remains.
Most of the time, fossils are not the actual remains of plants and animals. Another
type of fossil is an imprint. These simple fossils are the outline or depression left
by an organism. One example of an imprint fossil is a dinosaur footprint.
SILS
Changed hard parts are a different kind of fossil. Under the right conditions,
water that is rich in minerals dissolves the original organic matter. When this
material is preserved, the plant or animal is petrified, or turned to stone.
Mold fossils are one of the most common types of fossils. Mold fossils form when
a plant or animal is buried by sediment. When the organism decays, a mold of its
original shape is left behind.
Cast fossils are another common type of fossil. Cast fossils are formed when
minerals or sediments fill the space left by a mold fossil. A cast fossil takes the
shape of the original fossil. Even if the mold wears away or breaks apart, the cast
may be left behind.
Fossils are useful for more than just information about the plants and animals that
lived long ago. Fossils can also be used to determine the age of rock.
Index fossils are used for this purpose. Index fossils are the fossils of animals
that were abundant and known to have existed for only a short period of time.
Trilobites are an example of an index fossil. Scientists know that trilobites lived
only from about 540 millions year ago to 245 million years ago, during the Paleozoic
Era. They were abundant during that time. Geologists know that if they find a
trilobite fossil, that the rock surrounding it must be that old. If paleontologists
find an unknown fossil in the same rock as a trilobite, then the unknown fossil must
be from that time period, too.
Some other index fossils are ammonites, brachiopods, and graptolites. Almost
every geologic era has at least one
Relative dating can also be used to determine the age of fossils and rocks. In
relative dating, it is known that the oldest rock and fossils are on the bottom. This
is particularly useful when dating sedimentary rock. Relative dating only allows
scientists to say if one sample is older or younger than another. It does not allow
them to assign a particular age.
Scientists can also use radiometric dating to determine the age of fossils and
rock. In radiometric dating, scientists measure the amount of carbon-14 compared
to other forms of carbon. The amount of carbon-14 allows the scientist to figure
the age of the specimen.