Fossils
Fossil:
• the evidence in rock of the presence of a plant or
animal from an earlier time period.
Types of Fossils:
Body Fossil:
• the remains of actual organisms which include
bones, teeth, shells, or claws
Trace Fossil:
• a fossilized imprint of an organism’s footprints,
trails, burrows, nests, or eggs
Mold Fossil:
• a fossilized impression of the actual organism into
the surface around it
Index Fossil:
• the remains of species that existed on Earth for
relatively short periods of time, were abundant,
and were widespread geographically
• used to estimate ages of rock layers based on the
index fossils they contain
Examples of Index Fossils:
Ammonite: 250 – 65
million years ago
Trilobite: 520 – 250
million years ago
In these rock strata (layers) the Ammonite and
mammal fossils indicate similar ages of rock.
Law of Superposition:
• youngest rocks are on top
and oldest rocks are at the
bottom
(assumes that layers have not been disturbed)
Fossils provide the evidence for the geologic time scale.