Download Ch 12 - Mr. Neason`s Earth Science

Ch 12.2
Fossils: Evidence of Past Life
Lecture Outline
Types of Fossils
Preserved remains , such as frozen mammoth fossils, are just one type of fossil
The different types of fossils include;

petrified fossils

molds and casts

carbon films

preserved remains

trace fossils
Petrified Fossils
Fossils form when an organism’s remains become petrified or “turn to stone”.

In this process, mineral-rich water soaks into the small cavities and pores
of the original organism.

The minerals precipitate from the water and fill the spaces.

Sometimes, minerals replace the cell walls or other solid material
of an organism.
Molds and Casts
A fossil mold is created when a shell or other structure is buried in sediment and then
dissolved by underground water.
The Mold reflects only the shape and surface markings of the organism.
Cast Fossils are created in the hollow spaces of a mold are later filled with mineral
matter.
Carbon Films
Fossils called carbon films can preserve delicate details of leave and animal parts.

The formation of a carbon film begins when an organism is buried
under fine sediments.

Over time, pressure squeezes out liquids and gases and leaves behind a
thin film of carbon.

Black shale contains carbon film fossils.
Ch 12.2 Fossils: Evidence of Past Life
Lecture Outline
p. 2
Preserved Remains
Fossilization preserves all or part of an organism with little change.
Examples:
a.
the mammoth frozen in permafrost.
b.
Insects preserved in amber—the hardened resin or sap of ancient trees
c.
Fossils can form when remains are preserved in tar
Trace Fossils
Trace fossils are indirect evidence of prehistoric life.
Tracks are animal footprints made of soft sediment that later changed to sedimentary
rock.
Burrows are holes made by an animal in sediment, wood, or rock that were later filled
with mineral matter and preserved.
The oldest know fossils are believed to be worm burrows.
Conditions For Fossilization
All fossils geologists have found, arranged by their relative ages, make up the fossil
record.
But the fossil record includes only a fraction of the different kinds of organism that have
lived on Earth.
Why? Some organisms are more likely than others to be preserved as fossils.
Two conditions that favor preservation of an organism as a fossil are rapid burial
and the possession of hard parts.
For a fossil to form, the remains of an organism must be buried quickly by sediment.
Sediment protects the soft parts of a dead animal from being eaten by scavengers or
decomposed by bacteria.
Organisms have a better chance of being preserved if they have hard parts such as shells,
bones, and teeth.
Ch 12.2 Fossils: Evidence of Past Life
Lecture Outline
p. 3
Fossils and the History of Life
During the 1700s and 1800s scientists thought that fossils might provide clues to the
history of life on Earth.
Two major scientific developments helped scientists explain the fossil record:

the principle of fossils succession
(the fossil record showed that certain sets of organisms were
characteristic of different periods in Earth’s past.)

the theory of evolution
(living things had evolved, or changed, over time)
Fossil Succession
The principle of fossil succession states that fossil organisms succeed one another in a
definite and determinable order.

Any period can be recognized by its fossil content.
Theory of Evolution
Geologists had noticed that fossil from older rock layers were very different from the
fossils in younger rocks.
In 1859, Darwin set forth the theory of evolution, which states that life forms have
changed over time, or evolved, from simpler to more complex forms.
To explain why evolution occurs, Darwin proposed the mechanism of natural
selection.
In natural selection, individuals that are better adapted to their environment are more
likely to survive and reproduce than others of the same type. Organism that are less well
adapted are likely to become extinct.
Adaptation drives the process of evolution.
1.
Organisms possess certain traits called adaption, that affect their ability to
survive and reproduce.
2.
Well adapted to their environment survive
3.
They are able to past on their traits to later generations.
Ch 12.2 Fossils: Evidence of Past Life
Lecture Outline
p. 4
The fossil records show that over millions of years, mammals evolved adaptations for life
in the oceans.
Interpreting the Fossil Record
The principle of fossil succession and the theory of evolution helped geologists to
interpret the fossil records.
Geologists used fossils to improve the correlation of rock layers and reconstruct past
environments.
Fossils and Correlation
Geologists use index fossils and groups of fossils to correlate rock layers.
An index fossil is the fossil of an organism that was geographically widespread and
abundant in the fossil record, but existed for only a limited span of time.
The presence of an index fossil in rock layers at different locations means that the
Layers are roughly the same age.
Fossils and Past Environments
Fossils can be used to reconstruct ancient environments.
Because organisms evolve with adaptations suited to particular environments, fossils
provide clues to the characteristics of those environments.
Fossils can help geologists build more detailed picture of a past environment.
Fossils can indicate the former temperature of the water.

Present-day corals require warm and shallow tropical seas…like those
around Florida and the Bahamas.
When similar corals are found in ancient limestones, they indicate that
Florida-like marine environment must have existed when the corals were
alive.