Download Alpine Natural Sciences Academy Mr. Bordelon, M.S.

Alpine Natural Sciences Academy
Mr. Bordelon, M.S.
Clues to Earth’s Past
Fossils
A fossil is
evidence of life.
Evidence such
as the remains,
imprints, or
traces of onceliving organisms
preserved in
rocks are fossils.
Photos taken at The Roynon Museum of Paleontology
Escondido, CA
What Fossils Tell Us..
By studying fossils, geologist
help solve mysteries of
Earth’s past.
Fossils tell us when, where, and
how organisms once lived.
Fossil Formation
Necessary conditions…
First of all, the body of a dead organism
must be protected from scavengers and
microorganisms.
One way this can happen is for the
body to be quickly buried in
sediment.
However, quick burial alone is not
enough to make a fossil.
Organisms have a better chance of being
preserved if they have hard parts such as
bones, shells, or teeth.
Fossils are most
often found in
sedimentary rocks.
The heat and
pressure involved in
forming igneous and
metamorphic rocks
most often destroys
fossil material.
The hard parts in plants, such as the
cellulose in cell walls, made preservation
of this fossil possible.
Petrified Remains
Petrified (PEH truh fide) remains are hard
and rocklike. Some or all of the original
materials have been replaced by minerals.
Much of the original matter
in this petrified tree slice has
been replaced by quartz and
other minerals.
Carbonaceous Films
The tissues of most organisms are made of
compounds that contain carbon. Sometimes, the
only fossil remains of a dead plant or animal is
this carbon.
This fossil graptolite has been
preserved as a carbonaceous film.
Graptolites are extinct colonial
animals that lived in the oceans from
about 530 million to 320 million years
ago.
Molds and Casts
In nature, imprints are made when sea
shells or other hard parts of an organism
fall into soft sediments such as mud.
The object and sediments are then buried by
more sediments.
The sediments are squeezed and cemented
together into rock.
Holes in the rock let water and air reach the
shell or hard part, causing it to dissolve,
and leaving behind a hollow place in the
rock called a mold.
A cast (internal
mold) resembling
the original
organism forms
when a mold fills
with sediments or
mineral crystals
from solution.
Original Remains
Sometimes the actual organism or parts of
the organism are found—such as an
insect trapped in amber.
Other organisms, like
woolly mammoths, have
been found preserved in
frozen ground.
Trace Fossils
Fossilized tracks and other evidence of
animal activity are called trace fossils.
Tracks made in soft
mud, and now
preserved in solid rock,
can provide information
about animal size,
speed, and other
behavioral patterns.
Index Fossils
Index fossils are species that lived on Earth
for short periods of time, were abundant,
and were widespread geographically.
Because few fossils meet all
the conditions to be an index
fossil, groups of fossils are
usually used to date rocks.
Index fossils indicate age by
the layering of their deposits.
Fossils and Ancient Environments
Fossils also can be used to tell what the
environment of an area was like long ago.
For example, rocks in Antarctica contain
fossils of tropical plants!
Brachiopods are found throughout
the Midwestern United States,
indicating the presence of a
prehistoric shallow sea.
Summary
Fossils tell us about:
• Past life on Earth
• History of rock layers containing fossils
• Past environmental conditions
• Past climate conditions
• Extinct animal behavior
• Age (dating) of rocks
Source
Science Voyages: Earth & Physical Sciences. Glencoe McGraw-Hill, California
Standards Edition. 2001.