Download Fossils

Fossils
Fossils
• Knowing the nature of the life forms that
existed at a particular time helps
researchers understand past
environmental conditions
• Fossils are important time indicators and
play a key role in correlating rocks of
similar ages that are from different places
How Fossils Form
• Petrification– small
internal cavities and
pores of the original
structure are filled
with precipitated
mineral matter
• Frozen
• Mummification—dry
conditions
How Fossils Form
• Replacement—cell
walls and other solid
material are removed
and replaced with
mineral matter
How Fossils Form
• When a shell or other structure is buried in
sediment and then dissolved by
underground water, it forms a mold
• If the hollow spaces of a mold are filled
with mineral matter it forms a cast
How Fossils Form
• Carbonization—fine
sediment encases the
remains of an organism,
pressure squeezes out
liquid and gaseous
components, and leaves
a thin residue of carbon
• If the film is lost from
such a fossil, a replica of
the surface may show
detail (impression)
Other Types of Fossils
• Tracks—lithified animal footprints
• Burrows—tubes in sediment, wood, or
rock made by an animal
• Coprolites—fossil dung and stomach
contents
• Gastroliths—highly polished stomach
stones that were used in the grinding of
food by some extinct reptiles
Conditions Favoring Preservation
• Rapid burial
– If the remains are buried quickly by sediment,
they are protected from scavengers and
decomposition
• Possession of hard parts
– Shells, bones and teeth
Principle of Fossil Succession
• Fossil organisms succeed one another in
a definite and determinable order, and
therefore any time period can be
recognized by its fossil content.
Index Fossils
• A fossil that is associated with a particular
span of geologic time
• Rock formations do not always contain a
specific index fossil; in these instances
groups of fossils are used to establish the
age of the bed.