Chapter 3 Section 4
Looking at Fossils
• The remains of physical evidence of an
organism preserved by geologic processes is
called a fossil.
• Fossils are mainly in sedimentary rock
• Fossils can also be preserved in other materials
Fossils in Rocks
• Sometimes when organisms die they are
buried by sediments
• When the sediments become rock, hard parts
of the organism are preserved in the rock
Fossils in Amber
• Organisms become trapped in sap
• When the sap hardens the organisms are
preserved
• The hardened sap is called amber
Petrification
• Petrification is the process in which minerals
replace an organism’s tissues
• Permineralization is a process in which the pore
space in an organism’s hard tissue (bone or
wood) is filled up with mineral
• Replacement is a process in which the organism’s
tissues are completely replaced by minerals.
• Fossils can be found in asphalt
• Fossils can be frozen.
• In 1999 scientists removed a 20,000 year old
woolly mammoth
• Cold temps slow down decay. Many frozen
fossils are preserved from the last ice age.
Other Types of Fossils
• Trace fossils
• Mold
• Cast
Using Fossils to Interpret the Past
• The fossil records only offer a rough sketch of
the Earth’s history.
• The fossil record is incomplete because most
organisms never became fossils.
History of Environmental Changes
• The fossil record reveals a history of
environmental change
• Ex) marine fossils help scientists reconstruct
ancient coastlines and the deepening and
shallowing of ancient seas
• Scientists can reconstruct ancient climates by
using fossils
History of Changing Organisms
• By studying fossils scientists can interpret how
life changed over time
• The fossil record is incomplete.
Using Fossils to Date Rocks
• Index fossils are fossils of organisms that lived
during a relatively short, well-defined geologic
time span
• Index fossils help scientists date rock layers
Ammonites
• To be considered an index fossil the fossils
must be found in rock layers throughout the
world ex) ammonites
• Tropites are a type of ammonites that is
similar to a modern squid
Trilobites
• Trilobites help date rocks
• Paleontologists assume that any rock layer
containing a fossil of a trilobite is dated to be
about 400 million years old