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