earth science 3.4 : looking at fossils
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Earth Science 3.4
Looking at Fossils
1. Describe five ways that different types of fossils form.
2. List three types of fossils that are not part of organisms.
3. Explain how fossils can be used to determine the history of changes in environments and organisms.
4. Explain how index fossils can be used to date rock layers.
Objectives:
FossilThe trace or remains of an organism that lived
long ago, most commonly preserved in sedimentary rock.Most often preserved in sedimentary rock, but other
materials can also preserve evidence of past life.
Fossils in RocksWhen an organism dies, it either begins to decay
or is consumed by other organisms. Sometimes dead organisms are quickly buried by sediment, which slows down decay.Shells and bones are more resistant to decay than soft
tissues, so when sediments become rock, the harder structures are more commonly preserved.
Fossilized Organisms
Fossils in Amber Organisms occasionally become trapped in soft,
sticky tree sap, which hardens and becomes amber.Insect fossils have often been preserved in this way,
but frogs and lizards have also been found in amber.Petrifaction
Process in which minerals replace and organism’s tissues.
PermineralizationProcess in which the pore space in an organism’s hard
tissue is filled up with mineral.Replacement
Process in which an organism’s tissues are completely replaced by minerals.
Fossils in Asphalt There are places where asphalt wells up at the
Earth’s surface. These thick, sticky pools can trap and preserve organisms.
Frozen FossilsSince cold temperatures slow down decay, many
types of fossils have been found preserved in ice.
Trace FossilsNaturally preserved evidence of animal activityExamples:
Preserved animal tracksPreserved burrows or shelters that were made by
animalsCoprolite, which is preserved animal dung.
Molds and Casts are two more examples of fossils.Mold
mark or cavity made in a sedimentary surface by a shell or other body.
Cast type of fossil that forms when sediments fill the cavity left
by a decomposed organism.
Other Types of Fossils
The Information in the Fossil RecordOffers only a rough sketch of the history of life on
Earth. Incomplete because most organisms never became
fossils.Scientists know more information about organisms that had
hard body parts and that lived in environments that favored fossilization.
History of Environmental ChangesThe fossil record reveals changes in an area’s climate
over time.By using the fossils of plants and land animals, scientists
can reconstruct past climates.
History of Changing OrganismsBy studying the relationships between fossils,
scientists can interpret how life has changed over time.
Using Fossils to Interpret the Past
Scientists have learned that particular types of fossils appear only in certain layers of rock.
By dating the rock layers above and below these fossilsScientists can determine the time span in which the
organisms that formed the fossils lived.If a type of organism existed for only a short
period of time, its fossils would show up in a limited range of rock layers.These fossils are called index fossils.
Index fossils Found in the rock layers of only one geologic age,
and can be used to establish the age of the rock layers.
Using Fossils to Date Rocks
Ammonites Tropites, a marine mollusk similar to a modern
squid, lived between 230 million and 208 million years ago.
Trilobites Fossils of a genus of trilobites called Phacops are
another example of an index fossil.Trilobites are extinct and lived approximately
400 million years ago.When scientists find Phacops in a rock, they assume
that the rock is approximately 400 million years old.
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