earth history and clues from fossils

index fossils

An index fossil can be used to identify a specific period of time. Organisms that make good index fossils are distinctive, widespread, and lived briefly. Their presence in a rock layer can be used to identify rocks that were deposited at that period of time over a large area. The fossil of a juvenile mammoth found near downtown San Jose California reveals an enormous amount about these majestic creatures: what they looked like, how they lived, and what the environment of the Bay Area was like so long ago.


By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher the climate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coal beds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was much warmer. Recall from the chapter Plate Tectonics that Wegener used the presence of coal beds in Antarctica as one of the lines of evidence for continental drift.

geologic history

The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine. Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabed were uplifted.

environment of deposition

By knowing something about the type of organism the fossil was, geologists can determine whether the region was terrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give clues to whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allows scientists to learn about what happened to the region after the organism died; for example, whether it was exposed to wave action.

history of life on earth

That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocks resemble animals and plants that are living today. In general, fossils in older rocks are less similar to modern organisms. We would know very little about the organisms that came before us if there were no fossils. Modern technology has allowed scientists to reconstruct images and learn about the biology of extinct animals like dinosaurs!

clues from fossils

Fossils are our best form of evidence about Earth history, including the history of life. Along with other geological evidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and other major geological events. Since the present is the key to the past, what we know about a type of organism that lives today can be applied to past environments.

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charles darwin discovered

a) the galapagos islands

-->  b) shell beds high in the andes mountains

c) mountains and earth must be extremely old

d) all of the above

fossils help us learn more about

a) earth history

b) the history and evolution of life

c) environmental conditions in the past

-->  d) all of the above

compared with fossils in younger rocks, fossils in older rocks are

a) more similar to modern organisms.

b) all extinct.

-->  c) less similar to modern organisms.

d) sometimes more and sometimes less similar to modern organisms.

by knowing something about the type of organism a fossil was, geologists can determine

-->  a) what the environment of the region was like at that time.

b) what organism it evolved from.

c) what organism it evolved into.

d) what the environment of the region is like now.

finding clam shells in a rock indicates that the region was once shallow marine.

-->  a) true

b) false

_ can be used to identify a specific period of time.

a) trace fossils

-->  b) index fossils

c) body fossils

d) complete fossils

geologists find ancient coal beds in antarctica. the one thing they can really know from this is that

a) antarctica moved to its current position by plate tectonics processes.

b) the swamps that make coal beds existed under different circumstances in the past.

c) africa, south america and antarctica were once joined into a supercontinent.

-->  d) the climate was much warmer on that continent at the time the coal beds formed.

if a two index fossils are found 3,000 miles apart, geologists know that the two rocks they are in

-->  a) formed at the same time.

b) were once together and have now drifted apart.

c) are volcanic ash.

d) formed in subsequent time periods.

if a fossil shell has been worn down, geologists know that

a) the organism lived a rough life.

b) the organism died and was deposited in soft sediment rapidly.

-->  c) the shell was eroded after the animal died.

d) the shell is not representative of a once-living organism.

an index fossil must be distinctive, widespread and short-lived so that it can identify a specific period of time.

-->  a) true

b) false

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