Unit 13a Fossil record and geologic time scale

12.1 The Fossil Record

KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form.


Show What You Know!

Does everything that dies become a fossil?

A. Yes, if it doesn’t get buriedB. Yes, if it dies in waterC. No, only non-living things like shells and rocks can

become fossilsD. No


Show What You Know!

Does everything that dies become a fossil?

A. Yes, if it doesn’t get buriedB. Yes, if it dies in waterC. No, only non-living things like shells and rocks can

become fossilsD. No

It can only become fossilized under special circumstances, such as being buried quickly or in low oxygen environments where it will not be decomposed.


Fossils can form in several ways.

• Permineralization occurs when minerals carried by water are deposited around a hard structure, such as bone.


• A natural cast forms when flowing water removes all of the original tissue, leaving an impression.


• Trace fossils record the activity of an organism.


• Specific conditions are needed for fossilization. If these conditions do not exist, what will happen to the remains of the organism?


• If these conditions do not exist, what will happen to the remains of the organism?

• They will decompose naturally and nothing will be left.

• Only a tiny percentage of living things became fossils.


• Usually only the bones remain because the soft tissues decompose. Bones take much longer to decay, so they have more chance to get buried and preserved.


Soft Tissue Preservation is Rare

Preservation of things other than bones and casts is very rare. Usually when they are found, it is just in the hundreds or thousands of years old, not millions.

Here are some examples of this unique form of preservation.


• Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

Amber insects are by far the oldest preserved soft tissues, some being 100s of millions of years old


• Ice can encase and preserve soft tissues, such as Ouzi. He was murdered and then frozen in ice in Austria for 6000 years before hikers discovered his arm sticking out of the ice.


Bog People

• People who died in acidic bogs hundreds of years ago in Northern Europe were preserved by the extremely acidic water that prevented decomposition by microorganisms. But, they were stained by the brown tannins.


La Brea Tar Pits

• Natural tar-oil seeps in Los Angeles preserved animals who got stuck in it some 10,000-30,000 years ago during the ice age, such as mammoths.


Radiometric dating provides an accurate way to estimate the age of fossils.

• Relative dating estimates the time during which an organism lived.– It compares the placement

of fossils in layers of rock.– Scientists infer the order in

which species existed.


• Radiometric dating uses decay of unstable isotopes.

– Isotopes are atoms of an element that differ in their number of neutrons. How stable is an atom that has more neutrons?

– An unstable atom will begin to decay

neutrons protrons


– A half-life is the amount of time it takes for half of the isotope to decay. The half-life of isotopes is constant.

• Radiometric dating uses decay of unstable isotopes.

– Isotopes are atoms of an element that differ in their number of neutrons.

12.2 The Geologic Time Scale

KEY CONCEPT The geologic time scale divides Earth’s history based on major past events.


Index fossils are another tool to determine the age of rock layers. What do you use the index of a textbook for?

• Index fossils can provide the relative age of a rock layer. – existed only during specific spans of time– occurred in large geographic areas– If you found a fossil near an index fossil, what would you

know about the age of the new fossil?• Index fossils include fusulinids and trilobites.


The geologic time scale organizes Earth’s history.

• The history of Earth is represented in the geologic time scale.

• It was developed long before Darwin’s time when geologists noticed the layers of rock and fossils.

100250

550

1000

2000

PRECAMBRIAN TIME

Cyanobacteria

This time span makes up the vast majority of Earth’s history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils.


• Eras and Periods– Eras consist of two or more periods– most commonly used units of time on

time scale– associated with rock systems.


• Eras last hundreds of millions of years.

– Paleozoic


• Eras last tens to hundreds of millions of years.

– Mesozoic


• Eras last tens to hundreds of millions of years.

– Cenozoic


Show What You Know!

Can we determine the age of a fossil for certain?

A. No, since we cannot go back in time, we cannot tell how old things are

B. No, we can only take guesses based on where we find the fossils

C. Yes, we can get fairly close and reliable estimates using different forms of technology

D. Yes, but only on fossils that have formed since people started keeping a written history of events


Show What You Know!

Can we determine the age of a fossil for certain?

A. No, since we cannot go back in time, we cannot tell how old things are

B. No, we can only take guesses based on where we find the fossils

C. Yes, we can get fairly close and reliable estimates using different forms of technology

D. Yes, but only on fossils that have formed since people started keeping a written history of events


What is the most common type of fossilization? • A. Trace fossilization • B. Permineralization • C. Preservation in amber • D. Preservation in ice


What is the most common type of fossilization? • A. Trace fossilization • B. Permineralization • C. Preservation in amber • D. Preservation in ice• Correct Answer = B

Do relative dating methods determine the exact age of a fossil? • A. Yes, the exact age, within a margin of error, can be determined. • B. Yes, radioisotopes date fossils. • C. No, they only compare the order in which groups of organisms

existed. • D. No, they show that earlier fossils are found above later fossils in

rock layers.


What is the most common type of fossilization? • A. Trace fossilization • B. Permineralization • C. Preservation in amber • D. Preservation in ice• Correct Answer = B

Do relative dating methods determine the exact age of a fossil? • A. Yes, the exact age, within a margin of error, can be determined. • B. Yes, radioisotopes date fossils. • C. No, they only compare the order in which groups of organisms

existed. • D. No, they show that earlier fossils are found above later fossils in

rock layers.• Correct Answer = C


Why can't C-14 be used to determine the age of rocks? • A. The half-life of C-14 is too long. • B. The half-life of C-14 is too short. • C. C-14 does not decay in rocks. • D. Only C-12 is found in rocks.


Why can't C-14 be used to determine the age of rocks? • A. The half-life of C-14 is too long. • B. The half-life of C-14 is too short. • C. C-14 does not decay in rocks. • D. Only C-12 is found in rocks. • Correct Answer = B


Fossils form in many ways

– Permineralization– Trace fossils– Amber preserved fossils– Ice preserved remains

• Relative dating of fossils – in relation to the layers of rock and other fossils

• Radiometric dating of fossils – compares how much of a radioactive substance has decayed into another substance

• Index fossils are helpful in determining relative age of rock layers • Eras are the bigger measurement of Earth’s time and can include 2-

3 periods covering tens to hundreds of millions of years• Periods last tens of millions of years.

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Unit 13a Fossil record and geologic time scale