Transcript Slide 1

Lesson Overview
The Fossil Record
Lesson Overview
19.1 The Fossil Record
Lesson Overview
The Fossil Record
Fossils and Ancient Life
What do fossils reveal about ancient life?
From the fossil record, paleontologists learn about the structure of
ancient organisms, their environment, and the ways in which they
lived.
Lesson Overview
The Fossil Record
Fossils and Ancient Life
Fossils are the most important source of information about extinct
species, ones that have died out.
Fossils vary enormously in size, type, and degree of preservation.
They form only under certain conditions.
For every organism preserved as a fossil, many died without
leaving a trace, so the fossil record is not complete.
Lesson Overview
The Fossil Record
Types of Fossils
Fossils can be as large and perfectly preserved as an entire
animal, complete with skin, hair, scales, or feathers.
They can also be as tiny as bacteria, developing embryos, or
pollen grains.
Sometimes an organism leaves behind trace fossils—casts of
footprints, burrows, tracks, or even droppings.
Although most fossils are preserved in sedimentary rocks,
some are preserved in other ways, like in amber.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
Most fossils are preserved in sedimentary rock.
Sedimentary rock usually forms when small particles of sand,
silt, clay, or lime muds settle to the bottom of a body of water.
As sediments build up, they bury dead organisms that have sunk
to the bottom.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
As layers of sediment continue to build up over time, the remains
are buried deeper and deeper.
Over many years, water pressure gradually compresses the
lower layers and turns the sediments into rock.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
Usually, soft body structures decay quickly after death, so
usually only hard parts like wood, shells, bones, or teeth
remain. These hard structures can be preserved if they are
saturated or replaced with mineral compounds.
Sometimes, however, organisms are buried so quickly that
soft tissues are protected from aerobic decay. When this
happens, fossils may preserve imprints of soft-bodied
animals and structures like skin or feathers.
Lesson Overview
The Fossil Record
What Fossils Can Reveal
The fossil record contains an enormous amount of information
for paleontologists, researchers who study fossils to learn
about ancient life.
By comparing body structures in fossils to body structures in
living organisms, researchers can infer evolutionary relationships
and form hypotheses about how body structures and species
have evolved.
Bone structure and trace fossils, like footprints, indicate how
animals moved.
Lesson Overview
The Fossil Record
Dating Earth’s History
How do we date events in Earth’s history?
Relative dating allows paleontologists to determine whether a fossil
is older or younger than other fossils.
Radiometric dating uses the proportion of radioactive to nonreactive
isotopes to calculate the age of a sample.
Lesson Overview
The Fossil Record
Relative Dating
Lower layers of sedimentary rock, and fossils they contain, are
generally older than upper layers.
Relative dating places rock layers and their fossils into a
temporal sequence.
Lesson Overview
The Fossil Record
Relative Dating
To help establish the relative ages of rock layers and their
fossils, scientists use index fossils. Index fossils are distinctive
fossils used to establish and compare the relative ages of rock
layers and the fossils they contain.
If the same index fossil is found in two widely separated rock
layers, the rock layers are probably similar in age.
Lesson Overview
The Fossil Record
Relative Dating
A good index fossil species must be easily
recognized and will occur in only a few rock layers
(meaning the organism lived only for a short time).
These layers, however, will be found in many places
(meaning the organism was widely distributed).
Lesson Overview
The Fossil Record
Radiometric Dating
Relative dating is important, but provides no information about a
fossil’s absolute age in years.
One way to date rocks and fossils is radiometric dating.
Radiometric dating relies on radioactive isotopes, which decay,
or break down, into nonradioactive isotopes at a steady rate.
Radiometric dating compares the amount of radioactive to
nonreactive isotopes in a sample to determine its age.
Lesson Overview
The Fossil Record
Radiometric Dating
A half-life is the time required for half of the
radioactive atoms in a sample to decay.
After one half-life, half of the original radioactive
atoms have decayed.
After another half-life, another half of the remaining
radioactive atoms will have decayed.
Lesson Overview
The Fossil Record
Radiometric Dating
Different radioactive elements have different halflives, so they decay at different rates.
Carbon-14, which has a short half-life, can be used to
directly date very young fossils.
Elements with long half-lives can be used to indirectly
date older fossils by dating nearby rock layers, or the
rock layers in which they are found.
Lesson Overview
The Fossil Record
Radiometric Dating
Carbon-14 is a radioactive form of carbon naturally found in the
atmosphere. It is taken up by living organisms along with
“regular” carbon, so it can be used to date material that was
once alive, such as bones or wood.
After an organism dies, carbon-14 in its body begins to decay to
nitrogen-14, which escapes into the air.
Researchers compare the amount of carbon-14 in a fossil to the
amount of carbon-14 in the atmosphere, which is generally
constant. This comparison reveals how long ago the organism
lived.
Carbon-14 has a half-life of only about 5730 years.
Lesson Overview
The Fossil Record
Radiometric Dating
For fossils older than 60,00 years, researchers
estimate the age of rock layers close to fossil-bearing
layers and infer that the fossils are roughly same age
as the dated rock layers.
A number of elements with long half-lives are used for
dating very old fossils, but the most common are
potassium-40 (half-life: 1.26 billion years) and
uranium-238 (half-life: 4.5 billion years).
Lesson Overview
The Fossil Record
Geologic Time Scale
How was the geologic time scale established, and what
are its major divisions?
Lesson Overview
The Fossil Record
Geologic Time Scale
Geologists and
paleontologists have built
a time line of Earth’s
history called the
geologic time scale.
The basic divisions of the
geologic time scale are
eons, eras, and periods.
Lesson Overview
The Fossil Record
Establishing the Time Scale
Geologists used these
boundaries to
determine where one
division of geologic time
ended and the next
began.
Years later, radiometric
dating techniques were
used to assign specific
ages to the various rock
layers.
Lesson Overview
The Fossil Record
Divisions of the Geologic Time Scale
Eons are divided into eras.
The Phanerozoic Eon, for
example, is divided into the
Paleozoic, Mesozoic, and
Cenozoic Eras.
Eras are subdivided into
periods, which range in length
from nearly 100 millions of
years to just under 2 million
years. The Paleozoic Era, for
example, is divided into six
periods.
Lesson Overview
The Fossil Record
Life on a Changing Planet
How have our planet’s environment and living things affected each
other to shape the history of life on Earth?
Building mountains, opening coastlines, changing climates, and
geological forces have altered habitats of living organisms
repeatedly throughout Earth’s history. In turn, the actions of living
organisms over time have changed conditions in the land, water, and
atmosphere of planet Earth.
Lesson Overview
The Fossil Record
Physical Forces
Climate is one of the most important aspects of Earth’s physical
environment.
Earth’s climate has undergone dramatic changes over time. Many of
these changes were triggered by fairly small shifts in global
temperature.
During the global “heat wave” of the Mesozoic Era, Earth’s average
temperatures were only 6°C to 12°C higher than they were during
the twentieth century.
During the ice ages, world temperatures were only about 5°C
cooler than they are now.
These relatively small temperature shifts changed the shape of life
on Earth.
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The Fossil Record
Physical Forces
Geological forces have transformed life on Earth,
producing new mountain ranges and moving
continents.
Volcanic forces have altered landscapes and even
formed entire islands.
Local climates are shaped by the interaction of wind
and ocean currents with geological features such as
mountains and islands.
Lesson Overview
The Fossil Record
Physical Forces
The theory of plate tectonics explains how solid continental
“plates” move slowly above Earth’s molten core- a process called
continental drift.
Over the long term, continents have collided to form
“supercontinents.” Later, these supercontinents have split apart
and reformed.
Lesson Overview
The Fossil Record
Physical Forces
Where landmasses collide, mountain ranges often rise.
When continents change position, major ocean currents change
course.
All of these changes affect both local and global climate.
Lesson Overview
The Fossil Record
Geological Cycles and Events
Continental drift has affected the
distribution of fossils and living organisms
worldwide. As continents drifted apart,
they carried organisms with them.
For example, the continents of South
America and Africa are now widely
separated. But fossils of Mesosaurus, a
semiaquatic reptile, have been found in
both South America and Africa.
The presence of these fossils on both
continents, along with other evidence,
indicates that South America and Africa
were joined at one time.
Lesson Overview
The Fossil Record
Physical Forces
Evidence indicates that over millions of years, giant
asteroids have crashed into Earth.
Many scientists agree that these kinds of collisions
would toss up so much dust that it would blanket
Earth, possibly blocking out enough sunlight to cause
global cooling. This could have contributed to, or even
caused, worldwide extinctions.