Lesson Overview

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Lesson Overview

• 19.1 The Fossil Record

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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.

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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.

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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.

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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.

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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. This fish
fossil was formed in sedimentary rock.
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What Fossils Can Reveal

• The fossil record contains an enormous amount of
  information for paleontologists, researchers who
  study fossils to learn about ancient life.
• 1) Evolutionary relationships comparing body
  structures in fossils to body structures in
  living organisms.
• 2) Movement Bone structure and trace fossils,
  like footprints.

3) Type of environment Fossilized plant leaves
and pollen suggest whether the area was a swamp,
a lake, a forest, or a desert. 4)
Reconstruction of an ecosystem when different
kinds of fossils are found together.
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Dating Earths History

• How do we date events in Earths 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.

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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.

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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.

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Radiometric Dating (Absolute Age)

• 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.

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.
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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.
• Half life 5,730 years
• 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, so its only useful for dating fossils no
  older than about 60,000 years.

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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).

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Geologic Time Scale

• How was the geologic time scale established,
  and what are its major divisions?
• The geologic time scale is based on both
  relative and absolute dating. The major divisions
  of the geologic time scale are eons, eras, and
  periods.

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Geologic Time Scale

• Geologists and paleontologists have built a time
  line of Earths history called the geologic time
  scale.
• The basic divisions of the geologic time scale
  are eons, eras, and periods.

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Establishing the Time Scale

• By studying rock layers and index fossils, early
  paleontologists placed Earths rocks and fossils
  in order according to their relative age.
• They noticed major changes in the fossil record
  at boundaries between certain rock layers.

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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.

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Life on a Changing Planet

• How have our planets 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 Earths history. In turn, the actions
  of living organisms over time have changed
  conditions in the land, water, and atmosphere of
  planet Earth.

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Physical Forces

• The theory of plate tectonics explains how solid
  continental plates move slowly above Earths
  molten corea process called continental drift.
• Over the long term, continents have collided to
  form supercontinents. Later, these
  supercontinents have split apart and reformed.

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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.

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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.

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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.

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Biological Forces

• The activities of organisms have affected global
  environments.
• For example, Earths early oceans contained
  large amounts of soluble iron and little oxygen.
• During the Proterozoic Eon, however,
  photosynthetic organisms produced oxygen gas and
  also removed large amounts of carbon dioxide from
  the atmosphere.
• The removal of carbon dioxide reduced the
  greenhouse effect and cooled the globe. The iron
  content of the oceans fell as iron ions reacted
  with oxygen to form solid deposits.
• Organisms today shape the landscape by building
  soil from rock, and sand and cycle nutrients
  through the biosphere.