Lesson Overview

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Lesson Overview

• 19.1 The Fossil Record

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THINK ABOUT IT

• Fossils, the preserved remains or traces of
  ancient life, are priceless treasures. They tell
  of life-and-death struggles and of mysterious
  worlds lost in the mists of time.
• Taken together, the fossils of ancient organisms
  make up the history of life on Earth called the
  fossil record.
• How can fossils help us understand lifes
  history?

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Fossils and Ancient Life

• What do fossils reveal about ancient life?

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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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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.

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Types of Fossils

• Many fossils are just fragments of an
  organismteeth, pieces of a jawbone, or bits of
  leaf.

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Types of Fossils

• Sometimes an organism leaves behind trace
  fossilscasts of footprints, burrows, tracks, or
  even droppings.

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Types of Fossils

• Although most fossils are preserved in
  sedimentary rocks, some are preserved in other
  ways, like in amber.

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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

• The preserved remains may later be discovered
  and studied.

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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.

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Fossils in Sedimentary Rock

• 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.
• 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.

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What Fossils Can Reveal

• Fossilized plant leaves and pollen suggest
  whether the area was a swamp, a lake, a forest,
  or a desert.
• When different kinds of fossils are found
  together, researchers can sometimes reconstruct
  entire ancient ecosystems.

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Dating Earths History

• How do we date events in Earths history?

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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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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).
• Trilobites, a large group of distinctive marine
  organisms, are often useful as index fossils.

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Radiometric Dating

• Relative dating is important, but provides no
  information about a fossils 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.

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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.

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Radiometric Dating

• Different radioactive elements have different
  half-lives, so they decay at different rates.

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Radiometric Dating

• The half-life of potassium-40 is 1.26 billion
  years.

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Radiometric Dating

• 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.

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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.
• 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?

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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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Divisions of the Geologic Time Scale

• The time scale is based on events that did not
  follow a regular pattern.
• The Cambrian Period, for example, began 542
  million years ago and continued until 488 million
  years ago, which makes it 54 million years long.
• The Cretaceous Period was 80 million years long.

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Divisions of the Geologic Time Scale

• Geologists now recognize four eons of unequal
  length.
• The Hadean Eon, during which the first rocks
  formed, began about 4.6 billion years ago.
• The Archean Eon, when life first appeared, began
  about 4 billion years ago.

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Divisions of the Geologic Time Scale

• The Proterozoic Eon began 2.5 billion years ago
  and lasted until 542 million years ago.
• The Phanerozoic Eon began at the end of the
  Proterozoic and continues to the present.

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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.

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Naming the Divisions

• Geologists started to name divisions of the time
  scale before any rocks older than the Cambrian
  Period had been identified. For this reason, all
  of geologic time before the Cambrian is simply
  called Precambrian Time.

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Naming the Divisions

• The Precambrian actually covers about 90 percent
  of Earths history.
• In this figure, the history of Earth is depicted
  as a 24-hour clock. Notice the relative length of
  Precambrian Timealmost 22 hours.

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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?

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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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Life on a Changing Planet

• Earth and its climate has been constantly
  changing, and organisms have evolved in ways that
  responded to those new conditions.
• The fossil record shows evolutionary histories
  for major groups of organisms as they have both
  responded to changes on Earth and how they have
  changed Earth.

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Physical Forces

• Climate is one of the most important aspects of
  Earths physical environment.
• Earths 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, Earths average temperatures were only 6C
  to 12C higher than they were during the
  twentieth century.
• During the ice ages, world temperatures were
  only about 5C cooler than they are now.
• These relatively small temperature shifts
  changed the shape of life on Earth.

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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.

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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.