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Title: Lesson Overview


1
Lesson Overview
  • 19.1 The Fossil Record

2
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.

3
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.

4
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.

5
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.

6
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.
7
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.
8
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.

9
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.

10
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.

11
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.
12
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.

13
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).

14
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.

15
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.

16
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.

17
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.

18
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.

19
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.

20
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.

21
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.

22
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.

23
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.
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