Chapter 7: Fossils and Their Place In Time and Nature

Chapter Outlines

  1. Fossils: Memories of the Biological Past
    1. What Are Fossils?
      1. Fossils are the remains of organisms chemically changed into rock.
    2. Taphonomy and Fossilization
      1. Taphonomy is the study of what happens to an organism’s remains after death.
      2. Describes what circumstances are necessary for fossilization to occur.
      3. Fossilization requires an anoxic environment, where decomposition is limited.
    3. Types of Fossils
      1. Fossils are found in different types of rock.
        1. Sedimentary
        2. Volcanic
      2. Sometimes vestiges of DNA can be found within fossils.
      3. Chemical segments of bone can also be analyzed.
    4. Limitations of the Fossil Record: Representation Is Important
      1. The fossil records present a “snapshot” of life in the past.
      2. Recognition of this limitation is critical in interpreting the fossil record.
      3. The Fayum in Egypt has a rich record of early primate evolution ending at about 31 mya; later fossilization conditions may not have been as ideal as in previous millennia.
      4. The same is true of the human fossil record: the best preservation is in eastern and southern parts of Africa after 4 mya.
  2. Just How Old Is the Past?
    1. Time in Perspective
      1. Long expanses of time that paleontologist handle (e.g., billions of years) are difficult for many people to conceive of.
      2. It is necessary to place fossils in time and space to fully understand their significance.
    2. Geologic Time: Earth History
      1. Geology has established three eras of time in Earth’s history.
        1. Paleozoic
        2. Mesozoic
        3. Cenozoic
      2. Changes to the Earth’s continent Pangaea and oceans occurred as mammalian, primate and human evolution was occurring.
    3. Relative and Numerical Age
      1. Relative Methods of Dating: Which Is Older, Younger, the Same?
        1. Stratigraphic Correlation
          1. Relative age is determined by comparing the incidence of one event to another. Those at the bottom of a stratum are older than those nearer the top.
          2. Stratigraphic correlation matches up strata based on a criterion to place the scale of time in a large region.
        2. Chemical Dating
          1. Fluorine dating measures levels of fluorine in bones to determine relative age.
        3. Biostratigraphic (Faunal) Dating
          1. Faunal dating compares different fossil forms based on the first appearance of an organism through its extinction.
        4. Cultural Dating
          1. Cultural dating uses changes in material culture (such as stone tools) to establish a chronology.
    4. Absolute Methods of Dating: What Is the Numerical Age?
      1. Absolute (numerical) age is determined through methods that result in a numerical time scale.
        1. Dendrochronology counts the rings of trees; each ring marks one year of growth.
      2. The Radiometric Revolution and the Dating Clock
        1. Radiocarbon dating measures the half-life of carbon isotopes.
        2. This method is most accurate for only the last 50,000–70,000 yBP.
      3. The Revolution Continues: Radiopotassium Dating
        1. Radiopostassium dating measures the potassium radioisotope in volcanic rock associated with fossils.
        2. Potassium has a long half-life, and can be used to date fossils older than 200,000 years.
    5. Genetic Dating: The Molecular Clock
      1. Amino acid dating, based on the decay of protein molecules, is useful for fossils up to 200,000 years old and, in some settings, 1 mya.
      2. Fission track dating is based on changes in uranium-238 and is used to date volcanic ash and glass millions of years old.
      3. Paleomagnetic dating uses changes in the earth’s magnetic field to date rock.
      4. Electron spin resonance dating takes a measure of radioisotope buildup in fossils over time.
      5. Luminescence dating takes the amount of the sun’s energy in sediment, stone, or ceramic.
      6. The molecular clock measures changes in DNA over time and is used to examine the timing of the splits in primate and human evolution.
  3. Reconstruction of Ancient Environments and Landscapes
    1. In addition to constructing time scales, researchers also try to reconstruct ancient environments.
    2. The Driving Force in Shaping Environment: Temperature
      1. Temperature drives climate, which is linked to biology and chemistry.
      2. Ancient temperatures are reconstructed through study of ocean dwelling organisms.
      3. One of the most dramatic changes in temperature occurred around 6 mya and was a cooling period in Earth’s history.
    3. Chemistry of Animal Remains and Ancient Soils: Windows onto Diets and Habitats
      1. Chemical analysis of bones and teeth can reconstruct diets and habitats based on plants the animals ate.
      2. Different environments are associated with different types of carbon dioxide (e.g., C3 or C4).
      3. Time, space, habitat, and diet are important factors in the interpretation of the fossil record.
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