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Chapter 26 0809

Chapter 26 0809 - Chapter 26 The Tree of Life An...

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Chapter 26: The Tree of Life: An Introduction to Biological Diveristy Section One: The Fossil Record and Geologic Time I. Sedimentary rocks are the richest source of fossils Fossils : the preserved remnants or impressions left by organisms that lived in the past. they are the historical documents of biology. Fossil record : the ordered array in which fossils appear within sedimentary rocks. These rocks record the passing of geological time. A. The Formation of Fossils Sedimentary rocks form from layers of sand and silt that settle to the bottom of seas and swamps. As deposits pile up, they compress older sediments below them into rock. The bodies of dead organisms settle along with the sediments, but only a tiny fraction are preserved as fossils. B. Rates of sedimentation The organic material in a dead organism usually decays rapidly, but hard parts that are rich in minerals (such as bones, teeth, shells) may remain as fossils. Under the right conditions minerals dissolved in groundwater seep into the tissues of dead organisms, replace its organic material, and create a cast in the shape of the organism. C. Other Types of Fossils Casts : see above Trace fossils: consist of footprints, burrows, or other impressions left in sediments by the activities of animals. These rocks are in essence fossilized behavior. Example: Dinosaur tracks provide information about its gait. Soft Tissue Fossils If an organism dies in a place where decomposition cannot occur, then the entire body, including soft parts may be preserved as a fossil. Example: organisms that have been trapped in resin, frozen in ice, or preserved in acid bogs. II. Paleontologists use a variety of methods to date fossils A. Relative Dating Age of a fossil is given in relative terms to another fossil with terms such as: before or after, and early or late When a dead organism is trapped in sediment, this fossil is frozen in time relative to other strata in a local sample. Younger sediments are superimposed upon older ones. The strata at one location can be correlated in time to those at another through index fossils. These are typically well-preserved and widely-distributed species. Geologic time scale : (figure 25.1) Boundaries between geologic eras and periods correspond to times of great change, especially mass extinctions, not to periods of similar length. Eras > Periods > Epochs Four eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic The serial record of fossils in rocks provides relative ages, but not absolute ages, the actual time when the organism died.
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B. Absolute Dating Age of a fossil is given in approximate years 1. Radiometric dating This technique takes advantage of the fact that organisms accumulate radioactive isotopes when they are alive, but concentrations of these isotopes decline after they die. These isotopes undergo radioactive decay in which an isotope of one element is
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Chapter 26 0809 - Chapter 26 The Tree of Life An...

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