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Chapter Eleven
The Record of the Past
© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
Fossils and Their Interpretation
• A fossil is the remains or traces of an ancient organism.
– Several conditions must be met for fossilization to occur.
• The fossil record is not a complete record of the history
of living organisms on the face of the earth.
– It exhibits sampling error.
• Fossils can give us information about the size, shape,
and function of different muscles, the relative
importance of the senses, and approximate brain size.
– Additionally, information about disease, injury, growth, and
development may be found.
• Artifacts are important clues to hominin behavior.
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
Taxonomy and the Fossil Record
• The definition of a species in living animals is based
on reproductive success, a concept which would be
difficult to apply to the fossil record.
• The typological viewpoint is that basic variation of
type is illusory and that only fixed types are real.
Typologists are more likely to split two specimens
that differ into two separate species.
• The populationist viewpoint is that only individuals
have reality and that type is illusory. Variation
underlies all existence. Populationists would be
more likely to lump two specimens that differ into a
single species.
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
Taxonomy and the Fossil Record
(Continued)
• A paleospecies is defined in terms of morphology
rather than reproductive success. A chronospecies is
an arbitrarily defined division in an evolutionary line
based on time.
• Age, sex, and genetic differences all lead to variability
in the fossil record.
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
Geological Time
• Relative dating provides information on which fossils in a
sequence are older or younger.
– Stratigraphy is based on the principle of superposition, which
states that the lower strata are older than those above.
• Methods like flourine dating can help establish if two
bones are contemporary.
• Certain fossils or combinations of fossils are commonly
found and have become markers for particular periods of
time.
– These are known as index fossils.
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
Radiometric Dating Techniques
• Radiometric dating methods are based on the decay
of radioactive materials.
– Radioactivity means that the atom is unstable and will decay
into another type of atom.
• If we have a given number of atoms, we can say that
one-half will have decayed in a specific number of
years known as the half-life.
• Dating techniques include fission-track,
thermolumine-scense dating, electron spin resonance
dating, amino acid racemization, and the geomagnetic
time scale.
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
Radiometric Dating
• Potassium-Argon dating is based on the radioactive
decay of potassium 40, which has a half-life of 1250
million years. Radiocarbon dating is based on the decay
of carbon 14, which has a half-life of 5,730 years.
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
The Geological Time Scale
• The evolution of life has been greatly affected by earth dynamics.
– Shifting landmasses create new migratory routes and destroy
others.
– Plate tectonics are also responsible for many alterations in
climate.
• Geologists have divided the history of the earth into a hierarchy of
units
• Era
• Period
• Epoch
• The three large eras are the
• Paleozoic
• Mesozoic
• Cenozoic
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.
The Cenozoic
• The Cenozoic is known as the Age
of Mammals, as it is the time of the
adaptive radiation of the mammals.
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© 2006 The McGraw-Hill Companies, Inc. All Rights reserved.