Laboratory Manual for Introductory Geology 4e

eras within the Phanerozoic Eon reveal the complexity of their life forms: Paleozoic,
meaning ancient life; Mesozoic, meaning middle life; and Cenozoic, meaning recent
life. The end of an era is marked by a major change in life forms, such as an extinction
in which most life forms disappear and others fill their ecological niches. For
example, nearly 90% of fossil genera became extinct at the end of the Paleozoic
Era, making room for the dinosaurs, and the extinction of the dinosaurs at the end
of the Mesozoic Era made room for us mammals. Several period names come from
areas where rocks of that particular age were best documented: Devonian from Devonshire
in England, Permian from the Perm Basin in Russia, and the Mississippian
and Pennsylvanian from U.S. states.
The geologic time scale was originally based entirely on relative dating. We knew
that Ordovician rocks and fossils were older than Silurian rocks and fossils, but had
no way to tell how much older, or where exactly in geologic time they belonged.
The numerical ages shown in Figure 12.8 provide that information today, but were
calculated using methods discovered more than 100 years after the original scale
was constructed.
12.3.3 Fossil Age Ranges
Some index fossil ages are very specific. The trilobite Elrathia, for example, lived
only during the Middle Cambrian, whereas the trilobite Redlichia was restricted
mostly to the Early Cambrian. Others lived over a longer span, such as the trilobite
Cybele (Ordovician and Silurian) and the brachiopod Leptaena (Middle Ordovician
to Mississippian). But even index fossils with broad age ranges can yield specific
information if they occur with other index fossils whose overlap in time limits the
possible age of the rock. This can be seen even in the broad fossil groups shown in
Figure 12.8. Trilobite, fish, and reptile fossils each span several periods of geologic
time, but if specimens of all three are found together, the rock that contains them
could only have been formed during the Pennsylvanian or Permian periods.
EXERCISE 12.6
Dating Rocks by Overlapping Fossil Ranges
Name:
Course:
Section:
Date:
Brachiopods are marine bivalves (two-shelled invertebrates) that flourished during the Paleozoic Era, but have been
mostly replaced by modern clams. The figure on the next page shows selected Paleozoic brachiopod species used as index
fossils and their age ranges.
(a) Based on the overlaps in brachiopod age ranges shown in the graph on the next page, what brachiopod assemblage in rock
would indicate
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a Permian age?
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a Silurian age?
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an Ordovician age?
(continued)
12.3 BIOLOGICAL METHODS FOR RELATIVE AGE DATING
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