Page 240 - Laboratory Manual in Physical Geology

ACTIVITY have existed since late in the Triassic Period. If you found
a rock layer with bones and tracks of both dinosaurs
8.3 Using Index Fossils to Date and mammals, then the age of the rock layer would be
Rocks and Events represented by the overlap of the dinosaur and mammal
range zones (i.e., Middle Triassic to Late Cretaceous).
Notice that FIGURE 8.10 also includes the following groups:

THINK | How are fossils used to tell geologic
About It time and infer Earth’s history? ■ Brachiopods (pink on chart): marine invertebrate

animals with two symmetrical seashells of unequal
size. They range throughout the Paleozoic, Mesozoic,
OBJECTIVE Use index fossils to determine the

relative ages (eras, periods) of rock bodies and infer and Cenozoic Eras, but they were most abundant in
some of Earth’s history. the Paleozoic Era. Only a few species exist today, so
they are nearly extinct.
PROCEDURES
■ Trilobites (orange on chart): an extinct group of marine
1. Before you begin , read Relative Age Dating Based arthropods (animals related to lobsters). They are only
on Fossils below. Also, this is what you will need : found in Paleozoic rocks, so they are a good index fossil
____ calculator for the Paleozoic Era and its named subdivisions.
____ Activity 8.3 Worksheet (p. 221 ) and pencil
■ Mollusks (pink on chart): phylum of snails, cephalo-
2. Then follow your instructor’s directions for pods (squid, octopuses), and bivalves (oysters, clams;
completing the worksheets. two asymmetrical shells of unequal size).
■ Plants (dark green on chart).
■ Reptiles (pale green on chart): the group of vertebrate
animals that includes lizards, snakes, turtles, and
Relative Age Dating Based on dinosaurs. Dinosaurs are only found in Mesozoic
Fossils rocks, so they are an index fossil for the Mesozoic and
its subdivisions.
The sequence of strata that makes up the geologic record
is a graveyard filled with the fossils of millions of kinds of ■ Mammals (gray on chart): the group of vertebrate
organisms that are now extinct. Geologists know that they animals (including humans) that are warm blooded,
existed only because of their fossilized remains or the traces nurse their young, and have hair.
of their activities (like tracks and trails). ■ Amphibians (brown on chart): the group of vertebrate
animals that includes modern frogs and salamanders.
Principle of Fossil Succession and
■ Sharks (blue on chart): a group of fish with teeth but
Index Fossils
no hard bones.
Geologists have also determined that fossil organisms
originate, co-exist, or disappear from the geologic record
in a definite sequential order recognized throughout the Rock and Time Units of the Geologic
world, so any rock layer containing a group of fossils can be Time Scale
identified and dated in relation to other layers based on its

fossils. This is known as the Principle of Fossil Succession . The geologic time scale in FIGURE 8.10 shows the ranges

A fossilized organism that can be used to identify the of index fossils in relation to named units of time and
relative age of rock layers is called an index fossil . rock plus a scale of absolute ages in millions of years.
Notice that there are two levels of named time and rock
units in FIGURE 8.10 . Long eras of time are subdivided

Range Zones
into shorter periods of time. As noted in the table below,
The interval of rock in which the index fossil is found an era of time corresponds with an erathem of rock
is called its range zone and corresponds to a particular containing its characteristic index fossils. A period of
interval of geologic time. The range zones of some well- time corresponds with a system of rock containing its
known Phanerozoic index fossils are presented on the right characteristic index fossils.

side of FIGURE 8.10 . Relative ages of the rocks containing

these fossils are presented as periods and eras on the left side
of FIGURE 8.10 . Rock Units (Division of the Corresponding Geologic

By noting the range zone of a fossil (vertical black Geologic Record) Time Units
line), you can determine the corresponding era(s) or Eonothem of rock Eon of time (longest)
period(s) of time in which it lived. For example, all of Erathem of rock Era of time
the different species of dinosaurs lived and died during
System of rock Period of time
the Mesozoic Era of time, from the middle of the Triassic
Series of rock Epoch of time
Period to the end of the Cretaceous Period. Mammals
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