Download Index Fossils Activity

Index Fossils:
Can fossils tell time?
What are Index Fossils and how do they help tell time?
Definition: While every fossil tells us something about the age of the rock it's found in,
index fossils are the ones that tell us the most. Index fossils (also called key fossils or
type fossils) are those that are used to define periods of geologic time.
A good index fossil is one with four characteristics: it is distinctive, widespread,
abundant and limited in geologic time. Because most fossil-bearing rocks formed in the
ocean, the major index fossils are marine organisms, but certain land organisms are
useful in young rocks and in specific regions.
Any type of organism can be distinctive, but not so many are widespread. Many
important index fossils are of organisms that start out life as floating eggs and infant
stages, which allowed them to populate the world using ocean currents. The most
successful of these became abundant—and at the same time, they became the most
vulnerable to environmental change and extinction. That boom-and-bust character is
what makes the best index fossils.
Consider trilobites, a very good index fossil for Paleozoic rocks. Trilobites lived in all
parts of the ocean and were constantly evolving new species during their existence,
which lasted from Middle Cambrian time to the end of the Permian Period, almost the
entire length of the Paleozoic. Because they were mobile animals they tended to inhabit
large, even global areas. In addition, their fossils are large enough to study without a
microscope. Other index fossils of this type include ammonites, crinoids, rugose corals,
brachiopods, bryozoans and mollusks.
Other major index fossils are small or microscopic, part of the floating plankton in the
world ocean. These are handy because of their small size. They can be found even in
small bits of rock, such as wellbore cuttings. Because their tiny bodies rained down all
over the ocean, they can be found in all kinds of rocks. Therefore the petroleum industry
has made great use of index microfossils, and geologic time is broken down in quite fine
detail by various schemes based on graptolites, fusulinids, diatoms and radiolarians.
For terrestrial rocks, which form on land, regional or continental index fossils may
include small rodents that evolve quickly as well as larger animals that have wide
geographic ranges. These form the basis of provincial time divisions.
Index fossils are used in the formal architecture of geologic time for defining the ages,
epochs, periods and eras of the geologic time scale. Related fossil types include the
characteristic fossil—a fossil that belongs to a time period but doesn't define it—and the
guide fossil, one that helps narrow down a time range rather than nail it down.
Procedure:
• Cut out the layers of fossils.
• Sort them from past to more recent. (Hint: Organisms do not disappear for a
layer and the reappear.)
• Check your answers and write the time period on each layer.
Questions:
1. What are the 4 characteristics of a “good” index fossil?
2. Which layers were confusing and why?
3. What patterns (with evidence from the reading) can be determined from the
fossils? Consider diversity of species, possible environment, fossil
relationships etc.