Download Gey205_1

NAME: EDU SAMUEL EFAIWA
MAT NO. :14/SCI14/013
DEPT: GEOLOGY
GEY 205
1.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.The best index fossils have four characteristics: They are unique, prevalent, plentiful and
restricted in geologic time. Most fossils are from ocean rocks, so most major index fossils are
from marine organisms. However, young rocks found in certain regions are useful for revealing
the fossils of land organisms.
Trilobites are excellent index fossils for Paleozoic rocks. These mobile animals lived across
oceans and constantly evolved into new species from the Middle Cambrian time to the end of the
Permian Period, which is almost the entire Paleozoic period. These organisms inhabited large,
even global areas, and their large fossils can be studied without a microscope. Similar index
fossils include rugose corals, crinoids, mollusks, ammonites, bryozoans and brachiopods.
Other index fossils such as floating plankton are very useful because of their small to microscopic
size. Their tiny bodies rained down all over the ocean and are found in many kinds of rocks, even
wellbore cuttings. Index microfossils are a great asset to the petroleum industry, and have enabled
geologists to break down geologic time into fine detail.
Index fossils of terrestrial rocks formed on land reveal clues about quickly-evolving small rodents
as well as larger animals that ranged over wide geographic ranges.
Index fossils define the ages, epochs, periods and eras in the formal architecture of geologic time.
Related fossil types are the characteristic fossil, which belongs to a time period without defining it,
and guide fossils, which narrow down but don’t specify a time period.
2. Ammonite is very useful in the dating of rocks due to their large number and distribution. It is
the oldest rock in Nigeria and is used in dating. They are found in places such as: Odukparo,
Ogoja province, and airport.