Download index fossils

NAME : ELUFOWOJU OLUWASANU
DEPARTMENT : GEOLOGY
COURSE : INVETEBRATE PALEONTOLOGY
MATRIC NUMBER : DIRECT ENTRY
INDEX FOSSILS
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 molluscs.
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.
Also Known As: key fossils, type fossils
Diagram showing Index fossils embedded in stratas.
IMPORTANCE OF AMMONITES IN NIGERIAN ROCKS
INTRODUCTION
Ammonites evolved from the early nautiloids. Nautiloids first appeared during the
subsequent Ordovician period (500-450 mya) and became a very widespread and
diverse group assuming the ecological niche of top predator following the
extinction of the anomalocarids. These animals tended to have cone shaped or long
straight shells divided into chambers and evolved jet propulsion, suggested by a
notch underneath the aperture of the shell that is assumed to have held the
hypernome, (the siphon in which water is squirted out to move the animal through
the water, a feature which all living cephalopods have retained). The nautiloids
diversified into many different orders, some of them huge predators e.g.
Cameroceras, which grew up to 10meters long, before most of them became
extinct by the end of the Devonian period (350 mya). Only one order survived, the
Nautilida from which our five species of Nautilus alive today are descended.
The ammonoids themselves evolved from an offshoot of one of these extinct
groups of straight shelled nautiloids, the bactritida, during the mid-Devonian
period and began to evolve the coiled shells we are so familiar with in fossil
collections today. The bactritids are an obscure group and have been interpreted a
transitional form between nautiloids and ammonoids. Three different basic forms
of ammonoids evolved, initially the goniatites, in the mid Devonian which lasted
until the end of the Permian (250 mya). They were superceded by the ceriatites
during the Triassic (250-200 mya). The ammonites themselves really came to
dominance in the Jurassic following the extinction of the ceriatites (200-150mya).
IMPORTANCE IN NIGERIAN ROCKS.
1. BIOSTRATIGRAPHY.
Biostratigraphy is the differentiation of rock units based upon the fossils
which they contain.. Rocks in Nigeria can be differentiated based on their
characteristic ammonite content.
2. PALEOENVIRONMENT
Paleoenvironmental analysis is the interpretation of the depositional
environment in which the rock unit formed, based upon the fossils found
within the unit.
3 SOLUTION TO STRUCTURAL PROBLEMS.
It can be used to solve structural problems arising from overtunned bed of
rocks.
4 USED TO DETERMINE THE AGE, MATURITY,SOURCE TYPE AND
KEROGEN QUALITY.
Ammonites are good index fossils and evolved during the time when Nierian
oil formed maturel . They are good indicators of the duration or age of
Nigerian oil.
5 ORGANIC EVOLUTION
They can be used to trace back other species that have lived before them
thereby finding more facts about their origin.
OTHER IMPORTANCE ARE
FOR ROCK BUILDING
DATING, DIVISION AND CORRELATION OF STARTA.
ROCK BUILDING.
PALEOCLIMATOLOGY.
BIOGEOGRAPHY.
THERMAL MATURATION.
DIAGRAM SHOWING A WELL LABELLED AMONITE