Index Fossils - It is used for the determination of the age of organic rocks and other fossil

assemblages and help to establish relationships between rock units.

In 1796, William Smith first noted that rock wascharacterized by unique sets of fossil taxa.
Certain floral and faunal remains are often found to be restricted to a particular geologic horizon
– 'index fossils' or 'guide fossils’

Geologic range - refers to the total length of geologic time that an organism was or has been,
present on earth.

Animal Index Fossils

a) Ammonites - group of fossils molluscs possessing exogastrically coiled shell. Perisphinctes

tiziani is a Jurassic index fossil and Tropites subbullatus is a Triassic index fossil.
b) Gastropods - another groups of molluscs have asymmetrical bodies with usually dextral,
spirally twisted shell; Nerinea trinodosa is a Jurassic index fossil.
c) Pelecypods - a mollusc enclosed with bivalved shell and ventrally located tongueshaped
foot; Venericardia planicosta is a Tertiary index fossil.
d) Brachiopods - bilaterally symmetrical coelomate animal with bivalved shell; Billingsella
corrugata is a Cambrian index fossil and Dictyoclostus americanus is a Carboniferous
(Pennsylvanian) fossil brachiopod.
e) Crinoid - a marine invertebrate, usually possessing a cup-shaped body and five or more
feathery tentacles; Cactocrinus multibrachiatus of Carboniferous (Mississippian) is a good
example of index fossils.
f) Trilobites - extinct marine arthropods with more or less oval body divided into three lobes by
two longitudinal furrows; Paradoxides pinus and Bathyurus extans are index fossils of Cambrian
and Ordovician periods, respectively.

Characteristics of Index Fossils

1.An organism must have lived only during a short part of Earth's history.
2.Many fossils of the organisms must be found in rock layers.
3.The fossil must be found over a wide area of Earth.
4.The organism must be unique.

History of the Earth

• Scientists used assumption called uniformitarianism in order to relate what we know about
present-day processes to past events
• Uniformitarianism states that the natural laws we know today have been constant over the
geologic past.

Earth’s Early Evolution

Three major divisions of the Earth’s interior
a) the iron – rich core,
b) the thin primitive crust, and
c) its thickest layer, the mantle.

• The light materials – including water vapor, carbon dioxide and other gasses escaped to form
primitive atmosphere and shortly thereafter the oceans.

Geologic Time Scale

• The geologic time scale (GTS) is a system of chronological dating that relates geological strata
(stratigraphy) to time.
• The Precambrian Time (the time of hidden life) is the point of time in which our planet started
to form. This is subdivided into Hadean, Archean, and Proterozoic Eon.
Meanwhile, Phanerozoic Eon, living organisms like plants and animals existed. It constitutes
the time of multicellular life on Earth and the evolutionary process happened.
• Phanerozoic Eon is further divided into the Paleozoic Era (time of ancient life), Mesozoic Era
(time of middle life), Cenozoic Era (time of recent life)

The Precambrian time

• Ranges from about 4.6 billion years ago, when the earth formed, to about 544 million years
ago, when abundant microscopic life appeared.
• During mid-Precambrian, organisms such as blue green algae developed a simple version of
photosynthesis.
• Photosynthetic organisms require carbon dioxide to utilize the sun’s energy. They keep the
carbon dioxide and expel oxygen. With the release of free oxygen, a primitive ozone layer
began to develop which reduced the amount of harmful ultraviolet radiation reaching the Earth.
• most common Precambrian fossils are stromatolites, indirect pieces of evidence of algae.
Many of Precambrian fossils were preserved in hard, dense chemical sedimentary rock known
as chert.
• Fossils of plants date from the middle Precambrian, but fossils of animals date in the
Precambrian.

HADEAN EON
• Greek for “beneath the Earth”. It is the earliest time in Earth history and ranges from the
planet’s origin 4.6 billion years ago to 3.8 billion years ago.
• Solar System and Planet formed.

ARCHEAN EON
• Greek for “ancient”. There are few fossils among the rocks and they are not preserved well
enough to allow for finely tuned subdivision of this eon that spanned from 3.8 to 2.5 billion years
ago.
• life on earth started to appear since the oldest dated bacterial microfossils were 3.5 billion
years old.

PROTEROZOIC EON
• Greek for “earlier life”. Diverse groups of fossils have been found in sedi rocks, 2.5 billion to
543 million years ago.
• many important geologic events: appearance of stable continents, bacterial, and archaean
fossils, and eukaryotic cell

PHANEROZOIC EON
• Phaneros is Greek for “evident”. Sedimentary rocks cover the most recent 543 million years of
geologic time and contained abundant fossils.
• Four changes occurred that greatly improved the fossil record
1.The number of species with shells and skeletons dramatically increased.
2. The total number of individual organisms preserved as fossils increased greatly.
3. The total number of species preserved as fossils increased greatly.
4. The average sizes of individual organisms increased

Paleozoic Era
• Began about 544 million years ago and lasted about 300 million years, during which time sea
levels rose and fell worldwide
divided into six (6) major periods:
a. Cambrian Period
Almost all marine organisms came into existence as evidenced by abundant fossils.
b. Ordovician Period
All major groups of animals that could be preserved as fossils had appeared. This period marks
the earliest appearance of vertebrates – the jawless fish known as agnatha.
c. Silurian Period
The Silurian brought about the emergence of terrestrial life, the earliest being the terrestrial
plants with well-developed circulatory system (vascular plants). Air-breathing scorpions and
millipedes
were common during the period.
d. Devonian Period
known as the “age of fishes”. Lowland forests of seed ferns, scale trees and true ferns
flourished. Sharks, . e.
e. Carboniferous period
Warm, moist climate conditions contributed to lash vegetation and dense swampy forests. The
evolution of the first reptiles took place with the development of the amniotic egg, a porous shell
containing a membrane that provided an environment for an embryo.
f. Permian Period
The reptiles were well-suited to their environment that they ruled the Earth for 200 million years.
The two major groups of reptiles — diapsids and synapsids — dominated this period. Diapsids
gave rise to the dinosaurs. Synapsis gave rise to mammals.

Mesozoic Era
• Known as the age of reptiles, it is made up of three periods: Triassic, Jurassic, Cretaceous.
The most significant event was the rise of the dinosaurs.
• A famous Jurassic deposit is the Morrison Formation, within which the world’s richest
storehouse of dinosaurs was preserved. A major event - breakup of Pangea.
a. Triassic Period- “Age of reptiles” begins; first mammals; corals, insects, and fishes resemble
modern types.
b. Jurassic Period- the Rocky Mountains rise; first birds; palms and cone bearing trees
dominant; large dinosaurs thrive; primitive mammals develop
c. Cretaceous Period- First flowering plant; placental mammals developed; dinosaurs die out,
as do many marine animals at the end of the period.

Cenozoic Era
• known as the “age of mammals” “age of flowering plants” because angiosperms replaced
gymnosperms as the dominant land plants.
• made up of two periods: Tertiary and Quaternary.
From oldest to youngest the periods are broken up into the Paleocene, Eocene, Oligocene,
Miocene, and Pliocene
for the Tertiary period, and the Pleistocene and Holocene for the Quaternary period.

a. Paleogene Period

• Paleocene Epoch- beginning of “age of mammals”; flowering plants and small mammals
abundant; many different climates existed.
• Eocene Epoch- fossils of “dawn horse”; grasslands and forest present; many small mammals;
larger mammals such as whales, rhinoceros, and monkeys begin to develop.
• Oligocene Epoch- fossils of primitive apes; elephants, camels, and horses developed; climate
generally mild.
b. Neogene Period
• Miocene Epoch- many grazing animals; flowering plants and tress resemble modern types.
• Pliocene Epoch- fossils of ancient humans near end of epoch; many birds, mammals and sea
life similar to modern types; climate cools.
c. Quaternary Period
• Pleistocene Epoch- “The Ice Age”; modern humans present; mammoths and other animals
become extinct.
• Holocene Epoch- Humans are the dominant forms of life and civilization begins and spread