13. evolution Fossils_67563806_2025_09_23_09_39.pdf

Definition
Sir Charles Lyell defines fossils as:
"Any body or traces of body, animal or vegetation buried and preserved by
natural means."
Fossils can be
- entire organisms buried in snow,
- molds or casts of organisms,
- unchanged parts,
- replicas,
- footprints, or
- even leaf imprints on stones.
- First fossil were discovered After Lamarck but prior to Darwin.

- Many lineages on the evolutionary tree show stasis, meaning
they remain unchanged for a very long time.
- These lineages are conservative and do not undergo significant
changes over extended periods.
- King crab (Limulus), which has remained almost the same for
millions of years.
- Coelacanth fish, which has also changed very little since it
branched off from its ancestors.
- Due to their minimal evolution over time, these creatures are
often referred to as "living fossils."
Living Fossil

Unusual Fossils:
- Formed under unique conditions
where most of the organism is
preserved.
- Famous examples include:
1. Mammoths from Siberia.
2. Archaeopteryx: the first
bird, found in Solenhofen,
Germany.
Types of Fossils
Body Fossils:
- Fossils of hard parts
like shells, teeth, or
bones.
- Provide details of
shape and function.
- Range from
microscopic
organisms to large
dinosaurs.
Subfossils:
- Remains of
animals and plants
preserved in rocks
less than 10,000
years old.
- Includes frozen
animals like bison
or mummified
ancient humans.
- Formed after the
last Ice Age
(Holocene Epoch).
Microfossils:
- Fossils of microscopic
animals and plants,
usually less than 0.5
mm in size.
- Can form deposits as
large as 10 cm (4
inches).

Types of Fossils
Macrofossils:
- Fossils larger than 1 cm.
- Includes fossils of
advanced plants and
animals like clams, corals,
and vertebrates.
Trace Fossils:
- Fossils of footprints, trails, and
other traces left in mud.
- Examples: Dinosaur footprints,
worm trails, and clam burrows.
- Formed from daily activities like
walking, crawling, or feeding.
Coprolites:
- Fossils of animal droppings (faecal
matter).
- Range from tiny pellets of snails to
large droppings of crocodiles or
dinosaurs.
- Can give information on the diet of
ancient animals.
Bioclast:
- Fossils or fossil fragments enclosed in
sediments.
- Term is often used for thin fossil
sections observed under a microscope.

Pseudofossils:
- Objects of inorganic origin that
resemble organic forms.
- Found in sedimentary rocks but not
actual fossils.
Types of Fossils
Burrows and Borings:
- Burrows or holes made by animals in the ground, wood, or
rocks for shelter or food.
- Filled with sediment and preserved as fossils.
- Borings in shells or wood by other organisms are also
considered fossils.
Gastroliths:
- Stones found in the body cavities of certain
reptiles.
- Believed to help grind food in the stomachs
of extinct reptiles

Formation of Fossils
1.Natural Decomposition:
1. When an animal dies in the open, its body is quickly attacked by predators, scavengers, insects, and bacteria.
2. The flesh, organs, and even bones decay rapidly in open environments like hills, plains, and forests.
3. Plants also decay similarly.
2.Burial in Sediment:
1. Sometimes animals die near streams or in stream beds and are buried in sediment and water.
2. The soft parts decay, while the bones are gradually replaced by minerals.
3. Volcanic eruptions can also bury organisms in ash, preserving them.
4. Petroleum springs, amber, resin, sand, and ice can also preserve organisms.

Preservation in Peat:
- Peat is partially decomposed vegetable matter
that creates water-logged, oxygen-poor, and
bacteria-free conditions.
- Animals or plants buried in peat are preserved.
- Examples include bodies of Iron Age people
found in peat in Denmark and "Pete March"
from Staffordshire.
Entire Organism Preserved
In rare, favorable conditions, entire
organisms may be preserved in
excellent condition, including their
soft parts. The following are ways
entire organisms can be preserved:
Preservation in Resins and Amber:
- Insects can get trapped in sticky tree resin, which hardens into amber over
millions of years.
- The fossils, often insects and their eggs or larvae, remain perfectly
preserved, even showing minute details like colors.
- Example: Fossils from the Baltic Coast over 30 million years ago.

Fossils in Petroleum Springs and Asphalts:
- Animals trapped in oil-saturated soils, tar, or asphalt become
fossilized.
- Examples of fossils include extinct rhinoceros, insects, birds,
and small mammals.
- A famous site is Rancho La Brea in Los Angeles.
Preservation in Ice or Permafrost:
- In cold areas like Siberia's Arctic Tundra, animals may
freeze and remain preserved for thousands of years.
- Cold temperatures prevent decay from bacteria and other
forces.
- Woolly mammoths have been found in Siberia, with their
flesh and bones intact. These fossils were found in:
1. 1790 in Lena Delta.
2. 1901 in Siberia.

Types of Fossil Preservation:
1.Carbonization:
1. Organic materials like chitin, cellulose, or lignin leave behind a carbon film.
2. The carbon film preserves the outline and sometimes soft anatomy details.
3. Example: Fossilized ichthyosaurs.
2.Pyritization:
1. Soft parts of organisms are replaced by pyrites (a mineral) under certain conditions.
2. Example: Cephalopod arms preserved in pyrite from Devonian deposits.
3.Petrifaction of Hard and Soft Parts:
1. Petrified fossils include bones, teeth, and shells.
2. Even soft parts like muscles can be petrified under special conditions, preserving fine details like muscle fibers.
3. Example: Fossils from Glass Mountains in Texas and a 300-million-year-old petrified shark muscle.

Impressions
Formation: Impressions are similar to molds and casts but represent the
surface patterns of vanished organisms or their parts, left on soft materials
like clay.
Examples: Impressions of leaves, feathers of extinct birds, wing membranes
of flying reptiles, and skin of dinosaurs have been discovered.

Mummies
- Formation: In deserts, hot dry winds quickly dehydrate dead organisms, preserving them as mummies.
.
Coal Balls
- Formation: Coal balls are petrified plant organs that appear as spherical, black objects found in coal deposits.
- Composition: Formed by infiltration of calcium carbonate, magnesium carbonate, and iron sulfide into buried plant parts,
preventing their conversion into coal.

Life is divided into five eras: Archeozoic, Proterozoic, Palaeozoic, Mesozoic, and Coenozoic.
- Marine fossils in mountains show past underwater regions.
- Climate fluctuations are indicated by fossils of species in unusual locations (e.g., palms in Dakotas, musk oxen in
Arkansas).
Throughout these eras, fossils provide a record of the evolutionary transitions in various species, highlighting the
connections between ancient organisms and their modern descendants. For instance:
- Hyracotherium (Eohippus), an ancestor of the modern horse, lived around 55 million years ago during the Eocene
epoch. Over millions of years, it evolved from a small, multi-toed animal into the large, single-hoofed horse of
today.
- Archaeopteryx, a primitive bird from the Late Jurassic period, showcases features of both dinosaurs and birds,
representing a critical stage in avian evolution.
- Australopithecus afarensis (Lucy), living 3.9 to 2.9 million years ago, provides key insights into the evolution of
human bipedalism.

Hyracotherium (Eohippus) Archaeopteryx
Australopithecus afarensis (Lucy)

- In 2007, scientists discovered the fossil of Indohyus, a small deer-like animal in Kashmir, India, considered the
most recent terrestrial ancestor of whales.
Fossil of Indohyus
Indohyus

- Reptile fossils dating back 200-250 million years have recently been discovered in Brazil and Ghana,
parts of West Africa.
- During this period, these regions were part of Gondwana, one of the two large landmasses that formed
when the supercontinent Pangea began to break apart.

- The Burgess Shale in the Canadian Rocky Mountains is famous for its well-preserved Cambrian fossils, which date
back about 508 million years.
The Burgess Shale, Canada

3. Radiometry or Radioactive Clock Method
- Radiometric dating, introduced by Boltwood in 1907, relies on the steady disintegration of radioactive
isotopes in rocks.
- This method provides a more accurate estimate of fossil ages based on the half-life of radioactive
elements.
- Isotopes like Uranium-238, Potassium-40, and Carbon-14 are commonly used for dating.
Half-life: The time it takes for half of a radioactive substance to decay. For example, Uranium-238 has a half-
life of 4.5 billion years.
- By measuring the ratio of parent radioactive isotopes (e.g., Uranium-238) to daughter isotopes (e.g.,
Lead-206), the age of the rock can be determined.

- Half-life is the time required for half of the quantity of a radioactive substance to decay into a more stable form.
- During this period, the substance's activity decreases by 50%, and this process continues in subsequent half-lives,
leading to a predictable pattern of decay that allows scientists to estimate the age of materials and the rate at which
radioactive isotopes transform over time.
Half life:

15/16th of a radioactive sample decays in 40 days. Half-life of the sample is

- Describes forests that appear intact but have lost most of their large wildlife,
particularly large mammals, due to human activities like hunting and habitat
destruction.
- The forest may still exist, but its biodiversity has diminished significantly.
Empty forest:

[GATE 2015]
Which one of the following statements is TRUE when determining the age of a
fossil using carbon dating?
1. Carbon dating is based on carbon-13 to carbon-12 ratio in fossils
2. Carbon dating is useful for determining the age of only fossils older than
100,000 years
3. Older the fossil, lesser the carbon-14 to carbon-12 ratio
4. Older the fossil, lesser the carbon-12 to carbon-14 ratio

Which of the following describes 'Empty Forest'?
1. Absence of large trees
2. Less species diversity due to natural reasons
3. Habitat void of large mammals due to anthropogenic impacts
4. Loss of habitat
CSIR – JUNE 2024

Given below are statements on 'living fossils'. Select the correct
statements
1. Living fossils are impressions of extant organisms in old rocks.
2. Living fossils show high morphological divergence from fossil
records.
3. Living fossils are always an evolutionary link between two classes
of organisms
4. Living fossils are organisms that have remained unchanged for
millions of years.
CSIR – JUNE 2021

The radioactive isotope of an element has a half-life of
100 hours. How many hours will it take for 15/16 of the
source amount to decay?
1. 50
2. 400
3. 250
4. 1000
CSIR – JUNE 2021

Age of fossils is usually determined by
1. Sediment deposition
2. Mineral deposition
3. Radioactive decay of isotopes
4. Age of surrounding rock
CSIR – JUNE 2003

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