Paper: Evolutionary Biology B.Sc. Part II
Fossils and Fossilization
Fossils are the remains and traces of ancient plants and animals. Fossils are formed
when ancient plant and animal remains become hardened and fixed within sedimentary
rock or sometimes volcanic ash. Some of the most interesting fossils are formed when
huge tree stumps, perfectly formed pollen grains, shells, or the bones of ancient fishes
(see Fig. 8.2), reptiles or mammals, become saturated and hardened with minerals such
as silica or calcite, and in this way are “transformed into stone”. In general the type of
material in which the remains are buried usually depends on where the organisms lived
e.g. the bones and shells of marine animals are common as fossils because they fell on
the ocean floor after death. Under such conditions they were covered by soft mud which
later hardened to shales and limestone of later geologic time. The soft mud are less
likely to damage the organism (organic remains). Certain fine-grained rocks e.g.
limestone have faithfully preserved delicate specimens as birds, insects and jelly fishes.
George Curvier (1769-1832) is considered ‘Father of paleontology’, who studied fossils
scientifically to develop phylogenies.
Types of fossils:
Based on the mode of formation of fossils, they can be categorized in several types.
Fossilization is a rare phenomenon, which takes place under specialized conditions.
The study of natural process of death, burial, decomposition, preservation and
transformation into fossil is called taphonomy. Fossils are the only direct evidence of the
biological events in the history of earth and hence important in the understanding and
construction of the evolutionary history of
different groups of animals and plants.
1. Petrifaction: Petrifaction is molecule-by-molecule replacement of organic matter by
inorganic compounds, viz. silica, calcium carbonate or iron pyrites. It literally means
“turned into stone” and takes place in buried situations, particularly at the bottom of
lakes, ponds or sea, where there are sediments rich in calcium carbonate and silica.
Over millions of years, inorganic matter replaces the entire bony material, making an
exact replica of the original. By this time sediments transform into sedimentary rocks, in
which fossils can remain preserved for a long time. Most of the old fossils are petrified,
e.g. shells of mollusks, arthropods and fish skeletons.
2. Molds and Casts: A mold forms when hard parts of an organism are buried in the
sediment such as sand, silt or clay. The hard part completely dissolves overtime,
leaving behind a hollow area of organism shape.
A cast forms as a result of the mold. Water with dissolved minerals and sediments fills
the mold’s empty space or cavity. The cavity is known as incrustation and the mineral
sediments that are left in the mold make a cast. A cast is opposite to its mold. These
fossils are suitable for the study of the morphology of fossil plants.
, Paper: Evolutionary Biology B.Sc. Part II
3. Preservation of footprints: When animals walk on wet soil and sand, they leave trail
of footprints or limbless animals and worms may leave tracks and trails in mud. If these
footprints are covered by volcanic ash, they can be preserved for a long time as the clay
containing footprints and the volcanic ash covering it will harden to form different types
of rocks. Mary Leakey discovered footprints of prehistoric man along with those of
giraffes, elephants, guinea fowls etc. in Kenya.
4. Trace Fossils/ Ichnofossils : Ichnofossils/ Trace fossils are marks left by an
animal or plant that has made an impression. These fossils include nests, burrows,
footprints or any other markings of the animal’s time on the earth. The structure of the
animal or plant remains as a mineral form. The colors of the minerals that replace the
form can be dazzling. Sometimes they are made into art and jewellery.
5. Amber: fossilized resin of more than 20 million years old. The intermediate state of
amber is called copal (less than 20 million years) old. The resin, before
becoming amber can trap insects, arachnids, pollen… in this case is considered a
double fossil.
6. Carbon Fossils: All living things contain an element i.e. carbon. When an organism
dies and is buried in sediment, the materials that make the organism break down and
eventually only the carbon remains. The thin layer of carbon left behind can show an
organism’s delicate parts like leaves or plant e.g. fern fossil 300 million years old.
7. Chemical fossils: are fossil fuels like oil and coal, which are formed by the
accumulation of organic matter at high pressures and temperatures along with the
action of anaerobic bacteria (bacteria that don’t use oxygen for metabolism).
8. Pseudofossils: Sometimes watery solutions of various minerals speed through the
sediments and it takes the shape of some plant part or animal. Their study shows that
they are neither plants nor animals. Such fossils are called pseudofossils.
9. Impressions: Impressions of body parts, skin, feathers, leaves etc. are formed when
they are
pressed hard against the soft clay, which subsequently hardens to form rock. Fossil of
Archaeopteryx is such an impression. More bird fossils in the form of impressions have
been discovered in China recently, e.g. fossils of Sinosauropteryx, Caudipteryx and
Confusiusornis.
10. Living fossils: name given to today’s living organisms very similar to species
extinct. The most famous case is the coelacanth, it was believed extinct for 65 million
years until it was rediscovered in 1938, but there are other examples such as nautilus.
Index fossils: Fossils that are found in undisturbed sedimentary rocks and in short
geological time period generally lie in recognizable strata of older rocks below and
Fossils and Fossilization
Fossils are the remains and traces of ancient plants and animals. Fossils are formed
when ancient plant and animal remains become hardened and fixed within sedimentary
rock or sometimes volcanic ash. Some of the most interesting fossils are formed when
huge tree stumps, perfectly formed pollen grains, shells, or the bones of ancient fishes
(see Fig. 8.2), reptiles or mammals, become saturated and hardened with minerals such
as silica or calcite, and in this way are “transformed into stone”. In general the type of
material in which the remains are buried usually depends on where the organisms lived
e.g. the bones and shells of marine animals are common as fossils because they fell on
the ocean floor after death. Under such conditions they were covered by soft mud which
later hardened to shales and limestone of later geologic time. The soft mud are less
likely to damage the organism (organic remains). Certain fine-grained rocks e.g.
limestone have faithfully preserved delicate specimens as birds, insects and jelly fishes.
George Curvier (1769-1832) is considered ‘Father of paleontology’, who studied fossils
scientifically to develop phylogenies.
Types of fossils:
Based on the mode of formation of fossils, they can be categorized in several types.
Fossilization is a rare phenomenon, which takes place under specialized conditions.
The study of natural process of death, burial, decomposition, preservation and
transformation into fossil is called taphonomy. Fossils are the only direct evidence of the
biological events in the history of earth and hence important in the understanding and
construction of the evolutionary history of
different groups of animals and plants.
1. Petrifaction: Petrifaction is molecule-by-molecule replacement of organic matter by
inorganic compounds, viz. silica, calcium carbonate or iron pyrites. It literally means
“turned into stone” and takes place in buried situations, particularly at the bottom of
lakes, ponds or sea, where there are sediments rich in calcium carbonate and silica.
Over millions of years, inorganic matter replaces the entire bony material, making an
exact replica of the original. By this time sediments transform into sedimentary rocks, in
which fossils can remain preserved for a long time. Most of the old fossils are petrified,
e.g. shells of mollusks, arthropods and fish skeletons.
2. Molds and Casts: A mold forms when hard parts of an organism are buried in the
sediment such as sand, silt or clay. The hard part completely dissolves overtime,
leaving behind a hollow area of organism shape.
A cast forms as a result of the mold. Water with dissolved minerals and sediments fills
the mold’s empty space or cavity. The cavity is known as incrustation and the mineral
sediments that are left in the mold make a cast. A cast is opposite to its mold. These
fossils are suitable for the study of the morphology of fossil plants.
, Paper: Evolutionary Biology B.Sc. Part II
3. Preservation of footprints: When animals walk on wet soil and sand, they leave trail
of footprints or limbless animals and worms may leave tracks and trails in mud. If these
footprints are covered by volcanic ash, they can be preserved for a long time as the clay
containing footprints and the volcanic ash covering it will harden to form different types
of rocks. Mary Leakey discovered footprints of prehistoric man along with those of
giraffes, elephants, guinea fowls etc. in Kenya.
4. Trace Fossils/ Ichnofossils : Ichnofossils/ Trace fossils are marks left by an
animal or plant that has made an impression. These fossils include nests, burrows,
footprints or any other markings of the animal’s time on the earth. The structure of the
animal or plant remains as a mineral form. The colors of the minerals that replace the
form can be dazzling. Sometimes they are made into art and jewellery.
5. Amber: fossilized resin of more than 20 million years old. The intermediate state of
amber is called copal (less than 20 million years) old. The resin, before
becoming amber can trap insects, arachnids, pollen… in this case is considered a
double fossil.
6. Carbon Fossils: All living things contain an element i.e. carbon. When an organism
dies and is buried in sediment, the materials that make the organism break down and
eventually only the carbon remains. The thin layer of carbon left behind can show an
organism’s delicate parts like leaves or plant e.g. fern fossil 300 million years old.
7. Chemical fossils: are fossil fuels like oil and coal, which are formed by the
accumulation of organic matter at high pressures and temperatures along with the
action of anaerobic bacteria (bacteria that don’t use oxygen for metabolism).
8. Pseudofossils: Sometimes watery solutions of various minerals speed through the
sediments and it takes the shape of some plant part or animal. Their study shows that
they are neither plants nor animals. Such fossils are called pseudofossils.
9. Impressions: Impressions of body parts, skin, feathers, leaves etc. are formed when
they are
pressed hard against the soft clay, which subsequently hardens to form rock. Fossil of
Archaeopteryx is such an impression. More bird fossils in the form of impressions have
been discovered in China recently, e.g. fossils of Sinosauropteryx, Caudipteryx and
Confusiusornis.
10. Living fossils: name given to today’s living organisms very similar to species
extinct. The most famous case is the coelacanth, it was believed extinct for 65 million
years until it was rediscovered in 1938, but there are other examples such as nautilus.
Index fossils: Fossils that are found in undisturbed sedimentary rocks and in short
geological time period generally lie in recognizable strata of older rocks below and
