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Consortium for
Educational
Communication
Module on
FOSSILS AND
FOSSILIZATION
By
Dr. Rouf Hamza
Booda
Assistant Professor,
Department of Botany
Government Girls Hr. Sec.Shcool
Anantnagh
Contact No. 9018862744
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Text
Fossils are the mineralized or otherwise preserved remains
or traces of animals, plants, and other organisms. The totality
of fossils and their placement in fossiliferous (fossil-containing)
rock formations and sedimentary layers (strata) is known as the
fossil record. The earliest fossils are about 3100 billion years
old. They are of simple prokaryotic organisms like bacteria and
cyanobacteria. Earlier fossils were considered products of stars
and thunderbolts but Leonardo da Vinci and Bernard Palissy were
the first to report that they are the past organisms which lived on
the earth. 400-million-year-old fossil plants are the oldest known
examples of wood.
WHICH THING SHOULD BE REGARDED AS FOSSIL
Fossil definition needs careful consideration. The term “fossil”
is used for any trace of past life. Fossils are not only the actual
remains of organisms, such as teeth, bones, shell, and leaves (body
fossils), but also the results of their activity, such as burrows and
foot prints (trace fossils), and organic compounds they produce by
biochemical processes (chemical fossils). The remains of plants,
as in coal or lignite, are usually found in forms of carbon. The
remains of animals usually consist of the hard parts of bones,
teeth or shells. Traces are such things as footprints, coprolites (i.e.
fossil dung or faeces), dwellings, burrows and eggs. Conditions
different from those of the present normally excludes an organism
which has lived in the last 1000 years or so. The mummy of an
ancient Egyptian or Inca, or the skeleton of a bronze age donkey
would not be regarded as a fossil. Another defnition of fossils is
that they are the remains of animals and plants which lived prior to
historical times i.e prehistory. Their study is a specialised branch
of geology known as Palaeontology. Schof (1975) defined fossil
as any specimen that demonstrated the physical evidence of the
occurence of ancient life i,e holocene or older may be regarded
as a fossil. Sometimes a demarcation is made between a fossil
and a subfossil. Fossil is regarded as an older specimen which
has been fossilized before the end of Pleistone epoch; they are
completely mineralized and thus changed into fossil. Subfossil is a
recent dead body of an organism less than 6000 years old, and is
less mineralized. Steward’s definition of fossil as any evidence of
prehistoric life seems to be exclusive.
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Some fossil related terms are also common, viz. Pseudo fossils,
living fossils and index fossils.
Index fossils: These fossils are used in dating of rocks, or act as
indicators of petroleum and coal.
Living fossils: It is an organism which shows long geographical
record and is still living in a limited area. It is not a fossil but it
is the oldest representative of a fossil group of organisms which
dominated the earth in past. Living fossil is a term used for any
living species that closely resembles a species known from fossils.
It is as if the fossil had “come to life.” Best example is furnished
by Ginkgo biloba.
Pseudo fossils: Theses are actually non living things, like rocks
which give an illusion of fossils of plant or animals.
WHERE TO HUNT FOR FOSSILS: Lagerstatten are the world’s
best fossils sites. Fossils are found in rocks. Fossils are found in
sedimentary rocks - mainly lime stones, siltstones and mudstones
- that were deposited in former lakes, rivers, estuaries, seas and
caves. Sedimentary rocks are formed as a result of the deposition
of various kinds of sediments like gravel, sand and clay upon
the land surface or in the water bodies. These sediments are
finally converted into rocks after a long interval of time. Since
these rocks are formed in layers, living organisms may become
fossilized in them and are available to be dug out by chance. Some
of the common sedimentary rocks are lime stone, gypsum, shale,
diatomite, rock salt and coal.
The general process of fossilization:
Fossilization is the process of the formation of fossils, i,e conversion
of organisms or their parts into fossils.
How are fossils formed: Organisms pass through a number of
stages on their way to become fossils, the important stages are:
Death: Generally, this is the first stage in fossilization. Death can
occur as a result of burial and in this case biostratinomic processes
may not take place.
Biostratinomic
processes:
Processes
such
as
reorientation,
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disarticulation, fragmentation, and corrosion, which modify the
remains. Thus information is lost.
Deposition: Burial of the remains in sediment, where they are less
likely to be disturbed. Plant parts are best preserved in very fine
grained silts and shales, which are the lithified muds of ancient
deposits. Such sediments generally yield excellent fossils because
the small grain size preserves minute details of the leaf; coarsergrained sediments such as sands generally do not reproduce
delicate features. Plant parts composed of thick-walled cells or
those having fibers within their tissues have a better chance of
being preserved (fossilization potential).
Diagenetic processes: Chemical / physical changes, e.g.
permineralization and carbonization. These are necessary for
fossilization.
Optimum conditions for preservation
There are two major requirements for an organism to be preserved
as a fossil:
1. It must possess hard parts.
2. It must undergo rapid burial in a protective medium.
Various agents can be responsible for the preservation of organisms.
Commonly, sediments bury the remains and keep them from
being disturbed during the process of fossilization. More unusual
agents include lava or volcanic ash, and severe cold or aridity.
Temperatures below 4°C (39.2ºF) inhibit bacterial activity and
thus, decay of organic matter.
Firstly, the organism must be deposited in sediment. Between
the time of death and burial, biostratinomic processes alter the
remains. Before burial, an organic skeleton is normally subject to
reorientation, disarticulation, fragmentation, and corrosion. Once
it is buried, it undergoes diagenetic processes. Such changes are
necessary for preservation, because organic matter will not survive
for long before it is decomposed, and even hard parts, as bones,
teeth, calcified shells, Xylem, sclerenchyma, are normally prone to
destruction. A typical diagenetic process is mineralization, which
can occur with various minerals such as pyrite, phosphates or the
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various forms of silica.
Carbonization: Carbonization is a process occurring within the
sediment in the absence of oxygen. In such conditions, anaerobic
bacteria consume hydrogen and oxygen, therefore concentrating
carbon. The final products of carbonization are coal and
hydrocarbons, which can be considered accumulations of very
badly preserved fossils.
Permineralizations
Permineralization occurs when the plant tissues are infiltrated
with mineral-rich fluid. Minerals (commonly silica, carbonate,
phosphate or pyrite) precipitate in cell lumens and intercellular
spaces, thus preserving internal structures of plant parts in three
dimensions. When mineral matter actually replaces the cell-wall
and other internal structures, the preservation may be called
petrifaction. Following are important types of permineralization:
Silica permineralization (silification commonly occurs in areas
where silica-rich volcaniclastic sediments are weathering, for
example the famous upright trees in Yellowstone National Park.
According to Leo et.al (1976) concentrated watery solution of
minerals like salic acid infiltrates the tissues which is followed by
intracellular or intercellular polymerization of siloxane bonds and
the elimination of water. With the continous growth of polymer the
amorphous silica gets deposited from the solution and the water
moves out of the plant tissues. Ultimately this silica gets converted
into quartz and fossil gets petrified. Silica and especially its most
common crystallized form (the mineral quartz), is extremely
stable at the Earth’s surface, therefore silicified fossils are likely
to withstand modification or damage.
Permineralization with calcium carbonate (calcite or dolomite) is
particularly common in Carboniferous coal seams, where whole
regions of peat were permineralized. Called coal balls because of
their round or ellipsoidal shape
Carbonate preservation
Ideal conditions for carbonate preservation are normally found
in marine shelf sediments, with a high biomass of organisms
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that could potentially fossilize.
Marine waters are constantly
supersaturated with respect to calcium carbonate, and the
preservation of carbonate shells is a rather common instance.
Pyritization
Pyrites are iron-sulfur minerals. These pyritized fossils often
occur in the presence of sea water (a source of sulfur), and are
characteristic of plant tissues washed into marine basins.
Permineralization with phosphate: Phosphatization of soft parts
have an unsurpassed degree of preservation, with detailed
cellular structure. Muscle fibers are most often preserved through
phosphate replacement.
Charcoalification
Charcoalification can preserve tissues burned in wildfires or
buried in pyroclastic flows. Internal structure is often detailed.
Types of fossils and their fossilization:
There are six main types of fossils, based on the manner of
fossilization.
1. Coalified compressions: a fossil containing compressed
coalified remains of plants.
2. Authigenic preservations or cementation. Here surface
layer of the organism/part is preserved by cementation.
In Cementation compacted sediments are cemented by
cementing material, like silica or calcium to form sedimentary
rock. It shows only surface details of the fossilized organism.
Following types are its examples:
a). Impressions: Also called imprints are the external features
of the past organisms or their partsleft in hardned rock.
b). Moulds: These are encasements of hardened mud or
minerals that surround a fossilized organism.
c). Casts: They are hardened minerals deposited inside the
mould.
3. Infiltrated cellular preservations: Here cells and tissues of
plants or plant parts are preserved, it shows internal detail of the
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organism.
a). Petrifactions: Also called true fossils, because in
them cellular details are preserved due to replacement of organic
material particle by mineral matter.
b). Mummifications: In this type the entire organism is or
body part is preserved in some special preserving material so that
all its soft body parts are preserved perfectly and remain intact.
Thus organic matter has not been replaced by mineral matter in
such fossils,.e.g ice preserved fossils.
4. Hard part preservations: Here plants possessing hard parts
are preserved, e.g diatoms.
5. Structural fusinite: The coalified plants after charing in forest
fire form fossil charcoal, or fusain are called structural fusinites.
These fossils show celluar details though cell walls are generally
absent.
7. Chemical fossils: These include fossilized plant chemicals,
like chlorophyll, DNA and proteins.
1) Coalified compressions: It is a type of fossil that contains
compressed coalified remains of plants generally in the form of
black carbonaceous film in the rock. The compression retains the
original outline of the organism.
Compressions are plant parts that have suffered physical
deformation such that the three-dimensional plant part is
compressed to more-or-less two-dimensions. Compression mainly
occurs in plants, retaining coalified organic matter. Well fossilized
compressions of leaves shows leaf form, veins, stomata, epidermal
hairs, and cuticle. Coal is the best example of compression.
Fossilization: Plant part or plant gets buried in sediment layers
and under high pressure gets converted into coal. Generally
compressions retains their lower surface intact while as upper
surface gets distorted. Plant stems and other three-dimensional
plant structures do not preserve well under compression.
In carbonization, the decompostion of organic matter under
anaerobic water or sediment, the hydrogen, oxygen, and nitrogen
are driven off, leaving the carbon residue. This leaves a thin film
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layer of carbon on a rock as evidence of the original organism.
What are impressions
Impressions are two dimensional imprints of an organism that
donot contain any organic material. Impressions are essentially
compressions without organic material.
Impressions probably represent the most common type of plant fossil.
They are formed by dorsiventral or two dimensional plant structures
like leaves, and sporophylls. They occur when a plant part is covered
by sediment and the water is squeezed from the cells and tissues. Cells
that make up the plant are eventually degraded, perhaps as a result of
microbial activities, so that only a shallow negative, or imprint, of the
plant organ (leaf) remains. Impressions usually occur in fine-grained
sediments, such as clay or silt.
Impressions may also occur if, when layers of rock are split apart, the
organic material adheres to only one side of the rock. In this case, the
side with organic material is the compression, known as the “part”,
while the corresponding impression known as the “counterpart”.
What do impressions tell us:
The paleobotanist determines the shape, venation, and other
structural features of the leaf and notes any characteristics that
make it unusual. An impression is a clue left of the organism’s
biological activity; they can be formed while the organism was
moving or doing any other biological activity. Some examples
of impressions are the trace fossils such as footprints, traces,
remains of tunnels left by burrowing organisms, and the holes
made in shells by parasites
MODULE-6 MOLDS AND CASTS ARE INSIDE OUT FOSSILS OF
EACH OTHER
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Casts and Molds
When sediment is deposited into cavities left by the decay of
plant parts, a cast results. A mold is essentially a cavity left in
the sediment by the decayed plant tissue. Molds are generally
unfilled, or may be partially filled with sediment. In other words
after decay of organism when all that is left is an organism-shaped
hole in the rock, it is called a mold fossil or typolite. If this hole
subsequently had been filled with other minerals, it is called a cast
fossil and is considered a replacement fossil, since the original
materials had been completely replaced by new, unrelated ones.
Casts and molds may be found together with the cast filling the
mold.
Molds are formed when soft sediment surrounding the structure
lithifies or hardens before the structure decays.When the mold
fills in with sediment that subsequently hardens, a cast is formed.
Casts of an internal hollow structure like a pith cavity are also
common. Like compressions and impressions,casts and molds
record external (or sometimes internal) organ features well,
Unlike compressions/impressions, molds and casts often are truer
records of the original three-dimensional shape of the structure.
Fossilization of impression,cast and mould. After burial of the
organism in fine clay two processes occur viz decay of the
internal organic materialof the buried plant and cementation of
the surface sediment. Decaying material develops electric charge
which attracts colloidal particles having opposite charge from
the surrounding medium. Decaying material create a specific
geochemical situation around themselves. Sedimentary material
containing iron and carbonate surround the organism and become
cemented. This cemented layer preserves the surface features of
the organism. The Cast is formed by filling of mould with mineral
matter.
Petrifications: Petrifications means turning to stone
In these fossils tissues are fully or partially replaced or infiltrated
by mineral matter so that even cellular details are preserved.
These are regarded as best fossils for the study of the internal
organisation of the fossilized plant.
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Fossilization process: After burial of the plant in sediments
permeation of silicates,iron compounds and carbonates starts
between and inside the cells. According to Leo et.al(1976)
concentrated watery solution of minerals like salic acid infiltrates
the tissues which is followed by intracellular or intercellular
polymerization of siloxane bonds and the elimination of water.
With the continous growth of polymer the amorphous silica gets
deposited from the solution and the water moves out of the plant
tissues. Ultimately this silica gets converted into quartz and fossil
gets petrified.
Mummification: These represent actual remains of an organism,
which remain fossilized in ice,volcanic ash or old bogs. They are
of the following types.
a)
Ice embeded fossils: Fossils of mammoths and rhinoceros
have been found in the Siberian ice. They belonged to late
Pelistocene.Their flesh,skin and hair was intact.
b)
Preservation in oil saturated environment: Organisms
fossilized in oil saturated places preserve their organic
parts. Tree trunks found in such places are so well
preserved that it is possible to cut them with saw and
drive nails.
Resin embeded fossils: Amber is a fossilized resin which can
preserve insects and pollen grains. Fossilized pollen or spores are
called micro fossils or palynofossils. Fossilized insects of Oligocene
period have been found preserved in amber, thus amber acts as
a nature’s tomb. Amber forms when plant resin is exposed to
the air, causing its constituent molecules (terpenes) to cross-link
and harden. The substance is very resistant to decay, with the
oldest amber found dating to the Late Carboniferous (290 Ma).
Amber younger than 40 000 years is termed sub-fossil resin.
This is highly useful if the amber has plants or animals trapped
inside. Organisms are 3-dimensional with excellent morphological
preservation, but their internal structure is dissolved.
c)
Bog
ple
and
and
people: Peat bogs have yielded the famous Bog Peotwo thousand year old humans with intact skin, hair
clothing due to the presence of anti-bacterial tannic
fulvic acids. Because of the low pH, their bones have
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been dissolved.
Hard part preservations: Some resistant plant material like
silica and calcium compounds are immune to physiochochemical
changes, thus serving in fossilization of the organism. e.g Diatoms
containing siliceous and calcareous frustules are the best fossilized
plants.
Structural fusinite: The coalified plants after charing in forest fire
to form fossil charcoal, or fusain are called structural fusinites.
These fossils show celluar details though cell walls are generally
absent.
Molecular Fossils (Chemical fossils):
As more becomes known about the chemistry of modern
plants, paleobotanists have begun to examine the fossil record
for corresponding chemical data. For example, characteristic
breakdown products of chlorophylls and lignins have been found
in well-preserved fossil leaves. Lipids and their derivatives have
also been recovered from sediments. Some carbohydrate breakdown products may also survive in sediment. A special class of
these, oleananes, are formed by flowering plants, some ferns
and lichens. An increase in abundance of these molecules in
sediments of mid to Late Cretaceous age is used to document the
increasing abundance of flowering plants. In another stunning
example, genetic material was recovered from Tertiary leaves.