What are Fossils? Meaning & Layer by Layer Formation

A fossil is the preserved impression or remains of a once-living organism. Bones are the most common type of fossil. Shells, exoskeletons, hairs, skin imprints, and even petrified wood can all be used to create fossils. Animal footprints, tracks, trails, and even animal burrows can become fossilised. However, not all bone fragments qualify as fossils; only specimens older than 10,000 years qualify as fossils.

Keyterms: Fossil, Shells, Exoskeletons, Hairs, Skin imprints, Wood, Silicon dioxide, Skeleton, Calcium phosphate, Bone

Read More: Heredity and Evolution

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About Fossils

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• Only a small percentage of ancient organisms can be preserved as fossils, and usually, only organisms with a solid skeleton are easily preserved. The majority of invertebrate animal groups have a calcareous shell or skeleton(for example, molluscs, corals, bryozoans, brachiopods, and more).
• The other forms have shells of calcium phosphate (which occurs in vertebrate bones) or silicon dioxide. 
• These organic tissues can be retained in a bone or shell that is quickly buried after deposition, though they become petrified (converted to a stony substance) over time.
• Hard bits that have not been altered, such as brachiopods or clamshells, are relatively abundant in sedimentary rocks, some of which are very old.

Also Read: Embroyology

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How Do Fossils Form?

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Fossils can be formed in one or five of the ways as is written below:

• Natural Cast

A natural cast is the organism's "cast," as the name implies. This occurs when the organism's bones and tissues are eventually removed by flowing water, leaving only an impression in the sediment. Minerals eventually fill the void left by bones and tissues, restoring the organism's original shape. The most common type of mold fossil is the shells of extinct marine invertebrates.

• Amber Fossils

Resins exude from the barks of most trees. When an organism, such as insects or small animals like frogs and lizards, becomes trapped in resin, the process of fossilisation occurs. When the tree is buried underground, the resin hardens and turns to amber. Pollen and seeds, in addition to insects, can become fossils if trapped in resin.

• Preserved Remains

The rarest type of fossil remains are preserved remains. Muscle, internal organs, and even skin are frequently preserved in specimens. These fossils form when an organism dies and is buried in a peat bog. Being buried beneath volcanic ash can have the same effect. 

Bog bodies, or mummified human remains, have been discovered in peat bogs. Even though the bodies are thousands of years old, they exhibit exceptional anatomical details, including the probable cause of death.

• Trace Fossils

Trace fossils are made up of the activities of the organisms, such as footprints, animal excrement, burrows, and nests, rather than the organism itself.

• Permineralization

Permineralization is the process by which most fossils are formed. When an organism dies, the cellular spaces are filled with dissolved minerals carried by groundwater. 

The minerals eventually crystallise and harden into rocks that resemble the organism's shape. The most common types of fossils formed this way are shells, bones, teeth, and wood.

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Parts Of Fossils

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• The soft parts of plants and animals are rarely preserved. The preservation of Pleistocene mammoth carcasses in ice and the embedding of insects in amber (a process known as resin fossilisation) are unusual, but they are striking examples of soft tissue fossil preservation.
• Carbonization can preserve stems, leaves, and other vegetable matter by flattening such parts between the two layers of rock.
• The chemical removal of the component results in a carbon film on one layer of rock, while the same component affects the other layer of rock.
• Microfossils are fossils of both soft and hard parts that are too small to be seen with the naked eye. A few fossils are completely devoid of animal and plant parts, but they do represent evidence of organism activity. Traces left by such organisms, dubbed "trace fossils," include trails or tracks, borings, and preserved waste products.
• Because terrestrial remains are quickly destroyed, the vast majority of fossils may be kept in an aquatic environment. 
• Anaerobic conditions, such as those found at the bottom of seas or other bodies of water, are especially favourable for conserving minute features since no bottom fauna, other than anaerobic bacteria, is present to destroy the remains.
• In general, for an organism to be preserved, two conditions must be met: possession of hard portions that can be fossilised; and fast burial to avoid decomposition and scavenging.
• We may see a huge thickness of almost horizontal strata depicting the sediment deposition on the seafloor over hundreds of millions of years in some areas, such as the Grand Canyon in Northern Arizona.
• It is frequently obvious that each layer in such a succession comprises fossils that are unique from the layers below and above it. 
• Faunas or fossil floras occur in the same order in such layer sequences in various geographic regions, whether comparable or the same. It may be able to compile a continuous record of floras and faunas that have progressively more in common with modern life forms as the sequence's top is reached by comparing such overlapping sequences.

Read More: Evidence For Evolution

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Things to Remember

• A fossil is the preserved impression or remains of a once-living organism.
• Only a small percentage of ancient organisms can be preserved as fossils, and usually, only organisms with a solid skeleton are easily preserved.
• The majority of invertebrate animal groups have a calcareous shell or skeleton(for example, molluscs, corals, bryozoans, brachiopods, and more).
• A natural cast is the organism's "cast," as the name implies.
• Resins exude from the barks of most trees.
• When an organism, such as insects or small animals like frogs and lizards, becomes trapped in resin, the process of fossilisation occurs.
• The soft parts of plants and animals are rarely preserved.
• The preservation of Pleistocene mammoth carcasses in ice and the embedding of insects in amber (a process known as resin fossilisation) are unusual, but they are striking examples of soft tissue fossil preservation.

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Previous Year Questions

• Evolution of different species in a given area starting from a point and spreading to other geographical areas is called...[JKCET 2015]
• Vestigial organs present in an adult individual are example of basis of evidence of evolution...[JKCET 2007]
• The theory of random genetic drift was proposed by...[JKCET 2007]
• Biogenetic law as given by Haeckel states that….[AMUEEE 2012]
• This kind of change in the gene pool is called a...[JKCET 2012]
• Transformation of the early reducing atmosphere of the earth into an oxidizing atmosphere was mainly due to the activities of...[KCET 2008]
• The brain capacity of Homo habilis...[KCET 2019]
• According to Darwin, evolution is_______...[KCET 2010]
• By the statement "Survival of the Fittest" Darwin meant that...............[KCET 2011]
• The idea of Natural Selection as the fundamental process of evolutionary changes was reached….[KCET 2007]
• Coarcervates were experimentally produced by,…..[KCET 2004]
• Find the mis-match from the following pairs :….[KCET 2016]
• Which among the following was the biggest land dinosaur?...[KCET 2020]
• A good example for recapitulation theory is/are​
• Which one of the following organs are homologous?​
• An experiment to prove that organic compounds were the basis of life was performed by:
• Example of analogous organs is:​
• Which one is not a vestigial organ in man?​
• Age of fishes is also known as...[JKCET 2011]
• Mammal like reptiles evolved during….[AMUEEE 2012]