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A fossil is a preserved remnant, impression, or trace of an animal or plant from a past geologic age by some natural process. Just a small proportion of extinct species have been preserved as fossils, and often only those with a solid skeleton are capable of being preserved. A calcareous skeleton or shell is found in a large number of major groups of invertebrate creatures (e.g., corals, mollusks, brachiopods, and bryozoans).  The organic tissues can be preserved in a shell or bone that is buried soon after deposition, they eventually get petrified (transformed into a stone substance). In sedimentary rocks, unaltered hard pieces, such as the shells of clams or brachiopods, are abundantly available.

What are Fossils?



An impression or the preserved remains of a once-living organism are called fossils. Bones are typically the most prevalent kind of fossil. Shells, exoskeletons, hairs, skin imprints, and even petrified wood can turn into fossils. Animal burrows, tracks, and even footprints can also become fossilized. The term “fossil” is typically used to describe the bones, teeth, or shells of extinct animals. Together with their bones, large portions of animal bones dried dead skin, and hair has also been found. These organisms have, in some way, escaped the bacterial action that destroys organisms made of carbon. Fossils, which make up around 90% of the animal, contain pieces that have been preserved, and they tell us a lot about past life. 

In terms of evolution, fossils are important because they provide crucial information about an organism’s life history. One of the best ways to understand the anatomy of extinct animals, such as the dinosaurs, is through their fossil records.

Types of Fossils

Fossils can be classified into different subgroups. Types of Fossils are:

Index Fossils

An Index Fossil is also known as a guide fossil. Fossils of any plant or animal preserved in the Earth’s rock record are used to determine a certain period of geologic time or habitat. An effective index fossil needs to be unique or easily identifiable, abundant, have a broad geographic distribution, and have a short time range. The best index fossils are widespread, common, and easy to recognize at the species level. The boundaries for the geologic time scale and the correlation of strata are determined by index fossils.

Trace Fossils

The majority of trace fossils are tracks and burrows, although they can also be coprolites (fossil excrement) and feeding imprints. Trace fossils are especially significant since they represent a data source that is not restricted to creatures with easily fossilized hard components and because they depict animal behavior. Trace fossils record an animal’s activities at a specific time and hence reveal the animal’s habitat. For instance, mudstone trace fossils of animal burrows indicate that the animal lived in a muddy environment.

Transitional Fossils

A transitional fossil is one that belongs to a species that possesses qualities from both its ancestor and its descendant species. These fossils are the remains of an organism that existed between the earliest known form of a species and the current species. Transitional species and fossils provide insight into an evolutionary process. As an example, it is commonly known that humans descended from primitive apes millions of years ago. Transitional fossils are said to depict intermediate forms of a species, which changed and accumulated adaptations slowly, making them evidence for evolution.


A microfossil is a tiny fossil that is often studied only under a microscope and can either be a whole tiny organism or a piece of a bigger organism. The tiny remains of bacteria, protists, fungi, animals, and plants are known as microfossils. Since they must be removed from rock samples in a specific way and viewed under a microscope, microfossils are a group of fossil remnants researched as a particular subject. The reason why microfossils are classified differently from other types of fossils is entirely due to their relatively small size and techniques of examination.

Amber Fossils 

Most trees release resin from their bark. When an organism, such as insects or even small animals like frogs and lizards, is caught in the resin, the process of fossilization occurs. When the tree is buried underground, the resin eventually becomes hard and transforms into amber. In addition to insects pollen and seeds can also turn into fossils if they are caught in resin. Inclusions are additional fossils found in fossil resin that were drawn into the resin’s sticky substrate. They include various types of animals, plants, fungi, and microbes. Animal inclusions are often tiny invertebrates, mostly arthropods like insects and spiders, and only a few are vertebrates like small lizards.

Body Fossils

Body fossils are those that are discovered on an organism’s hard body parts, such as its bones, claws, teeth, outer skin, or scales. The majority of fossils found globally are body fossils, which are created from the remnants of extinct animals and plants. The majority of fossilized body parts are composed of hard structures like teeth, bones, shells, or woody trunks, branches, and stems. Body fossils come in a variety of sizes, from tiny organisms that can barely be seen under a microscope to gigantic dinosaurs.

Molecular Fossils 

A molecular fossil is simply preserved organic material that has mostly decomposed or been turned into fossils. Molecular fossil is extremely delicate  Because it is built on the same chemical structures that made up the organism. This type of fossil is extremely vulnerable to degradation as chemical processes break the bonds holding the molecule together.

Trace Fossils

The majority of trace fossils are tracks and burrows, although they can also be coprolites (fossil excrement) and feeding imprints. Trace fossils are especially significant since they represent a data source that is not restricted to creatures with easily fossilized hard components and because they depict animal behavior. Trace fossils record an animal’s activities at a specific time and hence reveal the animal’s habitat. For instance, mudstone trace fossils of animal burrows indicate that the animal lived in a muddy environment.

Carbon Fossils

Carbonized fossils are found commonly around coal seams. The conversion of anything into carbon is referred to as carbonization. these remains of organisms were exposed to high pressure over time, such as when they were buried beneath sediments on the seafloor. Compression is the term for the action of forcing two or more things together while also reducing their size and volume. A number of its constituents, including hydrogen, nitrogen, and oxygen, are finally drained away after the remains are compressed. This results in the formation of a thin coating of carbon residue or film on a two-dimensional rock.


Pseudo fossils are inorganic remnants, imprints, or markings that could be mistaken for real Fossils. Pseudofossils may be misleading because some mineral deposits have the ability to resemble living organisms by producing what appear to be complex or well-organized structures. Pseudofossils resemble fossils or other types of fossilized life, as they are created by inorganic (non-living) processes.

Fossilization Processes

Fossilization Process


A deceased organism typically begins to fossilize when it is buried before exterior deterioration takes place. The organism is covered in sand, or it may occasionally sink to the bottom of a swamp, lake, sea, or river. Upon burial, decomposition sets in and the body’s soft tissues are destroyed. The more durable parts develop into fossils. The organic remnants are safely stored and kept in good condition while water, sand, and mud solidify to rock.


The permineralization process is a type of fossilization that occurs when minerals carried by water fill in all the gaps in an organism or organic tissue. Mineral deposits create internal castings within these cavities. The organism must be covered by silt shortly after death for permineralization to occur; otherwise, scavengers or decomposition will consume the remains. When water transporting minerals from the earth, lakes, or oceans seep into the cells of organic tissue, a cast of crystals eventually forms. Permineralization is helpful while examining an organism’s internal systems, particularly in plants like the ancient Redwoods at Florissant Fossil Beds. Lahars or volcanic mudflows buried the ancient Redwoods in this area, which caused silica from the volcanic ash to largely precipitate inside the Redwoods’ cells.

Casts and molds

the original organism’s remnants fully vanish or are destroyed in some other way. External mold is the term for the residual organism-shaped hole in the rock. The resulting cast resembles the organism if the void is later filled with sediment. Endocast or internal mold occurs when the sediments fill an organism’s interior, such as the hollow of a skull or the interior of a bivalve or snail.

Authigenic mineralization

This is a unique method of creating molds and casts. If the chemistry is appropriate, the organism (or part of the organism) may function as a nucleus for the precipitation of minerals like siderite, leading to the formation of a nodule surrounding it. Very fine three-dimensional morphological information can be preserved if this occurs quickly before the organic tissue experiences severe degradation. 


Recrystallization is a process through which the minerals that made up the original shell or bone of a fossil transform into a separate material made of the same chemical elements. Fossilized shells made of aragonite commonly recrystallize into calcite, a more stable variety of the same material. Recrystallization takes place when another mineral replaces the shell, bone, or other tissue. In exceptional cases, the original shell is gradually replaced by minerals at such tiny scales that microstructural features are preserved despite the total loss of the original material.


Compression fossils, like those of fossil ferns, are produced when the intricate organic molecules that make up an organism’s tissues are chemically reduced. The fossil in this case is made of the original material, although this has undergone geochemical modification. This chemistry change is an indication of diagenesis. Compressions and impressions frequently take place in combination. For example, the phytoleim will frequently be attached to one part of the rock when it is broken open (compression), whereas the counterpart will only be an impression.

Soft tissue and molecular preservation

Cells and soft tissues should have decomposed after millions of years, and the preservation of cells and soft tissues after millions of years, in some dinosaur bones appears to be inconsistent. Cells and soft tissues within bone are often protected from total deterioration. A mineral concretion that forms around a bone protects the proteins inside from groundwater hydrolysis. A bone’s cells are protected from autolysis by iron infusion and coating during an important phase of the deterioration process. Collagen fibers in a bone are further protected by cross-linking and interaction with bone mineral surfaces. These protective elements have the potential to preserve soft tissue for millions of years.

Calcification and carbonization 

When an organism is carbonized or coalified, its remains are essentially converted to the chemical element carbon. A thin film that represents the original creature in a carbonized fossil is called a silhouette, and the original organic remains were typically soft tissues. Coal makes up the majority of coalified fossils, and the original organic material was mainly composed of wood.


Bioimmuration is generally defined as fossilization as a result of organic overgrowth which can preserve soft-bodied animals and soft sections of organisms with mineralized skeletons. Other creatures that are struggling for available living space frequently overgrow sessile species linked to hard or rigid substrates. On its underside, it may have a high fidelity (sub-micron size) impression of the overgrowing organism if it has a mineralized skeleton that is likely to become fossilized. The bioimmured organism may occasionally have a soft body and be retained in negative relief as a type of exterior mold. There are instances where a live, skeletal organism climbs upward as an organism settles on top of it, preserving the settler in its skeleton.

Fossil Record

When fossils are found, they are examined as the remains of a former life, their position in historical time is determined, and fossils from different parts of the world are compared. Then and only could a fossil record be created. This fossil record proves that many types of prehistoric species had highly distinct forms from any of the organisms currently living. Also, it displays the succession of species across time, demonstrating their change from one form to another.

The best way to display some characteristics of fossil data is in a table. The most recent fossils are found in the higher strata of the earth, while the oldest and most ancient ones are found near the bottom. Several thousands of fossils have been found, yet they only represent a tiny portion of the total number of creatures and plants that once walked the earth.

What is Taphonomy?

Taphonomy is the branch of paleontology that studies the processes involved in fossilization. This includes everything that occurs to an organism’s remains after it dies, up until the point where a geoscientist observes or collects them. It may include Soft tissue degeneration, the separation of skeletally rigid components, and alterations in the organism’s chemical composition following the burial. We can learn more about the environment in which the organism lived, died, and was preserved by studying these processes.

Taphonomic phenomena are divided into two phases: diagenesis, which takes place after the burial, and biostratinomy, which occurs between the organism’s death and its burial. Taphonomy has evolved to include the fossilization of organic and inorganic materials as a result of social and environmental factors.

Importance of Fossils 

Layer by layer, the earth’s crust creates fossils. The previous species of plants and animals are buried beneath the layer that is visible on the surface. Deeper fossils have been found to have more superficial structures than those found in the top layer. We have been able to understand the stages of evolution due to the complete fossil records of animals like horses, camels, and humans.

FAQs on Fossils

Question 1: Why studying fossils is important?


By studying fossils, scientists get behavioral information about the species. Common ancestor, connecting link and lineage of the species information get by studying the fossils.

Question 2: What are the different types of fossils?


The different types of fossils are:

  • Body Fossils
  • Molds and Casts
  • Petrification and Permineralization Fossils
  • Trackways and Footprints
  • Fossilized Feces

Question 3: How are the fossils formed?


Fossils are the dead remains of ancient plants and animals. When dead organisms are not fully decomposed, fossils are created. The organisms may become stuck in tree resin, volcanic lava, or hot mud, which, when it dries, holds the animal parts and creates fossils.

Question 4: What are trace fossils?


A trace fossil is reflected behaviour by the organism and serves as indirect proof of ancient life (excluding bodily components). Trace fossils include things like footprints, trails, gnawings, borings, burrows, eggs, nests, gizzard stones, and dung.

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Last Updated : 27 Mar, 2023
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