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Index Fossil

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Introduction to Index Fossil

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Index fossils are the fossils that are commonly found as well as widely distributed and are the fossils limited in the period. These are helpful in the determination of the age of organic rocks as well as other fossil assemblages. Not only this, but it also helps in establishing the relationships between the rock units and is also called guide fossils or indicator fossils. In this article, we will cover index fossils, index fossil definition & meaning along with index fossil examples and other related aspects which will be helpful to grasp this concept.

Fossils are the remains of ancient living things like plants and animals preserved from the prehistoric period by the prospect of natural conditions. It is generally a rock in which organisms are preserved. It is found in sedimentary rocks, asphalt deposits, coal, amber and other materials. The study of fossils is called palaeontology. The bones, shells, leaves and feathers become fossils. It can be large or small accordingly. 

Index Fossil Meaning

These are those fossils that are useful for dating as well as correlating the different strata where it's found. Some species lived in a particular space and environment and animal/plant life is preserved in rocks of the Earth that are a feature of a particular reach of geologic time or environment. These fossils are also known as guide fossils and indicator fossils which means those fossils which are used to define as well as signify geological stages. They are used by geological scientists. They are important aids to determine the correlation as well as the age of rock patterns. Geologists use large fossils to microscopic fossils to identify geologic time.


They use both large fossils/macrofossils as well as microscopic fossils or microfossils as indices to define as well as identify geologic periods.


If we talk about macrofossils, they have the advantage over the microfossils because they are easy to see and find in the field but actually, they are rare whereas the microfossils are used by mineral resources industries after extraction of required information from these fossils.

Index Fossil Definition

Few definitions of index fossils are mentioned below:

  • "It is a fossil usually with a narrow time range and wide spatial distribution that is used in the identification of related geologic formations." - Merriam-Webster

  • "It is a fossil that is found in many places and comes from the same time in history so that scientists can use it to decide how old certain rocks are." - Cambridge dictionary.

  • " It is a widely distributed fossil, of the narrow range in time, regarded as characteristic of a given geological formation, used especially in determining the age of related formations." ( Geology, Paleontology ) - dictionary 

  • " Those fossils of the organisms that are helpful in identifying the geological time of the rocks or layers of rocks in which they were found. " - General Definition.

Types of Index fossils

There are some types of index fossils which include Ammonites, Brachiopods, Graptolites, Nanofossils and Trilobites. 

  • Ammonites: The fossil of ancient marine animals is said to be known as an ammonite index fossil. During the Mesozoic era, they were common (245 to 65 million years ago ). After the Cretaceous period, they were not found as they went extinct during the K-T extinction ( 65 million years ago ).

  • Brachiopods: They emerged 550 million years ago. It was first founded in the Cambrian age.  These are mollusc-like marine animals) that appeared during the Cambrian (540 to 500 mya) some of them still survive. They are said to be the most common fossils.

  • Graptolites: They are thin and shiny. The markings on rock surfaces look like pencil marks. The name Graptolites comes from the Greek for "writing in the rocks." These are widespread colonial marine hemichordates. They lived during the period of Cambrian roughly from  540 to 505 million years ago) to the early to mid-Carboniferous (360 to 320 million years ago). It is found in shales and mudrocks. 

  • Nanofossils: These are microscopic fossils (the remains of calcareous nannoplankton, coccolithophores) from various eras. Because of their evolutionary rates, these are time-specific. Nanofossils are very plentiful and it is distributed widely in the areas. There are various numbers of beneficial nanofossils which include radiolarians & foraminifera. To date, marine sediments are the primary methods.

  • Trilobites: During the Paleozoic Era (540 to 245 mya); they were common. About half of the Paleozoic fossils are trilobites. At the beginning of the Paleozoic Era, they evolved whereas went extinct during the late Permian period i.e. 248 million years ago.

Merits

Various Merits of Index Fossils are Mentioned below:

  • These fossils are used by geologists as well as palaeontologists as significant aids in order to determine the correlation as well as the age of rock sequences or patterns

  • If we talk about microfossils, when required as well as accurate knowledge from the fossils is being collected, after that, these are generally used by oil prospectors along with other industries which are interested in mineral resources.

  • By usage of some physical or biological methods, stratigraphic correlations can also be made.

  • Besides the above points, lithologic similarity may also be ascertained between the various rock units by the usage of one or two marker beds.

  • It helped and played a vital role in establishing the Continental Drift Theory.

List of Index Fossils

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Index Fossil Example:  Some of the Common index fossils are given below along with their name, time they represent as well as images:

  • Name: Viviparus Glacialis

Time: Early Pleistocene 0.5 million

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  • Name: Pecten Gibbus

Time: Quaternary Period 1.8 million

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  • Name: Calyptraphorus velatus

Time: Tertiary Period

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  • Name: Scaphites Hippocrepis

Time: Cretaceous Period 145 to 66 million years ago

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  • Name: Perisphinctes Tiziani

Time: Jurassic Period

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Did you know?

Because of the evidence of the index fossils, Alfred Lothar Wegener was able to establish as well as present his " Continental Drift hypothesis " in 1912. According to this, all the continents of the world that we can see today have been originated from a single supercontinent i.e Pangea and breaking the Pangea into smaller pieces leads to drifting away of these pieces from each other around 200 mn years ago which leads to the formation of present continents. He found that fossil remains, as well as the structure of the rocks around the coastlines of South America, Africa and India, were similar to each other despite being separated by oceans such as the Atlantic and the Indian Ocean. Even fossils of tropical plants such as ferns, cycads etc are being found in the Arctic island, Spitsbergen. Thus, the presence of index fossils leads to the verification of the theory of Continental Drift.

Conclusion

To conclude, we can say that index fossils are considered as commonly found as well as widely spread & distributed fossils which are limited in the period. The architecture of geological time includes ages, epochs, periods, & eras, which index fossils helped to determine. They consist of remains of plants and animals along with other species in the layers of sedimentary rocks. If we talk about a good index fossil, two basic, as well as required features, are they should be abundant as well as vulnerable. This boom-and-bust character of fossils makes them a good fossil for geologists and palaeontologists. Here, we have covered the index fossil concept comprehensively along with index fossil example & index fossil definition, merits, etc. which will be helpful in Geography, Geology and Palaeontology, etc.

FAQ (Frequently Asked Questions)

Question1. What Do You Mean by Index Fossils?

Answer.  Index fossils are the fossils that are also known as a guide as well as indicator fossils that are found commonly and distributed widely and limited in time and any animal & plant that is preserved in the rock of the earth. It is used by geologists and palaeontologists. These are helpful in the determination of organic rocks and fossils assemblages as well as studying the time zones of the specific regions.

Question2. What are the Merits of Index Fossils?

Answer. These are used by palaeontologists & geologists to determine the correlation as well as the age of rocks and patterns. The fossils are collected to know the accurate knowledge about it. Stratigraphic correlation can also be made by the usage of physical & biological methods whereas lithologic similarity may also be ascertained between the various rock units by the usage of one or two marker beds. Besides these, these also helped and play a vital role in establishing the Continental Drift Theory.

Question3. What are the Characteristics of Index Fossils?

Answer.  An index fossil must be distinctive, easily acknowledgeable, liberal well as distributed widely and limited in a short period. It is the basis for defining boundaries and for the correlation of strata. They can be micro as well as macrofossils. Large fossils are easily identified but they are rare whereas microfossils can be used by other industries that are interested in resources. These should be abundant as well as vulnerable too.