What Are Fossils?

A fossil is defined as the naturally preserved remains and traces of either animals or plants that lived in the geologic past. Fossils can be of two types: body and trace. Body fossils are used as evidence to show that the organisms lived there while the trace fossils are used as a piece of evidence that the organisms were present such as their trails, footprints, etc.

These fossils can be further classified into four types of fossils, which are:

  • Mold fossils: These are the fossilized impressions made on the substrate.
  • Cast fossils: These are obtained when a mold is filled in. 
  • Trace fossils: Footprints, fossilized nests are the examples of trace fossils, which indicate that the organism was present in that area.
  • True form fossils: When fossils are formed from the actual animal and plant part.

How are fossils formed?

All living organisms are capable of leaving fossils but only the hard parts of plants and animals fossilize. Soft organisms such as jellyfish and worms have no fossil records as their preservation is difficult. But the hard parts of the organisms such as bones and shells can be preserved. 

Fossil formation happens when the plant or animals die in a watery environment and gets buried in mud and silt. As time passes, sediments are built over the top and get hardened into rocks. 

The other way in which fossil may form is when bugs get trapped in the tree sap, which then turns into a semiprecious material known as amber. Volcanic eruptions can also form fossils when the animals get trapped in the hot ash.

So, it is clear that for an organism to turn into a fossil, it must die where there is water and must be covered under mud and silt. When these fossils are found, they need to be preserved. Which brings us to the preservation of fossils. 

Types of fossil preservation

Preservation can be done in two ways and they are:

  • With alteration which includes petrification, carbonization, replacement, and recrystallization.
  • Without alteration includes a collection of indirect evidence and the use of molds. 

Preservation with alteration

  • Carbonization: This method of fossil preservation is used for plants and soft organisms. 
  • Petrifaction: This method is also known as permineralization and is used for preserving porous materials such as bone or shell which is made of calcium carbonate or silica. 
  • Recrystallization: The process in which a small molecule is crystallized inside a shell that is formed from one type of calcium carbonate into another type. 
  • Replacement: This process is used in shellfish and wood. Here, every cell is replaced by a new chemical structure. Silification is the most common type of replacement.

Interested to learn more about other concepts such as total internal reflection, stay tuned with BYJU’S.


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