What Are Fossils?
A fossil is an impression or the preserved remains of a once-living organism. Typically, the most common type of fossils is bones. Fossils can also be formed from shells, exoskeleton, hairs, skin imprints, and even petrified wood. Interestingly, animal footprints, tracks, trails and even animal burrows could become fossilized. However, not all fragments of bone qualify as a fossil; only if a specimen is older than 10,000 years, it qualifies as a fossil.
Fossils are important from an evolutionary perspective as they reveal important details and insights about an organism and how it lived. Animals like the dinosaurs died out millions of years ago and one of the best ways to study their anatomy is through their fossil records. Some animals, such as jellyfish do not have any hard parts such as bones, thereby fossils of these organisms are extremely rare. However, the bodies of these organisms do fossilize, given the right environmental conditions. Such fossils are called “soft-fossils”, and leave an imprint of their bodies on the rock.
How Do Fossils Form Layer By Layer?
Fossils can form in one of the five ways mentioned below:-

  • Natural Cast
  • Amber Fossils
  • Preserved Remains
  • Trace Fossils
  • Permineralization

Natural Cast / Mould Fossils
As the name implies, a natural cast is the “cast” of the organism. This process happens when flowing water eventually removes the organism’s bones and tissues and leaving just an impression in the sediment. Minerals eventually fill in the void left by the bones and tissues, thereby recreating the organism’s original shape. Shells of extinct marine invertebrates are the most common type of mould fossils.
Amber Fossils
Most trees have resins that exude from its barks. The process of fossilization happens when an organism – such as insects or even small animals like frogs and lizards are trapped in the resin. Eventually, the resin hardens and turns to amber when the tree is buried underground. Besides insects, the pollen and seeds can also become fossils if trapped in resin.
Preserved Remains
Preserved remains are the rarest forms of fossil remains. Specimens often have muscle, internal organs and even skin preserved. These type of fossils are formed when the organism dies and gets buried in a peat bog. Even being buried under volcanic ash can also have the same effect. Bog bodies, which are mummified human remains, have been found in peat bogs. They show exceptional anatomical details, including the probable cause of death, even though the bodies are thousands of years old.
Trace Fossils
Trace fossils usually don’t include the organism itself, but its activities – such as footprints, animal excrement, burrows and nests.
Most fossils are formed through permineralization. After the organism dies, its cellular spaces get filled with dissolved minerals which are carried by groundwater. The minerals eventually crystalize and harden into rocks that have the shape of the organism. Shell, bones, teeth and wood are the most common types of fossils formed this way.

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