Fossils (Formation, Types and Importance)

What are fossils?

The term fossil comes from the Latin word fossilis, which literally means ‘dug up’.
Fossils are the remnants, impressions, or traces of animals, plants or any other living organisms that existed in the past geologic age, millions of years ago and have been preserved in Earth’s crust.
Fossils not only refer to the bones, shell, teeth or any hard structures of plants or animals which have been preserved in the rocks, but also to any impressions or traces left by ancient plants and animals.
Fossils tell us about the history of life on Earth and provide more reliable evidence in support of evolution, which we refer to as the fossil record.
The branch of science that deals with the study of fossils is called paleontology.

How are fossils formed?

Fossils are formed in numerous ways, and the process is called fossilization.
Most of the fossils are formed in seas and oceans. When a living organism (such as a plant or animal) dies, it is quickly buried by sediment (such as mud, sand or volcanic ash) carried in by big rivers.
Decomposers decompose the soft parts of these organisms leaving only the hard parts like bones, scales and shell etc. in animals and wood or leaf in plants.
However, in special circumstances, the soft tissues of organisms can also be preserved.
With the addition of more sediments, volcanic ash or lava over the top of the buried organism, all the layers eventually harden into sedimentary rock through the process of petrification or lithification.
The rocks are worn back down and washed away with the continuous process of erosion which reveal the prehistoric life forms within the rocks and stones known as fossils.
5 common ways of formation of fossils are as follows:

1. Permineralization or Petrification:

It occurs when dissolved minerals carried by ground water fill up space inside the cells of plants and animals.
The dissolved minerals crystalize in these cellular spaces and eventually form rocks in the shape of the animal or plant.
This is the most common type of fossil formation which preserves teeth, bones, shells and wood.
One example of fossilization of dinosaurs by permineralization is given below:

- Stage 1: An aquatic dinosaur dies while swimming in a lake. The flesh and the muscles decompose, but the hard bones remain intact at the bottom of the lake.
- Stage 2: The dinosaur’s remains is covered up by sediments which over time forms a thick bed protecting the dinosaur bones from scavenging animals.
- Stage 3: The minerals transported in the ground-water gradually replace the bones and turn them to stone or rock by the process called permineralization. Over the centuries, more sediment accumulates due to which the pressure and compaction builds up and eventually the bones and sediment layers become the bed rock.
- Stage 4: The dinosaur bones in the form of fossils are finally uncovered through erosion or excavated by paleontologists.

2. Impression:

These are the fossils of the shells of many marine invertebrates.
Impressions of organisms are formed when their original bone or tissue are removed after their burial by processes such as ground water flow.
A cast is formed when the organisms remains are decomposed entirely, leaving an empty space in the shape of the organism.
A mould is formed when the minerals fill in this empty space and form a mineralized 3 dimensional shape of the organism.

3. Amber

Organisms like insects are preserved if they become trapped in tree resin.
The resin will eventually harden to form a golden amber that has been shown to preserve fossils up to 100 million years old.

4. Trace fossils

It records the activities of an organism which include nests, burrows, footprints and coprolites (fossilized feces).

5. Soft tissue:

Apart from bones and other hard tissues, soft tissues of organisms can also be preserved in the form of fossils.
Intact remains of organisms like preserved skin, muscle, bone, hair and internal organs can be fossilized.
Fossils of soft tissues form in special circumstances.
When an entire organism is rapidly encased in material such as ice or volcanic ash or buried in peat bogs or trapped in amber, there is low oxygen and stops the decomposition of organisms and also prevents their bodies from being scavenged by other organisms.
These types of fossils are very rare but exceptionally well-preserved and provide a lot of information about prehistoric forms of life.
Fossil sites with exceptional soft tissue preservation are often referred to as Lagerstätte (which means ‘storage place’ in German). An example of this kind of soft tissue preservation comes from the world’s best-preserved woolly mammoth (a baby mammoth named Lyuba) discovered from within ice in Russia that formed 40,000 years ago.

Importance of fossils:

Fossils provide more reliable evidence of evolution.
The history of evolution of life on the earth is constructed by the study of fossil plants and animals from the various strata of the earth.
The study of the fossils provide information on when and where the major groups of plants and animals arose, flourished, and either became extinct or evolved into new forms.
In many cases, they help to determine the environment of the past.

References:

https://australian.museum/learn/australia-over-time/fossils/how-do-fossils-form/

Online Science Notes