What is a Fossil?

We watch documentaries and read a plethora of materials on the history of animal and plant species every single day. But how did humans discover the origins of these animals and plant species?  It was with the help of fossils. Paleontologists have been able to harness vital information from fossils to give us clues to what environment, ecosystems and living things were thousands of years ago. But what are fossils and how are they formed?

Fossils are the remains or traces of living things that existed thousands or millions of years ago. Fossils can also be impressions of plant leaves in a rock, tracks left behind by animals, ancient dens, barrels or a whole animal or insect perfectly preserved and crystallized by ice. The oldest fossils come from mats of blue-green algae that existed over 3 billion years ago. On the other hand, the youngest fossils come from animals that existed before the start of recorded history, about 10, 000 years ago.


Paleontologists (scientists concerned with the study of fossils) understand that the kinds of creatures that existed on earth at different periods have transformed a bit over millions of years. Every unique form of life today, whether extinct or alive, is known as a species. Most fossils originate from species that no longer exist on earth today since they have become extinct. Most of these extinct species look a lot like species living today.

Most people think that fossils are only big dinosaur bones or decorative shells found in museums. In fact, if the thought that you’ve never stumbled across fossil creeps into your mind, you need to erase it immediately. We all depend on fossil fuels like oil, coal, and gas every single day to fuel our vehicles, light, cool or heat our homes. This means that almost every human has interacted with fossil.

How are Fossils Formed?

When living matter such as plants, animals, and other organisms die, they naturally decay completely. However, sometimes when conditions are appropriate, these remains of deal living matter are preserved as fossils. Fossils are created as a result of numerous physical and chemical processes. When an animal dies, it’s buried pretty quickly beneath layers of rock and dust known as sediment.

The pressure and heat as a result of buried animal and plant can cause the tissue of the dead organism including the soft body parts of fish, plant leaves, marine invertebrates, and reptiles to emit oxygen and hydrogen, leaving a residue of carbon behind. This process is known as carbonization or distillation. Carbonization gives birth to an extensive carbon impression of the dead organism in the sedimentary rock. This process is known as fossilization.

Permineralization, sometimes known as petrification, is the most common technique of fossilization. After decay of an organism’s soft parts in sediment, the hardest parts, mainly the bones, remain. The remains absorb water and minerals present in the absorbed water permeate into the spaces inside the remains, eventually forming crystals. These crystals, commonly called crystallized minerals, trigger the hardening of the remains together with the outer sedimentary rock.

There is another fossilization process known as replacement. Here, the minerals that constitute the bodily remains are replaced by minerals contained in ground water after the water has totally dissolved the initial hard components of the organism.

Fossils can also form from cats and molds. If the organism is totally dissolved or liquefied in sedimentary rock, it might leave behind an impression of its outer surface in the rock known as external mold. If the mold is filled with other minerals, it’s called a cast. When minerals and sediments fill up the internal cavity of a fossil such as skull and shell and the remains dissolve, internal mold forms.

Where are Fossils Formed?

Fossils can occur just about anywhere sedimentary rocks are exposed. Sedimentary rocks are formed when layers of sediments, for instance, sand, silt, clay, or mud harden over time. These kinds of sediments rest beneath oceans, swamps, and lakes. This explains why the vast number of fossils consists of remains of animals that existed in or near these water bodies.

Statistically, a greater percentage of fossils are marine invertebrates, which are animals that existed in the sea and did not have any backbone, but a hard outer shell. Examples of marine invertebrates include coral, ammonites, clams, brachiopods, and extinct creatures such as crinoids and trilobites. In other areas, the deposits are predominantly fish fossils and rare insects.

Types of Fossils

Fossils are classified on the basis of how they form. Lots of fossils originate from bones, shells or teeth that have been converted into stone through a process known as petrification. To extensively understand their formation, here is an outline of the different types of fossils:

  1. Mold Fossils

These are fossil impressions left behind after the dead plant or animal decomposes. When a plant or animal dies, it sinks beneath the ocean floor and is later covered by sediment. The pressure of the accumulated sediment squeezes out any water left in the bone. Later, chemical reactions decompose the bone leading to deterioration. Deterioration leaves behind an impression of the organism. This impression or void is referred to as a mold.

  1. Cast fossils

A cast fossil originates from a mold fossil. What separates a mold fossil from a cast fossil is that mold fossils form when the embedded dead plant or animal is eroded away by chemical or physical processes. A cast fossil occurs when the gaping hole in the mold fossil as a result of erosion is filled up with sediment. A cast fossil is 3 dimensional, while a mold must be present for a cast fossil to occur.

  1. Trace fossils

Trace fossils provide humans with indirect evidence of past life. They are tracks left behind by animals or plants. Trace fossils include footprints, burrows, tracks, borings and feces left behind by animals.

  1. True-form fossils

True-form fossils are fossils of entire bodies of plants or animals. They are usually formed when plants or animals are trapped in ice, tar or tree sap over many years. All the features of the organisms are preserved by the ice. These kinds of fossils can date back to millions of years. The soft tissues remain because they have been preserved by ice.

Facts About Fossils

  • In order for any plant or animal matter to qualify as fossil, it must be several thousand years old.
  • Softer materials normally decompose before they become fossils, which is why bones, shells, and teeth are highly likely to become fossils. In museums today, we have fossils of giant dinosaurs that once walked on the surface of the earth.
  • The oldest fossil around is of blue-green algae that existed in some rocks in South Africa about 3.2 billion years ago.
  • Sauroposeidon, is the biggest dinosaur fossil, which is estimated to be 60 feet tall and weighs up to 60 tons. 60 feet can be compared to a six-story building, and 60 tons can be compared to nine elephants.
  • The smallest fossils ever discovered are the blue-green algae, which also happens to be the oldest. The smallest dinosaur, known as Microraptor, was recently discovered in China. It’s about 12 inches long, the size of a cow.
  • During the division between the Tertiary periods (the K-T boundary) and the Cretaceous, approximately 65 million years ago, lots of species went extinct. This is according fossil records. The explanation many concur with is that an asteroid hit the surface of the earth, resulting in inferno deaths of many plant and animal species that lived at that time. Today, the boundary is clearly visible in the rock strata like a layer of iridium. It’s a radioactive material unavailable on the earth’s surface, but available in asteroids.
  • Many people still believe that fossil fuels originate from dinosaurs. The truth is they originate from fossilized remains of small organisms known as diatoms. Since it lasted millions of years for fossils to form, this kind of fuel is non-renewable.
  • Fossilized diatoms are alternatively utilized as abrasives. In fact, manufacturers of toothpaste incorporate this kind of fossil to make the paste abrasive enough to clean out our teeth.
Photo by: strecosa
Sonia Madaan

Sonia Madaan

Sonia is a High School Graduate and Runs the Writing and Editing Team for EarthEclipse.com. She is Extremely Passionate about Environment, Technology and Computing.
Sonia Madaan