How Do Fossils Form? A Journey Through Time

By: Austin Henderson  | 
A fossilized trilobite found in the Green River Formation in Wyoming.
The term "fossil" includes a number of different artifacts, including this fossilized trilobite found in the Green River Formation in Wyoming. Layne Kennedy / Getty Images

The term fossil describes a wide range of natural artifacts. Generally speaking, a fossil is any evidence of past plant or animal life preserved in the material of the Earth's crust.

But when most people talk about fossils, they mean a specific subsection of this group — fossils in which the shape of the animal or plant has been preserved, while the actual organic matter of its body is gone. So, how do fossils form, and what can they tell us about Earth's ancient history?


Understanding Fossils: A Quick Primer

In essence, a fossil is the preserved remains or impression of an organism that once existed on this planet. But fossils aren't just about dinosaur bones or ancient plants. They can also be footprints, trails or even dung, also known as coprolite.

Types of Fossils

  • Body fossils: These are what most people think of when they hear the word fossil. They include the preserved remains of ancient animal and plant bodies. Think dinosaur bones or plant leaves.
  • Trace fossils: Not the organism itself but a trace of its existence. This could be footprints, burrow marks, or even those aforementioned droppings.

Now that we're clear on what a fossil is, let's dive into the process of how they come to be.



The Making of a Fossil

  1. An animal dies. This might seem obvious, but it's the start of our fossil journey. When an animal dies, and is quickly buried in sediment (like mud or sand), the stage is set. Rapid burial helps in preserving the animal's body parts, particularly the harder bits like bones or shells. Soft-bodied creatures, though, are a bit of a rare event in the fossil world, due to their delicate nature.
  2. Sediment layers build up. Over thousands to millions of years, sediment layers accumulate over the buried remains. With time, these layers compact and turn into hard rock.
  3. Nature plays its part. As ground water seeps in, minerals fill the spaces and pores of the organism's remains. The original organic material might decay or dissolve, leaving behind a natural mould. If minerals, such as calcium carbonate, enter this mold, they solidify and form a natural cast fossil.
  4. The fossil forms. Eventually, erosion or human activities might expose these rocks, revealing the preserved evidence of life from ages past.
A fossil of a Microraptor from a 130-million year old forest that existed in what is now Liaoning Province, China is displayed at the American Museum of Natural History in New York City.
Getty Images


Nature’s Treasure Chest: Unique Fossil-forming Processes

Fossil creation isn't just limited to sedimentary rock magic. Nature has other tricks up its sleeve:

  • Amber: Some prehistoric insects met a sticky end, getting trapped in tree sap which, over millions of years, turned into amber.
  • Petrified wood: Minerals seeped into fallen trees, replacing the organic material cell by cell, preserving its structure but turning it to stone.
  • Tar pits and other sticky situations: Animals sometimes got trapped in sticky tar pits, bogs or quicksand, leading to remarkable preservation. Additionally, volcanic ash has been known to play its part in fossil preservation, particularly for land animals.


Why Should We Care About Fossils?

Fossils tell us which plants and animals existed in prehistoric times and provide clues about their environment, habits, and interactions. The fossil record — that mega collection of all fossils worldwide — serves as a chronicle of Earth's history.

Additionally, using techniques like carbon dating, paleontologists can estimate the age of fossils, helping piece together the timeline of life on Earth. This helps identify which animals lived concurrently and which predate others.


Fossil Fun Facts

Most Organisms Don't Become Fossils

It’s a very rare event, in fact. Quick burial is crucial, and the conditions must be just right.

Not All 'Fossils' Are Ancient

Some might be just a few thousand years old, while others, particularly those of marine animals trapped in sedimentary rocks, can date back over 500 million years.


The Jurassic Period Rules

If you think of dinosaur fossils, you might be thinking of the Jurassic period, which was a golden age for these giant reptiles.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.


Frequently Answered Questions

What makes something a fossil?
A fossil is a preserved remains of an organism that lived in the past.