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Fossils 101: How are Fossils Formed?

cartoon: Microorganisms on Strike

How are fossils formed?

If you think of the total number of organisms that have lived on the Earth since life evolved over 3.8 billion years ago, it's a wonder that fossils aren't more common.

The fact is, it takes a very specific set of conditions for plants and animals to be preserved as fossils. After all, if there were no efficient way of breaking down and recycling all those organisms after they died, imagine how crowded the planet would be!

For an organism to have a chance at becoming fossilized, it has to avoid being completely broken down by the scavengers and microorganisms that are responsible for decomposing dead matter. One way this can happen is if the organism is covered with sediment quickly after it dies.

Once the organism is covered, sediment continues to build up over the course of many years. In the meantime, molecules in the organism's body are gradually being replaced by minerals in the surrounding sediments. As more and more sediment builds up, pressure begins to increase and compacts the sediment into rock. This is why fossils are often flat and distorted.

At this point, the fossil is formed, but it may be hundreds or thousands of feet under the earth's surface. What has to happen for scientists to be able to find and analyze it?

After fossils are buried, how do they get to the surface to be discovered?

That's right, the only way for those sediments to reach the surface is for them to be pushed up during the process of mountain making and then worn away by the forces of erosion. Hard to imagine? It's happening all the time.

Imagine that the layers of sediment covering our fossil are sheets of paper that keep stacking up. Now imagine your stack of paper gets rammed by another stack of paper. The layers are going to buckle and the first sheets of paper you laid down, which were hidden from view, will be pushed up and visible at the surface now.

mountain building

Mountains have to be made and erosion has to work its magic

The same thing happens when continents collide. All those layers of sediment get bent out of shape as the continents shift around and bump into eachother. Older layers of sediment that were previously buried deep in the Earth get pushed to the surface. Then over the course of millions of years, water wears away at the rock, and if we're lucky enough for those layers to contain fossils, then it's only a matter of going out and finding them!

See how scientists found Tiktaalik