A fossil is a record of a plant or animal that lived thousands or even millions of years ago. Some fossils are very big, like dinosaur bones. Others are so tiny you need a microscope to see them. The scientists who dig for fossils and study them are called paleontologists.
How do fossils form?
Fossils are usually impressions of hard structures like wood, bones, or shells that don’t decompose quickly. Typically, fossils form when a dead organism gets covered in sand, mud, or clay. Over time, minerals from the soil surrounding a dead animal slowly replace parts of the animal. This usually only happens for the hardest parts of an animal, such as its bones.
Fossils can also be an impression of a leaf or an animal left in the mud. Over time, the mud can harden into stone, preserving the outline of an organism or its footprint.
Sometimes, a whole animal or plant will become a fossil. This happens when the dead animal is completely covered or buried. In this environment, there is no oxygen available for organisms that want to decompose the material. So, the organism will lie in place for millions of years. Each year, tiny pieces of the organism get replaced with minerals. Eventually, the entire organism is “turned to stone”.
One example of this is the La Brea Tar Pits in Los Angeles. Animals fell into the tar and couldn’t get out. They died, but no other organisms could eat or otherwise decompose their bodies. The remains of these animals were preserved for paleontologists to find today. That’s how we know that saber-toothed cats and wooly mammoths lived in the area thousands of years ago!
How can you tell how old a fossil is?
Scientists need to know how old fossils are. This helps them understand how species evolved and how the planet changed over time.
One way to date fossils is by the layer of rock where they were found. Since this process of adding new dirt on top of the earth happens at a similar rate over time, scientists can estimate how old something is by how deep it was buried. If a fossil is found in a rock formation that is ten million years old, then scientists can estimate that creature died approximately ten million years ago. This is called relative dating.
Another method is absolute dating. Scientists can use radioactive elements found in fossils and the surrounding rocks. Radioactive elements slowly break down into other, more stable elements over time, in a very consistent manner. By measuring amounts of radioactive material in a sample, scientists can figure out exactly how old a rock or fossil is.
The Story Told by Fossils
Not everything becomes a fossil. Some plants and animals decay completely before their bones can become fossilized. Some fossils are so broken that it’s hard to read their story. In fact, most fossils are only tiny fragments of the organisms they once belonged to.
We wouldn’t know about dinosaurs if we hadn’t dug up their fossilized remains and put all the pieces together. Sometimes paleontologists get lucky. For example, a tyrannosaurus rex fossil (named Sue) was found in South Dakota. This fossil contained about ninety percent of the bones from the original skeleton.
Still, there are gaps in the fossil record. For instance, we still don’t know exactly how whales evolved. We have found fossils of many of their relatives, including some ancient animals that looked like hippos with webbed feet. However, since whales evolved in the ocean, few of their fossil remains are available to study.
Fossils also teach us about the history of Earth. When paleontologists found fossils of the same plants and animals on every continent, they began to suspect that the continents were once pushed together into one giant supercontinent. This turned out to be true.
Written by Laura McCamy
Edited by Gabriel Buckley, MS Professional Natural Sciences
Illustrated by Meimo Siwapon