Fossils Tell a Story

Fossils are preserved remains or traces of ancient living things that give us clues about life on Earth long ago.

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Definition

A fossil is the remains, imprint, or trace of a plant or animal that lived thousands or millions of years ago, preserved in rock. Scientists called paleontologists study fossils to learn what creatures looked like, where they lived, what they ate, and how Earth has changed over time. Just like chapters in a book, each fossil adds to the bigger story of life on Earth.

Remember the rule

Dead thing + buried in sediment + time + pressure = fossil. The deeper the rock layer, the older the fossil!

Key words

Fossil: The preserved remains or imprint of a once-living thing found in rock.
Paleontologist: A scientist who studies fossils to learn about ancient life.
Preserved: Kept safe from rotting or breaking down so it can last a very long time.
Imprint: A mark or shape pressed into rock, like a footprint left in mud that hardened.
Sediment: Tiny bits of sand, mud, and rock that settle in layers and can bury and preserve living things.
Extinct: A type of plant or animal that no longer exists anywhere on Earth.
Organism: Any living thing, such as a plant, animal, or insect.
Trace fossil: A fossil that shows evidence of an animal's activity, like footprints, burrows, or bite marks, rather than its actual body.

Worked examples

A scientist finds a leaf pressed into gray rock. What can she learn from it?

→ She can learn that a plant with that leaf shape lived in that area long ago. If the plant only grows in warm, wet places today, she can guess the climate there was once warm and wet too. · Plant fossils tell us about ancient environments, not just ancient plants.

Workers digging in a desert find large dinosaur bones in the rock. The desert is very dry today. What story do the bones tell?

→ The bones tell us dinosaurs once lived there. Because many dinosaurs needed water and plants to survive, the area was probably a wetter, greener place millions of years ago. · Fossils show us that landscapes change dramatically over millions of years.

A fossil of a fish is found on top of a mountain. How did it get there?

→ Long ago that mountain was underwater or near a sea. Over millions of years, Earth's crust shifted and pushed the seabed up to form a mountain, carrying the fish fossil with it. · This is real — fish fossils have been found in the Rocky Mountains!

Scientists find fossil footprints of a large animal walking in a straight line, then the tracks suddenly stop. What story might those trace fossils tell?

→ The animal may have reached water and swum away, or a predator may have caught it. The tracks tell us about the animal's size, speed, and behavior even without finding any bones. · Trace fossils give clues about how animals acted, not just what they looked like.

A scientist finds fossils of the same type of small ocean creature in rock layers in Africa and in South America. What does this suggest?

→ Those two continents were probably once connected, allowing the same creature to live across both areas before the land slowly drifted apart over millions of years. · Fossils are important evidence for how continents have moved over time.

A child finds what looks like a shell shape in a rock at the park. Is it definitely a fossil?

→ It might be! Real fossils feel like rock because they have turned to stone over time. If the shell shape is part of the rock itself and not just sitting on top, it is likely a fossil of an ancient sea creature. · Many common fossils are shells from ocean animals that lived millions of years ago.

Common mistakes

Thinking fossils are the actual bones or body parts — fossils are usually minerals that slowly replaced the original material, turning it to stone over a very long time.
Thinking all dead animals become fossils — most organisms decompose and disappear; fossilization is rare and requires quick burial in sediment.
Confusing a trace fossil with a body fossil — footprints and burrows are trace fossils; actual bones and shells are body fossils.
Thinking fossils are only dinosaurs — fossils include plants, insects, fish, shells, leaves, and even bacteria.
Thinking the youngest fossils are deepest in the rock — it is the opposite: older fossils are deeper because they were buried first, and newer layers formed on top.

FAQs

How long does it take for a fossil to form?

Usually thousands to millions of years. It takes a long time for sediment to harden into rock and for minerals to slowly replace the original organism.

Can a human become a fossil someday?

It is possible but very unlikely. Fossilization is rare because it needs just the right conditions — quick burial, the right minerals, and no disturbance for a very long time.

Why do we find so many shell fossils compared to dinosaur fossils?

Shells are made of hard material that preserves well, and sea creatures are often buried quickly in ocean sediment. Dinosaur bones are bigger and rarer to find in the right conditions.

Do fossils only form in rocks?

Almost all fossils form in sedimentary rock, but organisms have also been preserved in ice, amber (hardened tree sap), and tar pits — these are special cases.

What is the difference between a fossil and a rock?

A rock is made only of minerals. A fossil is found inside rock but has the shape or material of a once-living thing, like a bone, shell, leaf, or footprint.

Why can't we find fossils of every animal that ever lived?

Most animals decompose completely before they can be buried and preserved. Only a tiny fraction of all creatures that ever lived left behind fossils.

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