A fossil is the preserved remains or traces of a living thing that died a long time ago. The word "fossil" comes from the Latin "fossus," meaning "dug up." Fossils are the main way scientists learn about life that existed before humans were here to watch it — including dinosaurs, mammoths, ancient fish, and plants from when the Earth looked very different from today.

This guide explains what fossils actually are, how they form, the different types, and how kids and families can find or make their own. It draws on sources from the Smithsonian National Museum of Natural History, the American Museum of Natural History, the Paleontological Society, and the Florida Museum of Natural History.

The short answer#

A fossil is the remains, impression, or trace of a once-living thing — usually older than 10,000 years — that has been preserved in rock or sediment. Fossils tell us what plants and animals were here before us, what they looked like, and sometimes how they lived and died.

How fossils form (the basics)#

Not every dead animal becomes a fossil. In fact, most do not. For a fossil to form, several specific conditions usually have to line up.

Step 1 — Quick burial#

The most important step. When a plant or animal dies, scavengers, weather, and bacteria break the body down within months or years. To become a fossil, the body has to be covered quickly — buried in mud, sand, ash, or sediment before it fully decomposes. This is why fossils are most common in places that were once underwater (oceans, lakes, rivers, floodplains) or near active volcanoes.

Step 2 — Soft parts disappear; hard parts stay#

Over thousands of years, the soft parts (skin, muscle, organs) decompose. The hard parts (bones, teeth, shells, wood) last longer. Most fossils are made from these hard parts.

Step 3 — Minerals replace the original material#

Over millions of years, water seeps through the sediment carrying dissolved minerals. The minerals fill the tiny spaces in the bones and shells, slowly replacing the original organic material with stone. This process is called mineralization or petrification. By the time it is complete, the fossil is essentially a stone copy of the original bone or shell, with the same shape but made of different material.

Step 4 — Discovery#

The rock containing the fossil has to come back to the surface. This can happen through erosion (wind and water wearing away covering rock over time), road cuts, mining, or quarry work. If the fossil never comes back to the surface, it stays buried and we never find it. Many, many fossils are still buried.

The five main types of fossils#

Not all fossils are bones. Five major categories.

1. Body fossils#

The most familiar kind. Preserved hard parts of organisms — bones, teeth, shells, claws, exoskeletons. Almost every famous dinosaur fossil falls in this category. A Triceratops skull, a T-Rex tooth, an ammonite shell — all body fossils.

2. Trace fossils#

Evidence of how an organism lived rather than what it looked like. Footprints, burrows, nests, trails, and bite marks all count. Trace fossils tell us behavior. A Triceratops trackway can tell us how it walked, whether it walked in groups, and how fast it moved. Dinosaur trackways are common in some parts of the U.S. (Utah, Colorado, Texas), preserving moments of behavior frozen in stone.

3. Coprolites#

Fossilized droppings. They sound funny, but they are scientifically valuable — coprolites can preserve undigested bone, shell, or plant fragments, telling us exactly what the animal ate. A coprolite from a T-Rex once contained shattered Triceratops bone fragments, direct confirmation of the predator-prey relationship.

4. Mold and cast fossils#

When a body decomposes inside sediment, sometimes the body itself disappears but the impression it left behind hardens. The empty space is a mold. If minerals later fill that mold, you get a cast — a stone replica of the original body shape. Many famous shell and leaf fossils are molds or casts rather than original material.

5. Amber inclusions#

Hardened tree sap that trapped small creatures — usually insects, occasionally small lizards or feathers — preserving them in remarkable detail. The Jurassic Park premise (DNA from amber-trapped mosquitos) is fictional, but the fossils themselves are real and beautifully preserved. Most amber comes from a specific geological time and a few locations (the Baltic region, the Dominican Republic, Myanmar).

What is NOT a fossil#

A few common misconceptions.

Bones from a recently dead animal (less than 10,000 years old) are not fossils — they are just bones. The age threshold matters.
A skeleton in a desert is not a fossil unless it has been mineralized. Bones that look ancient but have not been replaced by minerals are subfossils — interesting, but not technically fossils.
Old artifacts made by humans (arrowheads, pottery, tools) are not fossils — they are archaeology, a related but different field.
Petrified wood that is recent (the past few thousand years) doesn't count either — true petrification takes millions of years.

How old can fossils be?#

Fossils range from about 10,000 years old (the minimum for the word "fossil" to apply) to billions of years old. The oldest known fossils are microbial mats called stromatolites, around 3.5 billion years old.

For perspective:

Stromatolites — 3.5 billion years (single-celled life)
First multicellular life — about 600 million years ago
First dinosaurs — about 230 million years ago
Tyrannosaurus Rex — 68 to 66 million years ago
Last dinosaurs (non-bird) — 66 million years ago, end of the Cretaceous
First mammals replacing dinosaurs — beginning of the Paleocene, after 66 million years ago
Megalodon (Florida's most famous prehistoric resident) — 23 to 3.6 million years ago
Mastodons in Florida — 30,000 to 11,000 years ago
Earliest "fossil" threshold — 10,000 years ago

The full timeline is covered in our Mesozoic Era explained post.

Where are fossils found?#

Anywhere the right geological conditions existed. The most productive fossil-bearing rocks in the world include:

The Morrison Formation (Colorado, Utah, Wyoming, USA) — Jurassic dinosaurs
The Hell Creek Formation (Montana, USA) — Late Cretaceous dinosaurs including T-Rex
The Gobi Desert (Mongolia and China) — Velociraptor and Protoceratops
La Brea Tar Pits (Los Angeles, USA) — Ice Age mammals
The Burgess Shale (British Columbia, Canada) — Cambrian-era soft-bodied organisms

For Florida specifically: the state's Cenozoic sediments preserve marine fossils (megalodon teeth, shells, marine mammals) and Ice Age land mammals (mastodons, sloths). See our where to see dinosaur and prehistoric fossils in Florida guide for local destinations.

How kids can find or make fossils#

You do not need to go to Montana to engage with fossils.

Buy or borrow real fossil replicas#

Educational supply stores sell replicas of famous fossils — Triceratops horn casts, T-Rex tooth replicas, ammonites, trilobites — for $5 to $30 each. The Master Fossil Exhibition at our school events brings 30+ touchable fossil replicas to your campus.

Real fossil hunting in Florida#

Florida is one of the easier states for amateur fossil hunting. Megalodon teeth wash up on beaches in the Gulf Coast and Atlantic. Specific spots like Venice Beach are nationally famous for shark tooth hunting. The Florida Museum of Natural History's website has updated guidance on permitted collection sites.

Make your own "fossil"#

A fun home or classroom project for kids 4 and up:

Mix flour, salt, and water to form a thick dough
Press an object (a small toy dinosaur, a shell, a leaf) into the dough to leave an impression
Remove the object
Bake or air-dry the dough
The impression becomes a "trace fossil" of the object you pressed in

This is a simplified version of how mold fossils form in nature.

Set up a backyard fossil dig#

Fill a sandbox or plastic bin with sand. Bury 10 to 20 fossil replicas or dollar-store dinosaur skeleton parts. Give kids brushes and small picks. The activity teaches the patience and observation skills paleontologists actually use. A Ranger-staffed version of this is one of the five Zone 1 stations at our school events.

Why fossils matter#

Fossils are the only direct evidence of what life was like before humans. Without them, everything we know about dinosaurs, ancient mammals, and prehistoric ecosystems would be guesswork. Fossils let us reconstruct:

Body shape and size of extinct animals
What they ate (from teeth, jaw shape, and coprolites)
How they moved (from leg structure and trackways)
Where they lived (from associated rock formations)
How they were related to modern animals (from comparative anatomy)
How ecosystems changed over time (from species turnover across rock layers)

A child who understands what a fossil is and how it formed has the foundation for understanding all of paleontology, much of evolutionary biology, and the deeper history of life on Earth.

Frequently asked questions#

Are dinosaur bones we see in museums real?#

Often a mix. Most museum displays of dinosaur skeletons are casts (high-quality replicas) of real bones, because the real bones are too valuable, too fragile, or too heavy to mount as a complete skeleton. The original fossils are usually kept in research collections. Some museums display real bones with replicas filling in missing parts.

Can fossils be from plants?#

Yes. Plant fossils are common — petrified wood, leaf impressions, fossilized pollen and seeds. The plant fossils tell us what the ecosystem looked like (forest? swamp? desert?) and help date the rock layers.

What's the difference between a fossil and a rock?#

A fossil is something that was once alive, preserved through mineralization. A rock is a mineral or mix of minerals that was never alive. A fossil is contained within rock, but the fossil itself is the preserved organism.

Do fossils tell us about colors?#

Usually not. Color information is lost during fossilization because the pigment cells decompose. Some recent fossils preserve melanosomes (pigment-containing structures) and let us infer color — this has worked for some feathered dinosaurs. But for most dinosaurs, the color is informed guesswork based on modern animals.

How do we know how old a fossil is?#

Two main methods. Relative dating — figuring out which rock layer the fossil came from, and where that layer sits in the geological column. Absolute dating — using radioactive decay in the surrounding rock to assign a numerical age. Both methods combined give the dates you see in museums.

Can I find fossils in my backyard?#

Maybe, depending on where you live. Florida backyards rarely produce fossils, but Florida beaches sometimes do (especially Gulf Coast shark teeth). Backyards in fossil-rich areas (Montana, Wyoming, parts of Texas) sometimes turn up fossils during construction or gardening. If you find something that looks like a fossil, the Florida Museum of Natural History accepts photos for identification.

Bring fossils into your classroom or party#

Touchable fossil replicas anchor science lessons in a way photos cannot. Our school events bring 30+ replicas to your campus as part of the Master Fossil Exhibition station, plus a Fossil Dig Station where kids excavate their own fossils to keep. For South Florida schools and families, see the experience page or book a date.

What Is a Fossil? A Complete Guide for Kids and Parents