How Do Fossils Form?

What You'll Learn

• A fossil is the preserved remains or trace of a living thing from long ago
• Most fossils form when a plant or animal dies and gets buried under mud, sand, or ash
• Over millions of years, more layers pile on top, pressing down and turning sediment to rock
• Minerals seep into the remains and replace the original material, turning it to stone
• Fossils are usually found in sedimentary rock — rock made from pressed layers of sediment
• Not every living thing becomes a fossil — it's very rare! Most remains rot away before they can be buried
• Paleontologists are scientists who study fossils to learn about life from long ago
• Fossils can be bones, teeth, shells, footprints, or even plant impressions

Words to Know

Fossil

The preserved remains or trace of a living thing from long ago, turned to stone

"A dinosaur bone fossil is made of stone, not real bone."

Sediment

Bits of rock, sand, mud, or dirt that settle in layers

"Mud covered the fish and became sediment."

Sedimentary rock

Rock made from layers of sediment pressed together over time

"Fossils are found in sedimentary rock."

Paleontologist

A scientist who studies fossils to learn about ancient life

"A paleontologist dug up the dinosaur fossil."

Preserve

To keep something from decaying or being destroyed

"Being buried in mud preserved the shell."

Check What You Learned

Fun Facts

• A fossil is the preserved remains or trace of a living thing from long ago

  Source: National Geographic / Smithsonian

• Most fossils form when an organism dies and is rapidly buried under sediment — mud, sand, or volcanic ash

  Source: Smithsonian National Museum of Natural History

• Over millions of years, layers of sediment pile on top, pressing down and turning to sedimentary rock

  Source: USGS / geological references

• Minerals in groundwater seep into the remains and replace the original material bit by bit, turning it to stone — this process is called permineralization

  Source: Smithsonian

• Fossils are most commonly found in sedimentary rock, not igneous (volcanic) or metamorphic rock

  Source: USGS

• The fossil record shows that life on Earth is at least 3.5 billion years old — the oldest known fossils are of bacteria

  Source: NASA / Smithsonian