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🦴 How Fossils Form

How does a soft, living animal end up as solid stone millions of years later? Follow the whole amazing journey — burial, turning to stone, and discovery — that makes a fossil.

lessons

~30 min

to learn

🔬 Science

subject

Ages 6–12

level

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What you’ll learn

BurialExplain that fossils are rare and that remains must be buried quickly in sediment to begin fossilizing.Fossilization is rare and takes millions of years. It begins when an animal dies and its soft parts rot away, leaving the hard bones and teeth. To become a fossil, these remains must be buried quickly by sediment, usually in water, which protects them from rotting away.
Turning to StoneDescribe how layers of sediment form rock and how permineralization, molds, and casts turn remains to stone.Layers of sediment pile up and harden into sedimentary rock around the buried bones. Mineral-filled water seeps into the bone and hardens in a process called permineralization, turning it to stone while keeping its shape. Sometimes a bone dissolves to leave a mold, which minerals can fill to form a cast.
DiscoveryExplain how uplift and erosion expose fossils and how paleontologists safely dig them up.Buried rock is slowly pushed upward, and erosion by wind, rain, and rivers wears it away to uncover fossils. Paleontologists dig them out carefully, wrap them in protective plaster jackets, and bring them to a lab to clean and study, completing the fossil's long journey.

Questions this course answers

Why do most animals never become fossils?

Fossilization is very rare, needing special conditions and millions of years, so most remains never become fossils.

Which parts of an animal are most likely to become fossils?

Soft parts rot away quickly, so the hard parts like bones, teeth, and shells are most likely to fossilize.

What must happen soon after an animal dies for a fossil to form?

The remains must be covered quickly by sediment, often in water, before they rot away.

What forms when layers of sediment press down and harden over the bones?

Piled-up layers of sediment press down and harden into sedimentary rock, with the bones locked inside.

What is permineralization?

Permineralization is when mineral-filled water seeps into a bone's tiny holes and hardens it into stone.

What is a cast fossil?

A cast forms when minerals fill an empty mold and harden into a stone copy of the original shape.

Grounded in trusted sources

Natural History Museum (London)
Smithsonian's National Museum of Natural History
American Museum of Natural History
U.S. National Park Service
Britannica Kids

Every Wunder lesson is built from real, reputable sources — never invented.

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