How Fossils Form: From Bone to Stone in 5 Steps
Fossils form through rapid burial and mineral replacement, preserving less than one tenth of one percent of life.
Why must burial be fast?
Fossilization starts when an animal dies near rivers or seas and gets buried by sand and mud within days. Rapid burial blocks scavengers, bacteria and oxygen, saving hard parts like bones and teeth that survive over 10,000 years.
Sediment layers pile up year after year, pressing down with weight equal to hundreds of meters of rock. Water rich in silica and calcite seeps through pores and slowly replaces bone cells, turning a 70-million-year-old skeleton into solid stone.
What kinds of fossils exist?
Less than 0.1 percent of all organisms become fossils because soft skin and muscle usually rot away. Only teeth, bones and shells have a real chance, which is why dinosaur skeletons outnumber skin impressions by about 100 to 1.
Besides petrified bones, nature makes molds, casts and trace fossils like 30-centimeter footprints or fossilized dung. A mold forms when bone dissolves leaving a hollow, while minerals filling that hollow create a perfect 3D cast copy.
- Rapid burial within days blocks decay
- Sediment layers press over thousands of years
- Minerals replace bone in permineralization
- Fewer than 0.1 percent organisms fossilize
- Molds, casts and footprints preserve behavior