How Fossils Work

Bone to Stone: Building Fossils

Fossilized eggs on display at the Inner Mongolia Museum in the regional capital of Hohhot
AFP/Getty Images

Most of the dinosaur skeletons you see in museums exist because of sedimentary rocks. These fossils got their start when a dinosaur died in an environment that had lots of moving sediment, like an ocean, riverbed or lake. One such place is the benthic zone -- the deepest part of a body of water. This sediment quickly buried the dinosaur, offering its body some protection it from decomposition. While the dinosaur's soft parts still eventually decomposed, its hard parts -- bones, teeth and claws -- remained.

But a buried bone isn't the same thing as a fossil -- to become a fossil, the bone has to become rock. The organic parts of the bone, like blood cells, collagen (a protein), and fat, eventually break down. But the inorganic parts of the bone, or the parts made from minerals like calcium, have more staying power. They remain after the organic materials have disappeared, creating a fragile, porous mineral in the shape of the original bone.


STR/AFP/AFP/Getty Images An 8-million-year-old

Other minerals reinforce this bone, burning into a fossil. Water gradually makes its way into the bone, carrying minerals like iron and calcium carbonate picked up from the surrounding sediment. As the water penetrates the dinosaur's bones, some of these minerals precipitate into their microscopic pores. As this process continues, the bone becomes more and more rocklike. It's like filling a sponge with glue -- the sponge's physical structure stays the same, and the pores and pockets within it fill up. The glue makes the sponge sturdier and more resistant to damage. Large, thick bones, which have more room for mineral glue, make better fossils than small, flat bones.

Over the course of millions of years, the sediment around these reinforced bones becomes sedimentary rock. Erosion, tides and other natural processes continue to deposit more sediment, and this sediment becomes rock, too. As long as they can withstand the pressure from the surrounding rock, the bones remain safely hidden and preserved. After millions of years, some natural process, like the gradual shifting of the planet's surface, can reveal these layers of rock and the fossils they contain.

Sedimentary rock can also hold trace fossils, which record an organism's behavior. Some of the most well-known trace fossils are trackways, or the tracks of extinct animals. These form when an animal leaves its prints in soft but sturdy soil, which creates a mold. This mold fills with sediment, and both the mold and its filling harden over millions of years. Forces like erosion remove the upper layers of rock, revealing the preserved footprints underneath.

Sediment can also fill the mold and harden into a cast, or a reproduction of the foot that made the print. This can happen with other traces, too, like burrows and tunnels. Some other trace fossils include coprolites (fossilized dung) tooth marks on bones or wood, and nests. Sediment can even preserve plant life. Plants can make impressions in hardening sediment or become petrified wood after going through much the same process as fossilized dinosaur bones do.

This type of fossilization creates sturdy fossils, but it can only take place in specific conditions. Next, we'll look at the other ways life forms can be buried, encased or otherwise protected so their remains last millions of years.