Frozen in Stone: The Four Ways Fossils Form

Less than one-tenth of one percent of all animals that have ever lived left behind a fossil. For an animal to become a fossil, almost everything has to go right: the right burial, the right minerals, and a whole lot of time — usually at least 10,000 years. So how does it actually happen?

- The four main types of fossilization - Why soft body parts almost never fossilize - How a dinosaur bone slowly turns to rock - What trace fossils tell scientists that bones can't

The most common way large fossils like dinosaur bones form is called permineralization. When an animal dies and gets buried quickly in sediment (like mud, sand, or volcanic ash), water rich in dissolved minerals slowly seeps through its bones. Over thousands of years, those minerals — usually silica, calcite, or pyrite — fill in every tiny pore in the bone, until it is essentially rock in the exact shape of the original bone.

Sometimes the original bone or shell dissolves away completely after being buried, leaving a hollow, bone-shaped space in the rock called a mold. If that empty space later fills in with new minerals or sediment, it forms a cast — a rock copy of the original shape, like a plaster figure poured into a mold. Trace fossils are different altogether: they aren't body parts at all, but preserved evidence of an animal's activity, like footprints, burrows, or even fossilized dung, called coprolites.

A rarer and especially dramatic type of fossilization happens when tree resin (a sticky sap) traps a small organism like an insect, then hardens over millions of years into amber. Because amber seals out air and bacteria almost completely, it can preserve incredibly fine detail — scientists have found insects in amber with wing veins and even hairs on their legs still intact.

Skin, muscle, and organs decay within days to weeks after death, long before the slow mineral process of fossilization can begin. That's why the vast majority of fossils are hard parts only: bones, teeth, shells, and woody plant material. When soft tissue impressions are found — like skin impressions on a 'dinosaur mummy' — it's considered an extraordinarily rare discovery, usually requiring the animal to be buried almost instantly in fine sediment with no oxygen.