Fossils are what is left of plants and animals from long ago. A common rule counts preserved remains as fossils when they are older than 10,000 years. Remains that are only partly fossilised, often from the Ice Age or younger, are called subfossils.

Most animals and plants that die rot away and do not become fossils. Remains must be buried deeply and quickly to last. Burial slows decay because there is little oxygen, and otherwise bacteria that use oxygen break the remains down to rubble. In permineralization, minerals carried by water fill the tiny pores in wood, bone or shell. Petrified wood is a standard example, and the fossil ends up heavier than the original. Making a fossil takes a very long time, often thousands to millions of years.

The fossil record is skewed toward animals with hard parts, because soft tissues and organs do not usually fossilise. Many soft-bodied species are missing from it. As of November 2026, more than 99 percent of all species that have ever lived are thought to be extinct. This is an estimate, not a count.

There are two kinds of fossils: body fossils and trace fossils. Trace fossils record what an animal did, such as walking, burrowing or nesting, rather than what its body looked like. They are rarely moved from the place where they were made. Tracks tell how long a dinosaur's stride was, and the spacing of the prints shows it.

Fossil dating comes in two steps. Relative dating puts fossils in order, older and younger. Radiometric dating of the rocks around them then gives an age in years.

Whales once walked on four legs. Pakicetus was an early whale that lived on land. Ambulocetus, an early whale, had short legs, paddle-like feet and a long muscular tail, which suggest it lived a more watery life than Pakicetus.

Oviraptor means "egg snatcher", but the name is a mistake. A specimen found curled up on its nest, and others brooding clutches of eggs, show it as a nesting parent, not an egg thief.

As of November 2026, in national parks fossils may not be collected, and on other public land fossils taken for personal use may not be sold.

Fossils help show how living things changed over time. Fossils can show what the climate was like long ago. Fossils are our best evidence for the history of life on Earth.

Words to know

• subfossils: partly fossilised remains
• permineralization: minerals filling pores
• radiometric: using radioactive clocks
• extinct: died out
• brooding: sitting on eggs