A principle called faunal succession says each layer of rock holds its own record of life, and the layers follow each other in a known order. Rocks in different places that hold the same fossil species must be similar in age, and tracing rocks and fossils from one place to another is called correlation. Because fossils occur over wide regions of the Earth, they let geologists match up rock layers far apart. Fossil dating comes in two steps. Relative dating puts fossils in order, older and younger, and radiometric dating of the rocks around them then gives an age in years. Volcanic ash layers can be dated by radiometric methods. A fossil in a layer below a dated ash is older than the ash, and a fossil above it is younger.

Tiktaalik is better called a transitional form than a missing link. It shows a mix of fish and four-legged traits. Tiktaalik had fins with thin ray bones, scales and gills like a fish, but also a neck, shoulders and sturdy wrist bones like a four-legged animal. As of November 2026, Archaeopteryx has long been called a transitional form between reptiles and birds. As of November 2026, scientists still do not know if it could fly or only glide. Archaeopteryx had feathers, wings and a wishbone like a modern bird, but also teeth and a long bony tail.

There are two kinds of fossils: body fossils and trace fossils. Footprints and feather or skin prints can be trace fossils, and tracks are the marks that an animal leaves as it moves. Trace fossils record what an animal did, such as walking, burrowing or nesting, rather than what its body looked like, and they are rarely moved from the place where they were made. A trackway with many parallel tracks of different sizes can suggest that a group of animals walked together, so tracks can hint at herd behaviour. Tracks tell how long a dinosaur's stride was, and the spacing of the prints shows it.

William Smith used fossils to identify rock layers, and became the first person to map the geology of an entire nation, England and Wales. His idea of using fossils to identify rocks is still important today.

Mary Anning found the first complete skeleton of a Plesiosaurus, an ancient sea reptile, and her fossil discoveries helped scientists rebuild the history of life. An ichthyosaur skeleton she found made her famous. Accounts differ about the first ichthyosaur. Mary Anning is often credited with finding the first ichthyosaur fossils, but a museum notes it was not the first found, only the first to be scientifically studied and described.

Georges Cuvier, a French anatomist, firmly established that extinction is real. As of November 2026, more than 99 percent of all species that have ever lived are thought to be extinct, and this is an estimate, not a count.

Tiny sea creatures build their shells from elements in the ocean water, and their fossil shells help scientists learn about past climates. Fossils can show what the climate was like long ago. Fossils help show how living things changed over time. The fossil record is skewed toward animals with hard parts, because soft tissues and organs do not usually fossilise, and many soft-bodied species are missing from it. Fossils are our best evidence for the history of life on Earth.

Words to know

• correlation: matching rocks
• radiometric: using radioactive clocks
• transitional: in between
• trackway: line of tracks
• Plesiosaurus: ancient sea reptile