The presence of a fossil record in a fictional world is one way to age a world. Fossils by their nature speak to an age before the present and offer worldbuilders an easy way of including a segment of history alongside an interesting element. Dragon fossils, giant skeletons, alien remains, and more are all commonly found in fictional worlds.
The Creatures that Came Before
When designing fossils for a fictional world it is first important to consider the animal they came from. A fossil may be from a great beast (a dragon or an ocean leviathan) or may be from some ancient human or human-like species.
Fossils in your world may be of a size so vast entire towns are built within them. Or they may be so small and plentiful they create their own deserts.
Fossils may be collected as a hobby or for study. Consider if people in your world seek out fossils and the reasons they may do so. Fossils may also give rise to mythologies surrounding these strange beasts.
In several fictional worlds, most notably Jurassic Park, fossils and the DNA found associated with them are used to bring creatures back from extinction.