Preserved footprints are trace fossils. Some prehistoric creatures are known today solely because they left their footprints behind.
Footprints need a soft, moist, fine-grained surface—such as volcanic ash or mud—in order to be preserved.
Impressions of claws, the shape of a heel, or direction of a toe can provide new insights into evolution.
The footprint of a large mammal discovered in 1976 led to the surprising conclusion that humans were walking upright long before anyone had guessed.