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How Are Fossil Footprints Formed?

Fossil footprints are also called ichnofossils, or trace fossils. Fossil footprints can reveal the approximate weight of a prehistoric animal, how tall it was and how fast or slowly it moved. Scientists can determine whether the animal walked on two legs or four legs. The gait, or the way the animal walked, can usually be determined if there is a trail of footprints. Fossil footprints can be found in ancient layers of rock that were once mud flats or the soft sand near dried-up lakes and rivers.

Drying
- Fossil footprints can form by a drying process. Prehistoric animals that walked across the surface of a muddy area left footprints that have been found in rock that dried soon afterward. The mud dried before the footprint was disturbed by other animals or washed away by rain. The drying process preserved the footprint. Over time, layers of dirt and sand covered the dry mud footprint. The mud turned to stone over many thousands or millions of years, and the footprint is preserved in the stone.
Underprints
- A fossil footprint can be formed by an impression made deep in soft, muddy layers of sediment. Heavy animals, such as large dinosaurs, left behind footprints deep in layers of earth. When the animal walked across the soft surface, its foot sank deep into the layers of sediment. After the animal lifted its foot, the watery material in the sediment filled the deep impression in the mud. Underprints were protected from surface erosion by wind and rain, because they formed beneath the surface of the soil.
Casts
- A fossil footprint cast was created when an animal left a footprint on soft sand or sediment. The footprint was quickly filled with soft, muddy material and then covered with a different type of soil or sediment. The material that filled the footprint hardened to form a cast of the impression in the sediment. Fossil footprint casts that were covered by sand became embedded in sandstone. The fossil footprints are discovered when the sandstone is worn away by erosion, leaving the harder stone cast.
Human Footprints
- The fossil footprints of early humans have been discovered in Africa. The fossils found in Tanzania in Africa are about 3.6 million years old. Fossil footprints of prehistoric Aboriginal people found in Australia are about 20,000 years old. Early humans, or hominids, stood upright and walked on two feet much like modern humans. The Tanzania footprints, found at Laetoli in Africa, formed in an unusual way. The footprints were left in a wet layer of fine volcanic ash. Rain on volcanic ash created a substance similar to concrete, which preserved the footprints.

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