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Fossil footprints through geologic time
By Allison Vitkus, Karen Chin, and Martin Lockley
Tracks are made when an animal makes an impression in soft sediment like sand or mud, leaving an imprint in the ground. We have all observed animal tracks or trails in modern environments. But how can such tracks be preserved for millions of years?
Left: Modern bird tracks in sand. Photo by Tommy from Arad (CC BY 2.0). Right: Fossil bird tracks in Cretaceous sandstone (UCM specimen 199.10).1
Fossil trackway made by a dinosaur during the Jurassic Period (UCM 186.5 is a cast of the in place US National Park Service specimen).1
It is important to remember that footprints are fossilized under special conditions. Because most tracks are made in wet sediment, they are often preserved near bodies of water or in areas with shallow water, such as lakebeds. This means that fewer animal tracks are preserved in habitats that are not near bodies of water. However, tracks can also be preserved when animals walk on sand dunes moistened by dew.
Different modes of track preservation
Fossil footprints do not always represent the original animal track; multiple types of fossil tracks can be made from a single step. The fossil of the actual footprint, the indentation that is left behind when an animal steps in soft sediment, is called a true track. Sometimes details like skin impressions are preserved in true tracks. The layers of sediment beneath the actual track layer that the animal stepped on are also affected. The dent in these sediments caused by the pressure of the animal's weight is called an undertrack. Undertracks have less detail preserved than true tracks.
True tracks, natural casts, undertracks, and track infills are all types of fossil tracks that can be created from a single step. Figure adapted from Lockley and Hunt (1995).
Sediment that fills an original footprint (true track) and becomes lithified (hardened in stone) represents a natural cast of the track. Natural casts are like three-dimensional replicas of an animal's foot. As such, they can preserve informative foot details (such as skin impressions) just like true tracks. Sometimes depressions in the layer of sediments deposited above tracks can also be filled. These are called track infills. Like undertracks, track infills have less morphological detail. However, in some cases track infills can be used to find fossil footprints which have not been completely exposed by erosion and help paleontologists find true tracks.
Left: True track made by a dinosaur during the Jurassic Period (UCM 192.3/State of Colorado specimen).1 Middle: Natural cast of a track made by an ornithopod dinosaur during the Cretaceous Period (UCM 209.55/US Army Corps of Engineers specimen).1 Right: Plaster cast of a track made by an ancient reptile during the Triassic Period (UCM specimen 140.30).1 Notice the skin impressions on the toes.
Lockley, M.G., and C.A. Meyer. 2000. Dinosaur Tracks and Other Fossil Footprints of Europe. Columbia University Press, New York. 360 pp.
NASA Images. http://www.nasa.gov. Accessed March 5, 2013.
Tucker, M.E. 2001. Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks, 3rd Edition. Wiley-Blackwell. Hoboken, New Jersey.
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