University of Bristol EARTH SCIENCES

The preservation of footprints depends greatly on the nature of the substrate being walked upon. For example, you would be unlikely to find a track preserved in what was a very dry, sun baked sediment. Equally deep water sediments are unlikely to be disturbed by terrestrial vertebrates. Dinosaur tracks are occasionally preserved in the soft and incoherent surfaces of ancient sand dunes [caused] by slumping or drifting sand." (Thulborn, 1990). Footprint casts have even been found in coal seams formed by dinosaurs trampling on plant debris that became fossilized. Surely one of the most unusual footprints is that of a print found in some trampled dinosaur eggs (Zhao, 1979 in Thulborn, 1990).

Wet ground on or near land represents the best substrate for preserving tracks. "Dinosaur footprints are usually found in coastly laid clastic sediments such as sandstones, siltstones or mudstones." (Thulborn, 1990). "Tracks are most common in deposits that accumulated along lake shorelines, in wetlands and swamps associated with coastal plains, in floodplain environments adjacent to rivers, and around ephemeral or playa lakes in arid or desert environments." (Lockley & Hunt, 1995).

Since shoreline deposits usually represent shifting water lines, sediment can be carried and trapped in the footprint. Once filled, the print is protected from dissolution by the natural cast of sediment.

The Process of Preservation

Deposition can occur over a few metres or hundreds of miles. Large ancient floods covered miles of dinosaur and other tracks and traces. Such sites are known as "megatracksites". (Lockley & Hunt, 1995).

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Tracks are found in the rocks between two layers of deposition on bedding planes. This contrasts with fossil bones and shells which typically occur within the deposited matrix.

Footprints are most likely to be created between cycles of deposition. Periods without deposition, often known as hiatuses, can represent hours, days or even years. Longer periods between deposition may even reflect millions of years, and are known to geologists as "unconformities". (Lockley & Hunt, 1995). The preservation of footprints relies on burial. Exposed prints will be subject to forces of erosion, especially wind and rain. Therefore we are more likely to find ichnites in rocks laid down by reasonably regular cycles of deposition.

"Natural casts tend to be more durable than natural moulds." (Thulborn, 1990). Moulds can be easily overlooked since they often erode out from underneathsandstone ledges. Casts can be easily missed as well. Sunrise and sunset are the best times to look, when long shadows make subtle differences on rock surfaces more obvious.

Three localities around the world known for producing dinosaur footprints are

• the Wealden (late Cretaceous) S.E.England,
• Lark Quarry, Queensland, Australia where "footprint moulds occur in a claystone overlain by sandstone. (Thulborn, 1990).
• Paluxy River, Texas, USA. "Cretaceous limestone moulds filled with clay." (Thulborn, 1990).

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References

Lockley, M.G. & Hunt, A.P. (1995) Dinosaur Tracks and other fossil footprints of the Western United States.Columbia University Press, New York.

Thulborn, R.A. (1990) Dinosaur Traces. Chapman & Hall, London.

Zhao, Z. (1979) Discovery of the dinosaurian eggs and footprint from Neixiang County, Henan Province. Vertebrata Palasiatica, Vol.17: pp.304-9.

Footprint Morphology

Track Taphonomy