|University of Bristol|
Three factors contribute to a footprint (Phil L.Manning, pers.comm.):
Obviously, the size and shape of the trackmaker will affect the type of print made. Larger animals will generally produce deeper and more persistent prints.
The stature and build of the legs and feet will affect the area over which pressure is exerted. Wider feet will have a "snowshoe" effect and therefore leave less of an impression. Column-like legs with compact feet will carry the mass of the body over a smaller area. This will exert a high pressure and produce a more profound disturbance of the substrate.
The prints made by giant brontosaurs are particularly spectacular. "The largest are over three feet long and two feet across, and deep enough to hold sixty gallons of water." (Bakker, 1986).
Soft tissue structures that form characteristic footprint shapes, quite distinct from the skeletal frame of the foot may produce unusual or easily misinterpretted prints. Robert Bakker (1986) suggested that some dinosaurs, in particular the duckbilled hadrosaurs, may have had plump paws rather like camel's feet. The resultant print is suggestive of a webbed foot. Therefore a suggestion that the trackmaker had webbed toes for some type of aquatic lifestyle may be a misinterpretation.
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The nature of the trackmaker's locomotion will also affect the shape of the footprints. Running animals may leave quite different footprints to standing or walking individuals of the same species. Increasing speed will alter the angle at which the foot contacts the ground. It will also affect the force with which the foot strikes the substrate.
Prints made by a running animal represent prints made with greater power than walking prints, but where the force is exerted for a briefer period of time. Such nuances in the physical creation of the print will alter its morphology.
A footprint is usually made in three phases, termed, touch-down, weight bearing and kick-off (Thulborn & Wade, 1989). These are phases known in living tetrapods that have been applied to dinosaur prints. Many animals perform a small rotation of their feet during the progress of a footfall (Phil L.Manning, pers.comm.). This rotation, within the print, will naturally obscure some of the finer details of the foot structure. However, if recognised it may tell us a great deal about the trackmaker's locomotion.
The study of a tracksite in Lark Quarry, Queensland led Richard Thulborn and Mary Wade (1989) to some inferences about ornithopod and coelurosaur locomotion. They found retroscratches, which are marks created by the foot slipping back through the footprint, were more common in coelurosaur tracks than for ornithopods. They suggested coelurosaurs had less flexible toes. They further suggested that because coelurosaur trackways often featured many gaps or missing footprints the coelurosaur must have had broad feet that acted like snowshoes. The impact of their feet on the substrate was therefore less noticeable than the ornithopods.
The webbing of footprints, as described in the Morphology section can be the result of kinematic processes also. In some cases webbing may be caused by the foot sliding in the footprint during one or more of the phases of locomotion (Phil L.Manning, pers.comm.).
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The nature of the ground will naturally affect the final print made because the foot and the substrate interact to produce a track. Varying degrees of substrate-particle size and moisture saturation will affect how well a print is represented after the animals has moved on. Sticky sediments, for example, can cause lumps of substrate to be pulled from the print and cast back into the track or over the trackway. Also, "withdrawal of the foot from wet sediment often creates a suction effect so that the walls of the footprint tend to be drawn inwards" (Thulborn, 1990).
Substrates are rarely homogenous and therefore we would expect footprints to vary in quality along a trackway (Thulborn, 1990).
Substrate greatly affects the preservation of footprints as we shall see in the next seciton.
Padian and Olsen (1989) demonstrated the problems of reconstructing foot structure from footprints. They used a modern theropod dinosaur, a Rhea, to make tracks in potter's clay. They then attempted to reconstruct the bird's foot structure using palaeontological methods. Various errors in bone structure were apparent when compared with the actual bone structure of the bird.
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Bakker R. (1988) The Dinosaur Heresies. Penguin Books, London.
Padian, K. & Olsen, P.E. (1989) Ratite footprints and the stance and gait of Mesozoic theropods. In Dinosaur Tracks and Traces (ed. Gillette, D.D., Lockley, M.G.), pp.231-241. Cambridge University Press, Cambridge.
Thulborn, R.A. (1990) Dinosaur Traces. Chapman & Hall, London.
Thulborn, R.A. & Wade, M. (1989) A footprint as a history of movement. In Dinosaur Tracks and Traces (ed. Gillette, D.D., Lockley, M.G.), pp.39-50. Cambridge University Press, Cambridge.