What are tracks and trails ?

Tracks and trails are two other types of fossils that can be found under the trace fossils category. As with all other trace fossils, tracks and trails also tell more about the organism's behavior rather than the organism itself. These are typically formed when an organism moves over the surface of soft sediment and leaves an impression of its movement behind.

Tracks are the markings of movement that vertebrates leave (i.e. "foot prints"). When birds or other animals search for food, they would often walk over mud flats of rivers or seas. Because these grounds are typically moist, the organisms leave an impression of their feet (claws, paws, etc.). If these impressions are covered with sediment before they get washed away, they become preserved as fossils. This is easy to visualize: just walking through wet mud or wet sand on a beach barefoot would leave your footprints behind.

Trails are similar to tracks, but their patterns are more irregular as trails are markings commonly characterized as impressions left by snails or worms crawling, jelly-fish dragging its tentacles, or the markings left by the movements of crustaceans or sea urchins. Trails are often made on the soft sediment beneath the water surface.

Conditions and location for preservation

Tracks and trails are most commonly found in shallow marine sedimentary rocks; thus they are always subject to tidal washes. Furthermore because tracks and trails are the impressions left by organisms over soft sediment, the sediment must either harden before anything can disturb the markings or be buried by new sediment and remain undisturbed thereafter in order for these impressions to be preserved. A simple analogy to illustrate is one's footprints along the beach. If you have ever walked along the beach, you notice that you leave footprints, but you also notice that once the waves come in, those footprints are gone and so is the evidence of your presence. The same scenario applies in tracks and trails fossilization. Ancient organisms leave their "footprints" in much the same way and thus evidence of their existence.

Foot print formation and preservation

There are four basic stages in the formation of footprint fossils. First, a footprint is formed when an organism steps into soft mud (usually silty sediment that is deposited in shallow temporary pools.) Then the impression is covered with loose sand so that the footprint is filled. The sand eventually consolidate into sandstone and finally the rock split open along the bedding surface to reveal the original footprint in the shale and its cast in the sandstone.

Many footprints have been found to have had occurred from the Mississippian epoch up to present time. The most notable and numerous footprints that have been found belong to the reptiles of the Triassic period---yes, the dinosaurs. Many reptile tracks have been found in the Upper Triassic shaly sandstone of the Connecticut Valley.

What do these fossils tell us ?

Like most trace fossils, tracks and trails do not tell us the specifics on the organism in question; however, they do tell us something about its behavior. In addition, because tracks and trails cannot be preserved out of place (i.e., they are always found in situ), we can be quite sure that where the fossils are found was where the organism use to dwell. Thus, tracks and trails are often used to interpret, among other things, the environmental conditions that existed around that the organism.

Some tracks or trails left by organisms show a systematic pattern where an area of sediment surface is densely and evenly covered with no overlaps. This is interpreted to indicate systematic searching behavior and thus invariably interpreted as feeding traces. Conversely, some tracks or trails are random in their pattern. These types of markings are more often regarded simply as evidence of the organism's locomotion without any implied purpose (e.g., feeding traces.)

Sometimes, tracks and fossils do tell us something about the organism, for example, dinosaur footprints. Examining the footprints left behind by dinosaurs give us a hint as to the size of the dinosaur. Most of the time there is a certain proportionality between an organism's body and its legs (feet.) If the footprint left by the dinosaur is huge, we can only fathom the size of the whole dinosaur. The actual footprint provides us with a better estimate than any museum model.

Index

Amber || Casts & Molds || Compactions || Compressions || Coprolites & Gastroliths

Drying & Dessication || Freezing || Impressions || Molecular Fossils || Permineralization

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