Fossils: Window to the past
Tracks, Trails &
What are tracks and trails?
Tracks and trails can categorized as
trace fossils. As with all other trace fossils, tracks and
trails tell more about the organism's behavior rather than the organism
itself. These traces 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 or
paws). If these impressions are covered with sediment before being washed away,
they become preserved as fossils. This is easy to visualize: 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.
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, as
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,
in order for these impressions to be preserved. A simple illustrative analogy
is footprints on a beach. When you walk along the
beach, you leave footprints. But notice that when
waves come in, the 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.) Next, the impression
is covered with loose sand so that the footprint is filled. The sand eventually
consolidates into sandstone and finally the rock splits open along the bedding
surface to reveal the original footprint in the shale and its cast in the
Footprints have been found to have formed 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: 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, we can be quite sure that the fossils are found was where the organism lived. 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 indicates systematic searching behavior and thus
is 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 trails do tell us something about the organism.
In the case of dinosaur footprints, the footprints 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
deduce that the whole dinosaur is also huge. The actual footprint provides
us with a better estimate than a museum model.