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"There is no branch of detective science which is so important
and so much neglected as the art of tracing footsteps,"
- Sir Arthur Conan Doyle, creator of Sherlock Holmes,
1891.
This statement might well be applied to ichnology, the science
of fossil trackways and traces. Tracks are much more abundant
than bony remains and, in many cases, extinct animals are
known only from their trackways. An animal may leave behind
millions of foot prints, but only one skeleton.
Trackways are indisputable evidence of the posture and locomotion
of the track maker - how they stood, walked, ran, or even
swam. Palaeontologists can use a trackway to help identify
what type of animal made it. For example, amphibian trackways
are broad and marked by a short stride, indicating the inefficient
locomotion of these creatures on land. The trackways of reptiles
generally are narrower and the stride longer than in the case
of amphibian trackways, indicating more efficient locomotion.
It is possible to distinguish the major groups of animals
based on characteristic foot shapes, but it is difficult,
if not impossible, to identify exactly what animal made a
particular footprint. One of the difficulties is that foot
bones are small and rarely found. For this reason, trackways
are given their own names. Animals known by their footprints
are called ichnofauna. Often, the name of the animal and the
name of the trackway it made are different.
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Trackways
offer unique and important insights into the behavior of extinct
animals. They reveal details about the lives of ancient animals
that are not obvious from skeletal remains. The frequency of
tracks can serve as a census of the community and offer a glimpse
of the ecosystem. Tetrapod trackways weaving between the Walchia
stumps at Brule in Nova Scotia are a good example of this.
The speed of the animals can be calculated from the pattern
and spacing of its footprints. Trackways also tell us about
the environment of the track makers. In particular, the depths
of tracks are good indicators of the water content of the soils,
but they can also be used to determine the slope of the ancient
landscape, the position of an ancient shoreline and, in cases
where they were made underwater, current direction. Often, trackways
occur on mudflats around the margins of lakes.
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In order for an ancient animal to leave its track, conditions
had to be ideal, both before and after it passed by. The ground
had to be soft. Usually it was a muddy surface. The weight of
the animal impressed the footprint into the muddy ground (A).
The sun then baked the footprint until it dried out and became
hard. Then the tracks must be filled with another layer of sediment
(B). The difference in consistency between the two layers is
important in preserving the trackway.
Eventually, the trackway was covered by many layers of sand
and mud. The weight of this material turned the mud and sand
into stone over many millions of years. In a reversal of the
process, erosion by water and wind, exposed the footprints.
The sunken part of the footprint is called a natural mould.
Natural casts are the raised material that originally filled
in the footprint. The cast becomes apparent when the two layers
of stone split apart (C). |
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