Environmental Engineering Reference
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incomplete material, and gave a great boost to ichnology among sedimentolo-
gists, but a more recent approach is to use all parts of a trace fossil holistically.
A trilobite specialist, who ignored important characteristics of trilobites because
it made fragments of trilobites easier to identify, would not be taken seriously;
many ichnologists are now making the extra effort needed to add detail and
make finer distinctions where necessary. Where this hampers exact identifica-
tion, open nomenclature is used. Examples of domichnia include
Skolithos
,
Arenicolites, Diplocraterion, Bergaueria, Lockeia,
and
Thalassinoides.
6.3 Locomotion Traces (Repichnia)
Traces, the chief function of which is movement from one place to another, are
called crawling traces or repichnia (
Seilacher, 1953
). A better term is
locomo-
tion traces
because this category includes walking and running traces as well as
those made by crawling. Swimming traces (natichnia;
M¨ ller, 1962
) can be
included here but are often so distinctive morphologically that they are given
their own category.
Locomotion from one place to another generally produces a straight or curved
trace of which the sides are more or less parallel. Excess movement of sediment is
inefficient for this purpose, and locomotion traces tend to be shallow. Movement
may be effected by appendages, as in arthropods, echinoderms, and most verte-
brates, or by the whole body, as in earthworms, often creating a characteristic
bioprint of the tracemaker. Where the animal's body is supported above the sub-
strate by appendages, their contact with the surface leaves a series of imprints
(
tracks
) that together make up a
trackway
. Where the animal's body touches
the surface, the resulting continuous trace is called a
trail
.
Locomotion burrows
represent the subsurface passage of an animal through the sediment without feed-
ing on it. Historically, there has been some confusion over burrows that are now
thought to represent combined feeding and locomotion, such as
Cruziana
(
Goldring, 1985
). If the tracemaker moved a considerable amount of sediment
while moving on or just below the substrate, then feeding may have occurred
as well as locomotion. There is no firm distinction between traces at the substrate
surface and those just below it, which has caused some confusion as well.
Locomotion traces commonly cross over themselves or one another
(
Rindsberg and Martin, 2003
), in some cases making simple traces appear as
if they were branched. One animal may retrace its own path or even follow
another's, and predators commonly follow the path of their prey. This can lead
to compound traces requiring skilful interpretation.
The bioprint of trackways is more distinctive than that of most trace fossils
and so is used extensively in the diagnosis of ichnogenera. The repetition of
imprints in characteristic series allows them to be identified as the work of ani-
mals with two, four, six, eight, or more appendages. Care must be taken to avoid
mistakes; for example, decapod crustaceans such as crabs have ten legs, but
walk on only eight, holding the pincers above the substrate. Also, because some
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