Environmental Engineering Reference
In-Depth Information
6. ETHOLOGICAL CLASSIFICATION
Ethology
is the study of behavior. One of
Seilacher's (1953)
crowning achieve-
ments was to recognize that trace fossils could be classified according to their
ethological function because similar functions can result in similar morphology.
A U-shaped burrow made by a polychaete might look much like one made by a
crustacean. One might say that this is a classification made from the tracemaker's
point of view. Seilacher further recognized that these functions depend on the
environment—that is, on the use of available resources and avoidance of danger.
By taking the focus of attention away from identifying tracemakers and instead
analyzing the functions of trace fossils in whole communities, Seilacher made it
possible to study
ichnofacies
—archetypal facies based on their trace-fossil content.
In Seilacher's original ethological classification, traces are categorized into five
main groups: resting traces (cubichnia), dwelling traces (domichnia), crawling or
locomotion traces (repichnia), grazing traces (pascichnia), and feeding traces
(fodinichnia). The singular of each of these terms ends in -
ichnion
. These are still
the most commonly used terms, but additional terms have been added over the years
to incorporate other behavior (
Table 1
). The morphology of trace fossils and
comparison with modern traces are used to deduce ancient behavior as follows.
For a fuller treatment of the underlying concept, see
Frey and Seilacher (1980)
.
6.1 Resting Traces (Cubichnia)
Resting traces, or cubichnia, include traces made by organisms that have dug
into the substrate in order to rest, hide, or perform other relatively stationary
actions such as respiration or rehydration for a relatively brief period of time
(
Martin, 2006
). For instance, a ghost crab may dig into the sediment, leaving
only its eyes and antennae showing (
Martin, 2006
). Sea stars, brittle stars, crus-
taceans, and flatfishes commonly bury themselves in this way (
Sch¨fer, 1972
).
In hard substrates, limpets and sea urchins are examples of animals that bore
shallow pits that might be considered as resting traces (
Bromley, 1970
), because
they meet the morphological criteria. Predators such as stonefish may hide
under sediment to ambush their prey. Reflecting this diversity of functions,
Lessertisseur (1956)
distinguished informally between resting and hiding
traces; these may, however, prove difficult to distinguish in the fossil record.
In most cases, a thin veneer of sediment suffices to conceal the animal;
so, cubichnia tend to be much shallower than they are broad (
Frey and
Seilacher, 1980
). If the animal is startled, then it can emerge rapidly from the
sediment. No lining or other reinforcement is usually provided; none is needed
because the sediment is supported by the tracemaker's body, except perhaps
around respiratory organs to maintain an unimpeded water flow. Because the
animals burrow into the sediment once and then remain nearly motionless,
the trace may retain the sculpture made by the digging appendages—feet, fins,
claws, or podia—which are clues not only to the function of the trace but also to
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