Environmental Engineering Reference
In-Depth Information
substrates tend to be lined to prevent collapse and to fend off other burrowers as
well as to control water flow.
The dwellers of domichnia can be among the longest lived members of a com-
munity, with burrows changing through time. The burrow morphology of a juve-
nile may be differently shaped from that of an adult; for example, the burrow of
the fiddler crab
Uca pugnax
is initially a simple shaft and is only later extended as
a U-shaped burrow (
Basan and Frey, 1977
). The diameter of a burrow or boring
may increase through time (
Bromley, 1970, 1996
); the size of sculptural elements
also increases. Branches may be added to a burrow system, and others filled up or
closed off (
Sch¨fer, 1972
). Walls and linings may be reinforced, opened for
expansion, or repaired. Burrows may also be shifted by removing material from
one side of the inner wall while adding it to the other, eventually resulting in a
planar structure called a
spreite,
which can deepen or enlarge a dwelling burrow.
A spreite may also be present in feeding burrows.
Large domichnia may be inhabited by different organisms at the same time
(e.g., callianassid shrimp and the innkeeper worm
Urechis caupo;
Bromley,
1996; Fischer and MacGinitie, 1928
), and they may be inherited by later gen-
erations of individuals. After the abandonment of the dwelling trace, it typically
collapses or is filled passively. The trace fossil represents the end product of all
these processes. The modifications made through time by the maker of a dom-
ichnion should be included in its description.
The water circulation of open burrows with more than one aperture, includ-
ing U-shaped burrows and some network burrows, has been extensively studied
by biologists. Most researchers have focused on animals that irrigate their
burrows by active motions of gills or appendages, such as the polychaete
Chaetopterus variopedatus
(
Bromley, 1996
). Irrigation of the burrow increases
the flux of oxygen and nutrients, allowing burrowers such as the echiuran
Urechis caupo
to be filter feeders (
Bromley, 1996
). Where one aperture is raised
higher into the water column than the other, the difference in current velocity
generates a current through the burrow without active irrigation by the trace-
maker (
Vogel, 1978
).
The uppermost parts of dwelling burrows are usually not preserved because
they are removed by physical erosion, or because other animals rework the sub-
strate. Where preserved, the openings (
apertures
) can be highly distinctive and
useful in ichnotaxonomy. They are often narrowed to prevent entry by sedimen-
tary particles or unwanted visitors and to enhance the circulation of water
(
Howard, 1978; Vogel, 1978
). Others with a feeding function open out into fun-
nels, as in
Monocraterion
(
Jensen, 1997
). Many are reinforced more strongly
than the deeper parts of the burrow, in some cases with chimneys protruding
above the substrate as in
Diopatrichnus
(
Kern, 1978
).
In the 1970s, the ichnological consensus was to avoid utilizing easily eroded
parts of trace fossils in ichnotaxonomy (
F
¨
rsich, 1974
) and to emphasize fea-
tures that could be seen in a small piece of the fossil (e.g., for
Ophiomorpha
,
Frey et al., 1978
). This made trace-fossil identification easier, particularly with
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