Environmental Engineering Reference
In-Depth Information
Examples of grazing traces include the dabblings of shorebirds as they feed on
animals and microscopic plants in the sediment and the raspings of land snails
on bark and rocks as they feed on algae.
6.5 Feeding Traces (Fodinichnia)
Feeding traces are deposit-feeding traces, that is, the traces made by animals
ingesting subsurface sediment. Most are burrows rather than borings, as rock
is not very nutritious, but feeding traces in wood exist, such as bivalve-made
Teredolites
. Usually, fodinichnia incorporate a dwelling structure that may
be open during the life of the animal and which may be filled passively. The
feeding part of the structure may be actively filled or left open during the life
of the animal. Feeding traces may have horizontal, oblique, or vertical orienta-
tion, and they are extensive. In the simplest case, they consist of simple burrows
of indefinite length, such as
Planolites
or
Macaronichnus;
but feeding traces
may branch or shift by excavating one side of the lumen (the open part of
the burrow) while depositing material on the other. This shifting results in a
spreading planar structure called a
spreite,
which is also seen in dwelling traces.
Feeding traces include some of the morphologically diverse and distinctive
trace fossils, such as linear
Nereites,
branched
Chondrites,
and helical
Zoophycos
.
6.6 Farming Traces (Agrichnia)
Farming traces are a category based on the discovery that some marine inver-
tebrates use mucus-lined burrows as nets to capture meiobenthos (e.g., echiuran
Urechis caupo,
Fischer and MacGinitie, 1928
; polychaete
Paraonis fulgens,
R¨der, 1971
). This inspired the conjecture that some rather complex burrows
might even be microbial farms (
Ekdale et al., 1984
), similar to the galleries
in which leafcutter ants (e.g.,
Atta colombica
) cultivate fungi. The tracemakers
irrigate the burrows, which are constructed in such a way as to maximize the
inner surface area. Some of these burrows even pump sulfide-rich pore water
through the burrow to encourage the growth of sulfide-metabolizing bacteria;
these burrows may include downward extensions to tap this pore water (e.g.,
burrows of the bivalve
Solemya
). Apertures may be few or numerous. Much
is still unknown about these systems; their biology has been studied in
shallow-marine environments, but most of the ichnotaxonomic work has been
directed to deep-sea environments, where these traces are well preserved in tur-
bidites. Meandering
Cosmorhaphe,
coiled
Spirorhaphe,
zigzag
Belorhaphe,
and netlike
Paleodictyon
are possible examples.
A related category, chemichnia, has been proposed for farming traces that
apparently make use of redox differences between oxic and sulfide-rich tiers
in order to cultivate microbes (
Bromley, 1996
). It would go far to explain
the oddities of
Chondrites
and
Zoophycos,
which are currently classified as
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