Environmental Engineering Reference
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from particular burrowing or boring techniques adapted to early consolidated
substrates such as firmgrounds and hardgrounds. For instance, some borings
are driven by chemical means, such as
Trypanites
and
Entobia
, and are typical
in carbonate substrates. In general, climatic control has an influence on the dis-
tribution of some trace-fossil groups. Decapod crustaceans are important bur-
rowers and are most abundant in carbonate substrates from tropical and
subtropical zones (
Dworschak, 2000
). As such, burrows like
Ophiomorpha
,
Tha-
lassinoides
,
Spongeliomorpha
,
Psilonichnus
,
Pholeus
, and
Macanopsis
may be
more common in carbonates than in siliciclastics.
Evolution is an important aspect in the generation of trace fossils, as differ-
ent tracemakers change throughout the Phanerozoic. A relevant example is the
occurrence of crustacean burrows in Mesozoic and Cenozoic deposits, whereas
similar trace fossils such as
Thalassinoides
in Paleozoic strata were constructed
by trilobites (e.g.,
Cherns et al., 2006
) or unidentified animals. Other trace fos-
sils (e.g.,
Zoophycos
) are known from shallow-marine carbonates until the
Jurassic but migrate to deeper environments during the Cretaceous and Ceno-
zoic (
Seilacher, 2007
). On an ichnospecific level,
Rhizocorallium commune
(herein regarded as a senior synonym of
R. irregulare
) occurs from the Cam-
brian through Holocene and apparently is the product of polychaetes, whereas
R. jenense
(
sensu stricto
) characterizes Triassic and younger deposits and may
be the product of arthropods. Radular bite traces known as
“Radulichnus
” are
common in Proterozoic and Cambrian matgrounds (
Seilacher, 2007
) but also
appear in the Triassic, where similar environments prevailed after the end-
Permian mass extinction (
Knaust, 2010b
). In contrast, Cretaceous to modern
Radulichnus
occurs in hard substrates (
Bromley, 2004
).
Given the simple case of a homoclinal ramp, general trends can be observed
from proximal (backshore, supra- to intertidal) to distal (shelf, subtidal) set-
tings, although exceptions may occur. These trends include an overall decrease
of burrow size and depth, an increase in ichnodiversity and bioturbation inten-
sity, and a change from predominantly horizontal to vertical burrows (
Laporte,
1969
). The distribution of potential tracemaker benthic organisms is mainly
controlled by the availability of organic matter within the substrate as food
resource. Suspension feeders (e.g., many bivalves, sponges, brachiopods) prefer
a stabilized (firm and hard) substrate in a proximal position (supratidal to shal-
low subtidal), whereas deposit feeders (e.g., some worms, some echinoids,
some bivalves, many gastropods) thrive on and in more distal silty and muddy
bottoms where they find a higher organic content.
2.2 Sediment Types
Carbonates differ from siliciclastics in some important ways. Carbonates are
predominantly biogenic sediments and as such are mainly generated by organ-
isms and their reworked skeletal components. Such deposits are also more prone
to bioerosion by means of chemical dissolution than are siliciclastics.
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