Environmental Engineering Reference
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burial and preservation of fossils (taphonomy) are of particular interest in strati-
nomy. For a recent work on the taphonomy of trace fossils, see
Savrda (2007)
.
The physical properties of the substrate are of paramount importance in the
classification of trace fossils for biological reasons (
Bertling et al., 2006
).
Organisms are generally restricted to living in either soft substrates or hard sub-
strates, but not both; the distinction is so sharp that burrows and borings are
automatically classified separately, with very little overlap. Further, burrowers
in soft substrates usually are restricted to relatively narrow ranges of grain size
(mostly mud or sand). Animals that live in relatively pure sand are commonly
unable to burrow or even respire in muddy substrates. However, because a com-
plete gradation exists between muddy and sandy substrates and their inhabi-
tants, it is best to consider them all together when classifying their traces.
Borers are mostly restricted by the chemical composition of the substrate—
carbonate, silicate, bone, or wood—and most specialists focus on only one of
these substrates for study. For example, an extensive literature deals with bor-
ings in wooden ships, pilings, and other marine constructions. It has been shown
that some borers remove the substrate chemically and others mechanically; a
chemical process that works in limestone may be effective in loosening the
grains of a calcareous sandstone, but not siliceous sandstone. Accordingly, there
are sound biological reasons to classify borings made in these four kinds of sub-
strates separately, but always using common sense to recognize larger patterns
(
Bertling et al., 2006
).
4.2 Toponomic Classification
Toponomy is the study of form. In this classification, initiated by
Seilacher
(1953)
and modified by
Martinsson (1965, 1970)
, trace fossils are classified
according to their preservation within sedimentary beds, particularly with
regard to a more resistant casting medium such as sandstone (
Fig. 1
). Traces
are made at the surface of a substrate, within a homogeneous substrate, or at
the contact between substrates of different lithology. In the simplest version
of toponomic classification (
Martinsson, 1970
), the observer merely notes
the current relationship of lithologies without considering how they originated.
A trace fossil preserved at the top surface of a bed is called an
epichnion
(plural,
epichnia
); one preserved at the base is called a
hypichnion
(plural,
hypichnia
);
a trace fossil preserved within the bed is an
endichnion
(plural,
endichnia
).
Epichnia and hypichnia together are referred to as
semireliefs,
while trace fos-
sils preserved “in the round” are called
full reliefs
. Semireliefs that protrude
from the bed are said to have positive relief, or alternatively may be called con-
vex, in opposition to concave trace fossils with negative relief. For example, a
convex hypichnion is a trace fossil preserved by casting on the base of a bed and
which protrudes downward from it. Needless to say, if a sandstone slab is found
as talus, then directional indicators must be used to determine the original
orientation (
Reineck and Singh, 1980; Shrock, 1948
).
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