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insects ( Genise, 2000 ), arthropod trackways ( Minter et al., 2007 ), and many
kinds of borings, including those of sponges, bryozoans, and bivalves
( Bromley, 1970 ). In some cases, specialists have gone so far as to classify trace
fossils in parallel withmodern biological species, for example, boring bryozoans
( Pohowsky, 1978 ) and vertebrate trackways. In the case of boring bryozoans, the
borings accurately represent the shapes of several internal organs; moreover,
the animals die and decay within minutes so that the standard technique for
studying them is to remove the tissues as a matter of course before study of
the borings (J.D. Soule, Allan Hancock Foundation, University of Southern
California, Los Angeles, oral communication, 1982). This is a more ambiguous
case than that of vertebrate trackways, which at best can represent only a small
part of an animal's body and behavior, but which nevertheless often have been
harnessed as representatives of the missing animal itself.
It is always worthwhile to ask what made a trace fossil, but at present the
makers of most trace fossils cannot be identified unambiguously. Fossil foot-
prints represent only a small part of the morphology of the animal. Even in
the most convincing case, that of boring bryozoans, this is an error. It is true
that, a few decades ago, boring bryozoans could be best classified by the mor-
phology of the boring; comparison between living and fossil specimens was
thus facilitated. But as molecular analysis replaces morphology in the study
of living organisms, the classification of modern and fossil borings will become
disconnected again. Fossil borings lack the necessary DNA.
5.2 Ecological Classifications
Trace fossils can also be classified according to their ecological functions. The
most used ecological classification is Seilacher's ethological classification, which
will bediscussed indetail. It isonlyone ofmanypossible ecological classifications,
however. The tiering and ichnoguild concepts also merit brief discussion here.
Ichnologically, a tier is a vertical zone within the substrate that has a charac-
teristic assemblage or association of traces ( Ausich and Bottjer, 1982 ). Each bur-
rowing or boring organism penetrates only so far into the substrate and may even
determine the texture of that tier.Newsubstratesmay be characterized by a pioneer
community; substrates reaching a climax ecosystem may have more fully devel-
oped tiers. The preservation of tiers depends on rates of deposition, and particularly
on the frequency of sudden burial, which preserves well-defined tiers that might
otherwise be erased during more continuous deposition ( Howard, 1978 ).
Traces can also be classified ecologically into ichnoguilds representing sim-
ilar behavior without regard to their biological affinities ( Bromley, 1996 ).
Traces may be grouped according to functional morphology, food source, or
use of space (tiering).
While these concepts are useful in grouping trace fossils found in particular
studies, they have not yet yielded general classifications of trace fossils.
Seilacher's ethological classification is another matter.
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