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animals do not make a hard-and-fast distinction between moving at the surface
of the substrate (which can be soupy) and the sediment immediately beneath it.
There is thus a continuum between surface trails and subsurface locomotion
burrows that makes hash of fine theoretical distinctions between epifaunal
and infaunal behavior. The terms exogenic and endogenic are useful because
they do neatly distinguish between what the modern observer can and cannot
see on a substrate, but the taxonomist should classify an animal's behavior from
the animal's point of view.
In short, the ichnotaxonomist should classify a trace fossil based on its orig-
inal form, as deduced rigorously from its appearance in a large number of exam-
ples, and not rely solely on the current appearance of each individual following
erosion (
Fig. 2
). This advice should not be interpreted as a license to name trace
fossils based on imaginary characters!
5. BIOLOGICAL CLASSIFICATIONS
5.1 Phylogenetic Classification
In the phylogenetic classification, traces are called after their makers, for exam-
ple, a
Phoronis
burrow, the coprolite of
Tyrannosaurus rex,
or more informally,
a bird footprint or buffalo wallow. This classification is the most intuitive and is
also potentially the most informative, and it is used in virtually all treatments of
modern traces (e.g.,
Eiseman et al., 2010; Elbroch, 2003; Elbroch and Marks,
2001
). The aspects of a trace that are useful in identifying its maker may be
called its
bioprint
(
Rindsberg and Kopaska-Merkel, 2005
). However, in the
absence of the organisms and distinctive bioprint, traces cannot always be iden-
tified correctly as to their maker even in modern environments, especially those
that are most unfamiliar to us, such as the deep sea (
Ekdale, 1977; Wetzel, 1981;
Wetzel and Uchman, 2012
). In ancient examples, it is even more difficult to link
a trace fossil with its maker. The two are only rarely preserved together, and
even where a body fossil is found within a trace fossil, a logical case must
be constructed to test whether the organism made the trace instead of simply
harboring or being deposited there.
Another problem is that trace fossils commonly do not contain the right kind
of data to allow them to be classified in a manner even roughly paralleling that
of their biological makers. At the present state of the art, it is not possible to say
that all
Diplocraterion
belong to any one clade, on the contrary. Neither is it
possible to classify all the trace fossils made by a single organism together
as one taxon, because they can be very diverse, a point that is discussed further
under
Section 6
.
The trace fossils of some organisms are more apt for biological classification
than others, especially in cases where the trace fossils are complex and are well
preserved. The effort is worthwhile for an increasingly long list of trace fossils,
notably, trilobite trails and resting traces (
Cruziana
and
Rusophycus;
Seilacher,
1970
), the footprints of fossil vertebrates, the communal burrows of social
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