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among the most challenging trace fossils to study, yet also some of the most
rewarding, because they are potentially so informative about the history of ter-
restrial environments (
Hasiotis, 2003
). Because of their complexity, calichnia
are more easily linked to tracemakers than most other trace fossils, adding to
the history of life as well.
6.9 Other Ethological Categories
Many other categories have been proposed, some of which are used more or less
frequently by researchers. Among them are swimming traces (natichnia;
M¨ ller,
1962
), flying traces (volichnia;
M¨ ller, 1962
), predation traces (praedichnia;
Ekdale, 1985
), equilibrium traces (equilibrichnia;
Bromley, 1996
: 197;
Frey
and Pemberton, 1985
), aboveground traces (aedifichnia;
Bown and Ratcliffe,
1988
), and the list shows no sign of ending (
Buatois and M
´
ngano, 2011
). Some
of these categories represent behaviors that are uncommon in the geological
record, while others represent special cases of broader categories. At this point,
one has to wonder whether additional terminology is worth the effort of memo-
rization of two new terms, formal and informal, for each concept.
6.10 Overlapping Functions
Most tracemakers produce traces that fit readily into Seilacher's ethological
classification. However, many trace fossils represent more than one function
and thus cannot be shoehorned into a single ethological category. Many feeding
burrows include an open dwelling burrow, for instance, and the distinction
between surface-feeding and simple locomotion is not always unambiguous,
as in trilobite trace fossils (
Goldring, 1985
). The burrows of bivalves are com-
monly considered as dwelling burrows by those who emphasize the traces' rel-
ative permanence, but they are not open burrows and their makers are generally
capable of moving from place to place at need; so, they are classified also as
resting traces by other researchers.
In an ideal taxonomy, each ichnogenus should fit into only one ethological
category, and most researchers have followed this guideline. Where single
specimens incorporate complex behavior, as in
Hillichnus
(
Bromley et al.,
2003
), the observer has the choice of giving more than one name to the trace
fossil along its path or subsuming all the behaviors under one name. In prac-
tice, the first choice is the most common, because most organisms apparently
perform only one behavior at a time, and the different behaviors typically
result in traces of different morphology that are not usually found connected.
This is the case for the grazing trace
Cruziana
and the resting trace
Rusophy-
cus,
which are sometimes found connected;
Seilacher (1970)
emphasized the
identity of the trilobite tracemakers while others emphasized the difference in
ethology. In the case of
Hillichnus,
a bivalve tracemaker incorporated behav-
iors as different as locomotion, feeding, and respiration so intimately into
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