Ichnotaxonomy: Finding Patterns in a Welter of Information - Trace Fossils as Indicators of Sedimentary Environments - page 43

according to the number of foot imprints per set and the number of digits per
foot imprint. This had the advantage of grouping the trace fossils of some clades
together, for example, most footprints of birds (two tracks per set, three forward
and one backward digit imprint per track). However, it also had the effect of
grouping trace fossils of distant clades together (e.g., trails of diverse inverte-
brates) while separating others (e.g., trackways of horses, cattle, and dogs).
Morphologically based classifications that ignore biological relationships are
unlikely to have predictive value.
If so, the most obvious way to base a classification of trace fossils would be
one based directly on biological relationships. Biologists and sedimentologists
alike ask, why can we not name trace fossils for the organisms that made them?
But in most cases we do not know, nor can we obtain, this information. The trace
fossils must be understood on their own terms.
2. OBSERVING TRACE FOSSILS
To classify trace fossils, we must first observe them. Many studies have gone
astray because the researcher did not know what to look for, or even how to look.
Like any other fossils, trace fossils have discernible features, some of which are
shared among populations and others of which are unique to individuals. In the
classification of trace fossils, we are usually more interested in shared traits than
in unique ones, most of which represent slight variations of behavior. We try to
observe as many specimens as possible to determine the full range of behavior
represented by these fossil objects. The traits or features that observation shows
are shared by many objects can be called morphological characters .
Generally, there are many more characters to observe in body fossils than in
trace fossils, which makes the classification of trace fossils simpler but also
more ambiguous. Even the best footprint does not preserve all the features of
the body that made it, and a burrow or boring usually shows even less. Fortu-
nately, these trace fossils do show aspects of tracemaker behavior that may not
be inferable from body fossils.
How does the geologist differentiate between sedimentary structures made
by organisms from those made by inorganic processes? This is not always sim-
ple, and particular cases have been debated for decades. However, some guide-
lines apply. Bearing in mind that animals of any one species generally have
much the same shape, usually have only a limited size range, and often belong
to cohorts of the same age and size, the burrows, footprints, and borings made
by animals are likely to have a limited range in size and shape as well. Sediment
riddled by burrows may at first resemble a conglomerate, but a section of the
burrowed sediment is likely to present only a few consistent sizes and shapes
of cross-sections, while the conglomerate may have pebbles of more diverse
size and form. In particular, burrows in the third dimension will generally show
that they are more or less cylindrical, in contrast to pebbles, which should be
relatively spheroidal. The angle of intersection between burrow and outcrop