Ichnotaxonomy: Finding Patterns in a Welter of Information - Trace Fossils as Indicators of Sedimentary Environments - page 55

pairs of appendages may penetrate more deeply into the sediment than others,
not all of the appendages may be registered in the underprint, that is, the deeper
parts of the trackway. This can lead to a gross overestimate of ichnotaxa in an
assemblage.
The imprints of vertebrate trackways are large enough to show great detail
where favorably preserved. Even individual scales may be represented, and ver-
tebrate ichnologists have become accomplished at identifying footprints with
particular clades of makers. However, footprints are not simple molds of feet,
but the active product of feet that change shape and direction while moving
through the substrate. The analysis of the motions that produce footprints—
tracking—has occupied humans since before history ( Brown, 1999 ) and has
lately entered the computer era with regard to ancient footprints ( Gatesy
et al., 1999 ). To give an example of a commonly encountered pitfall, consider
the motion of a foot of which five digits penetrate the substrate to different
depths. If the rock bearing this footprint is broken open at a shallow level, all
five digits will be represented in the trace. However, if broken at a deeper
level, only three or even two of the digits may be represented. Historically,
ichnogenera were erected on the basis of the number of digits per footprint,
and it has taken time to recognize that some footprint taxa represent incomplete
versions of others. Repichnia include all trackways and most trails, for example,
Chirotherium, Petalichnus, Climactichnites, and Cochlichnus (in its strict
sense).
6.4 Grazing Traces (Pascichnia)
Ecologists differentiate surface and subsurface deposit-feeding because they
require different life strategies and adaptations. In Seilacher's (1953) original
conception, the traces made by surface and subsurface deposit-feeding were
called grazing traces and feeding traces, respectively. Unfortunately, most of
his examples, such as Nereites and Paleodictyon, are now recognized as having
been made at depth within the substrate. The concept remains valid, but at pre-
sent, there are few named examples.
Grazing traces are horizontal or subhorizontal andwere originallymade on or
just below the sediment/water interface. Accordingly, their preservation poten-
tial is low and depends on rapid burial without much erosion. They may be
accompanied by other surface traces such as locomotion traces and the apertures
of dwelling burrows, which are usually not preserved (see Section 6.2 ).
Grazing traces are mostly rather shallow in comparison to their width. In
most cases, they consist of continuous trails, but discontinuous grazing traces
can be made by animals that support their bodies (or at least their eating organs)
off the substrate. After processing the surface sediment, the makers of most
grazing traces avoid crossing their own and other individuals' paths where pos-
sible. Segments of the traces commonly parallel one another. Efficient grazing
may result in back-and-forth or spiral patterns of trails ( Seilacher, 1977 ).