Geoscience Reference
In-Depth Information
the sampled population. This usually means that their
distribution is random. A random arrangement is best achieved
by using random numbers as the basis of positional
coordinates. Because invariably not all the fossil assemblage is
accessible, subsequent statistical processing of the data has to
be robust: placing minimal reliance on individual variables
possessing normal (Gaussian) distributions, or, preferably, by
being non-parametric. The biota within a quadrat either need to
be recorded through measurements and sketches in the fi eld or
by taking detailed photographs. The latter method has the
advantage that the image analysis software can be used to
analyse the biota and there is a permanent record.
5
Three-dimensional sampling
While exposure of a single bedding plane may yield high-
resolution data, there are numerous limitations to its
application in practice. Inevitably the splitting properties of the
rock matrix will preclude the exposure of large planar areas
without the likelihood of cutting across several bedding
surfaces. Also, full excavation of individual specimens damages
adjacent bedding surfaces and breaks down the sampling
regime on those surfaces. A more realistic approach is to accept
the fact that single bedding planes cannot be excavated reliably
and attempt instead to sample volumes of sedimentary rock. In
this method blocks of rock can be excavated in the fi eld. Where
conditions and permission allow, cut from the rock using
carbide-tipped chainsaws (Figure 5.11), or for small volumes a
disc rock saw. In order to minimize loss of sampling resolution
the bedding-plane package sampled must fall within an interval
of sediment that was deposited under essentially constant
physical conditions, sometimes termed a 'sedimentation unit'.
Figure 5.11
Cutting volumes of
clays containing fossils using a
carbide-tipped chainsaw. The
advantage of this technique is that
the samples can be taken back to
the laboratory for detailed analysis.
(Robert A. Spicer, The Open
University, UK.)
Determining quadrat size - rarifaction or species/area
(or volume) curves
Fossil size and distribution pattern determine quadrat size. If
the quadrat dimensions are larger than those of the fossil
remains then sampling will be meaningful (Figure 5.12a, p. 94).
Suitable quadrat sizes can be determined for the particular
fossil by constructing taxon/area (or volume) curves (Figure
5.12b). Usually this curve is constructed from a pilot study and
may not be applicable to all parts of the fossil assemblage. For
this reason a reasonable safeguard is to double the minimum
size for the main investigation.
Pattern
Figure 5.12c represents three extreme two-dimensional
distribution patterns of a taxon. The density is the same in
each case. Quite different assessments of abundance will be
obtained, using a quadrat of the dimensions shown, depending
on the distribution and number of samples taken.
