last appearance of taxa based upon the quality of the intervening record. The chief method
for this is gap analysis which was originally developed to provide confidence limits on
stratigraphic range data in local sections (Marshall 1990), but can be (Marshall 1990) and
has been (Bleiweiss 1998) applied to global datasets provided that the distribution of fossil-
bearing horizons within the observed range is random. The technique provides a means of
determining limits of probability on how far outside the known stratigraphic range of a
taxon the true first (and/or last) appearance might occur—this is proportional to the
density with which the taxon has been found throughout its known stratigraphic range
(Marshall 1990). It follows that the greater the number of horizons from which the taxon
has been recorded, the less likely it is that the true range lies far beyond the limits of the
known range, and vice versa. Gap analysis calculates, at a given level of confidence (e.g. 95
or 99 per cent), an interval within which the true end point (appearance or
disappearance) of a stratigraphic range lies (Marshall 1990). At its simplest, the calculation
assumes constant fossil recovery potential, but techniques have been developed
to incorporate variable recovery potential that may result, for example, from biases in
facies preservation arising from changes in relative sea level (Holland 1995, 2000;
Marshall 1997; Tavaré et al. 2002).
We have calculated 95 and 99 per cent confidence limits for the fossil record of each of
the main groups of fossil and living invertebrate chordates and jawless and jawed
vertebrates using the combined micro- and macrofossil record. The values are presented
in Table 10.1 and are graphically expressed in Figure 10.3.
Table 10.1 Confidence intervals calculated on the basis of the internal relationships of the plesions
included in the main analysis.
‘Sil n’ is the number of records within the interval Cambrian-Silurian, and ‘Sil P>0.95’ and ‘Sil P>0.
99’ are 95 and 99 per cent confidence intervals on the first appearance of the plesion respectively,
based upon the Cambrian-Silurian interval. ‘Ord n’ is the number of records within the interval
Cambrian-Ordovician, and ‘Ord P>0.95’ and ‘Ord P>0.99’ are 95 and 99 per cent confidence
intervals on the first appearance of the plesion, respectively, based upon the Cambrian-Ordovician
interval.
a The fossil record of hagfishes and lampreys is limited to the Carboniferous and, as a result, the
confidence interval calculations are based on their full stratigraphic range, rather than limited to the
pre-Devonian as are the other calculations.
PHILIP C.J.DONOGHUE ET AL. 201