stratigraphically important groups, the graptolites seem to have the best record, with the
steepest part of the curve between 1925 and 1950. The foraminifera and nautiloids have
the next best records with the steepest part of the curves between 1950 and 1975. All the
other groups have the steepest parts of their curves between 1975 and 2000, suggesting
that stratigraphic ranges of their constituent families are still being extended back in time
on a regular basis. Perhaps range extension is not so unexpected in groups whose
stratigraphic occurrence is considered significant and therefore recorded accurately and
repeatedly.
DiscussionThe prime problem with techniques such as this is to derive a standard against which a
specific curve can be compared. The Zoological Record curve reflects total zoological effort,
not stratigraphic effort on fossil data alone. This may explain why most curves for fossil
groups lie to the left of the Zoological Record curve. Furthermore, the Zoological Record
curve only starts in 1864 which is later than about half the curves for fossil taxa.
However, this does seem to represent a promising approach. If a curve could be derived
that reflected stratigraphic effort reasonably accurately, one could use it to predict the
probability that a taxon of a given size would have a record beginning at any specified
time.
An alternative approach is to consider the shape of the curve for all fossil groups. At least
then one could say which taxa were better (or worse) than the average for the fossil
record as a whole. However, it would still be impossible to say whether or not the data
for the fossil record as a whole are better or worse than predicted on an hypothesis of
random discovery. Note, too, that the fossil data used here were not collected for this
particular purpose and are somewhat suspect.
Conclusions(1) Ghost ranges may be artefacts of cladistic methodology or the result of genuine gaps in
the fossil record.
(2) The stratigraphic order in which fossils occur accurately reflects the order in which
they evolved, irrespective of the completeness (or otherwise) of the fossil record.
(3) It follows from the above that it is unwise to use cladistic methodology to test the
completeness of the fossil record, but congruence with known stratigraphic occurrences is
a powerful test of cladograms.
(4) If the fossil record contains many ghost ranges, earliest stratigraphic occurrences of
fossil families would continually be extended back in time as new discoveries are made.
(5) Such collector curves offer a potential test of the reliability of the fossil record, but
deriving a standard for comparison is not easy. Here I have used the size of annual
volumes of the Zoological Record at decadal intervals from 1864 to 1993.
(6) Most fossil groups have curves that lie to the left of the Zoological Record curve, but
a few start very late and/or parallel the Zoological Record curve closely.
(7) A few fossil groups show signs of flattening out towards the present day despite the
increased effort in recent years shown by the Zoological Record curve.
102 CHRISTOPHER R.C.PAUL