Reliability of the fossil recordSo, how can we be sure that the order in which species occur in the fossil record is the
correct order in which they actually evolved? In Figure 5.3A the vertical lines represent
the entire period of existence of species E and F (i.e. their durations in the sense of Foote
and Raup, 1996), not just their stratigraphic ranges. Assume that species E is a Cambrian
trilobite and species F is a Pleistocene snail, so there are literally hundreds of millions of
years between the extinction of the trilobite and the evolution of the snail (=interval 2).
The trilobite cannot possibly supply skeletal parts to be included in the fossil record after
it became extinct. Equally, there cannot be any snail shells available for fossilization until
the base of the Pleistocene only two million years ago. It follows that there is no
possibility whatsoever that the first preserved specimen of the snail can occur in the fossil
record below the first (or for that matter the last) preserved specimen of the trilobite. It is
true that reworked fossils may extend apparent stratigraphic ranges (Figure 5.3B).
Nevertheless, the only way reworked fossils can affect the order of first occurrences of
fossils is if we fail to recognize them as being reworked and if we know of no examples
from their original stratigraphic range. While by no means impossible, the combination of
these two circumstances will be relatively rare.
If species coexisted then there is a possibility that the stratigraphic order of first (or
last) occurrences may be incorrect with respect to their real times of origin (or
extinction). Figure 5.4 illustrates an example where the stratigraphically earliest known
record of the earlier species (V, solid circle) occurs after that of the later species (W, solid
square). However, as soon as we discover an example of species V from level one (open
circle), it is no longer possible for the stratigraphic order to be wrong. We can never
know when this happens, but nevertheless sometimes it must occur and, of course, the
more specimens we collect the more likely we are to discover an example of species V in
level 1.
So when species did not coexist they cannot be preserved in the fossil record in the
wrong order with respect to the order in which they evolved. However, when two
Figure 5.3 A. Total durations of two fossil taxa that do not overlap. Note that their first occurrences
in the fossil record cannot be in the wrong order with respect to their real order of origin. B.
Reworked fossils merely extend apparent ranges (dashed line); they do not alter the order of first
occurrences.
96 CHRISTOPHER R.C.PAUL