The other molecular estimate that exhibits poor correspondence to palaeontological
data, the divergence of crown-gnathostomes, predicts a Late Cambrian event and, by
inference, a hitherto unrecorded interval of crown-gnathostome evolutionary history that
spans the Late Cambrian-Middle Ordovician. Significantly, this interval coincides with
recent discoveries of a swathe of new vertebrate taxa, which are assignable to the
gnathostome total-group, but no further, based upon the available evidence (Sansom et al.
2001). It is quite possible that these new taxa include further Ordovician representatives
of crown-gnathostomes (e.g. Skiichthys Smith and Sansom 1997). The Late Cambrian-
Middle Ordovician gap in the record overlaps well with gaps in the records of other
groups, for example, the echinoderms (Smith 1988a). This hints at a secular bias in the
fossil record as a whole that probably reflects the fact that imperfections in the fossil
record are rooted in imperfections in the rock record (cf. Holland 1995, 2000; Smith
2001; A.B.Smith et al. 2001).
Implications for established hypotheses and scenariosUnderstanding early chordate evolution using an incomplete
fossil recordIf nothing else, molecular clocks have provided the stimulus for palaeontologists to look at
their datasets anew and provide justification for cherished methodologies. This, in turn,
has provided the impetus for the development of old and new methods for assessing the
completeness of the fossil record. These internal assessments provide a means of
determining degrees of confidence in subsets of a dataset, provide cautionary limits in
reading the evolutionary history of particular clades, and provide predictions in our
attempts to recover missing components of the record.
The greatest concern of palaeontologists with regard to the mismatch of molecular
clocks and the fossil record may be that it indicates not only that the fossil record is
substantially incomplete but also, critically, that it is the initial period of the evolutionary
history of these clades that is missing. In such a case, would not the usefulness of the fossil
record, in uncovering the sequences of character change between extant clades, be
compromised? The simple answer to this question is no. The chief value of the fossil
record is that it reduces error in inferring the sequence of character changes that underlie
the establishment of living clades—this has been integral to testing and rejecting models
such as, for example, the origin of paired appendages within vertebrates (Coates 1994).
Whether or not we have a complete sample of the anatomical designs that have been
realized is not relevant; with the fossil record we have a more complete, and continually
expanding, understanding of chordate evolution than would be possible using only the
living biota. Furthermore, fossils help to prevent the identification of homoplasy as
homology in living members of distantly related groups, and identify homologies that
might not otherwise be recognized because of the hundreds of millions of years of
evolutionary change that has occurred subsequent to the divergence of the clades.
Our understanding of early chordate evolution may well be incomplete but it does not
follow that it is incorrect. Further attempts to reconcile the fossil record with the living
biota will lead to further refinement not only to the temporal scale of early chordate
216 THE ORIGIN AND EARLY EVOLUTION OF CHORDATES