DiscussionAdvocates of the gap between fossil and molecular evidence for the origin of major clades
being explicable by the general poverty of the fossil record are sometimes content to rest
their case merely on showing—on modelled or other grounds—that such a gap is
plausible. Nevertheless, ideally one would want to go beyond this merely theoretical
stratum of the problem and ask what is meant to be taking place during this period of
time. Is the lack because of low diversity? Or low abundance? Or poor preservability? Or
lack of rock? In such a way, it is possible to see that the question of the poverty of the
fossil record can be analysed further into its constituent components, each of which can then
be critically examined and considered in the context of the particular radiation in question.
Figure 9.2 Stem- and crown-group appearances of bilaterian phyla. Cambrian timescale as in Budd et
al. (2001)—note the condensation of the post-Cambrian timescale. For data, see Table 9.1. The
terminations of stem lineages are essentially arbitrary. The shaded areas show the coverage of
Ediacaran and Burgess Shale exceptional preservation. Note the important ‘missing interval’ where
this model predicts many stem-lineage taxa of the bilaterian phyla should be evolving: a clear test of
the model.
184 DATING THE ORIGIN OF BILATERIA