molecular evolution (e.g. Bromham et al. 2000), Aris-Brosou and Ziheng Yang (2000),
using a Bayesian approach on 18S rRNA data, found just that: a burst of rapid evolution
during a bilaterian radiation dated at around 560 Ma. If this pattern is confirmed, then the
apparent conflict between fossil and molecular dates would of course have been resolved;
but more important, the discovery of a relationship between rates of molecular evolution
and periods of high morphological evolution would have profound consequences on how
the two are related. Finally, we would like to stress once more that all of the approaches
outlined above rely absolutely on good phylogenies being available, an approach that
would be surely endorsed by many of the protagonists in the Cambrian explosion debate.
SummaryThe origins of the bilaterians is a classic problem in the fossil/molecular clock dating field.
Here, we have argued that:
(1) The known fossil record has not been misunderstood, and that there are no convincing
bilaterian candidates known from the fossil record until just before the beginning of the
Cambrian (c. 543 Ma), even though there are plentiful sediments older than this that should
reveal them (including cases of exceptional preservation).
(2) Phylogenetic and functional grounds cast doubt on the stem-groups of bilaterian
phyla being totally planktonic or too small to leave any sort of body or trace fossil.
(3) Questions of rock and facies preservation only apply to short-term gaps in the
record, and become increasingly less viable as the length of time to be explained away
increases (the rock record is less complete at the small scale than at the large scale).
(4) Models of diversity increase at the base of radiations fail to take into account the
fact that rate of preservation is related to abundance, rather than to diversity, and that
abundance is likely to increase extremely rapidly at the base of a radiation.
(5) The particular tectonic setting of the Early Cambrian would imply easy shelf-to-
shelf transfer of marine taxa between continents, and the lack of competitive exclusion
and previous community structure would both assist in rapid spread of taxa.
Furthermore, the changes in substrate (Dornbos and Bottjer 2000; Yuan et al. 2002) and
development of ecological linkages (Butterfield 1997) would both produce positive
feedback results. All of these points give strong grounds for doubting that there was an
extensive period of pre-fossil bilaterian history.
(6) Available tests, in terms of broad faunal succession, stem- and crown-group order of
appearance, comparison with recovery after extinctions and with modelling of taxa with
poor preservational potential (primates), all strongly suggest that the lag times for the
best-preserved taxa in the Cambrian are unlikely to be more than a few million years.
(7) Early molecular estimates of the time of origin of the bilaterians (e.g. Wray et al.
1996) have been subsequently subjected to considerable discussion and analysis. Current
models, including a recent one based on fashionable Bayesian methodology, that do not
rely on a strict clock assumption for the rate of molecular change have largely indicated
younger dates for the origin of the Bilateria; although this does not necessarily imply that
these models are more accurate than previous ones.
GRAHAM E.BUDD AND SÖREN JENSEN 187