The overall faunal succession of the Precambrian-Cambrian interval therefore gives the
impression of first yielding taxa of poriferan-cnidarian grade (?Ediacarans, plus the first
mineralized taxa), followed by taxa that are reasonably assignable to positions deep in the
protostome clade, followed by crown-group members of the phyla. All things being
equal, one would not expect this succession to occur unless the radiation that generated
these taxa was broadly contemporaneous with their first fossils.
This impression of stratigraphic and phylogenetic congruence is reinforced by
examining the first appearance of inferred stem- and crown-group taxa within particular
clades (Figure 9.2). This diagram should be approached with some caution.The
systematics of many Cambrian groups is highly problematic (Budd and Jensen 2000), and
the distinction into stem- and crown-group forms is not simple. Many taxa, such as the
echinoderms and molluscs, have had wildly differing phylogenetic schemes presented for
them, which would greatly alter the time of appearance of the crown-group. For
example, crown-group echinoderms might appear in the Middle Cambrian or the Lower
Ordovician, according to taste (see discussion in Budd and Jensen 2000). We have here
taken a conservative approach, assuming taxa to be in the stem-group unless definitive
evidence (i.e. a derived character shared with an extant form) is demonstrable. Although
this biases the record towards recovery of early stem-groups, we would argue that this
bias is defensible on grounds of parsimony. If it is not known whether or not a taxon
possesses a particular derived character, then, all things being equal, it is more
parsimonious to assume that it does not possess it than to assume that it does. We hope
that dissatisfaction with our results will in some measure spur increased efforts to
resolving the systematics of Cambrian fossils.
In our analysis, a remarkable pattern is seen: no crown-group taxa appear before the
first stem-group taxa in any of the 10 phyla considered. If stem-group bilaterian radiation
occurred deep in the Proterozoic, then one would expect this signal to be lost, and that
crown-group taxa would be as likely to appear before stem-group taxa. We regard this
pattern as strong evidence that, not only was bilaterian radiation taking place in the latest
Proterozoic, as suggested by the broad faunal succession, but that stem-group phyletic
radiation was a phenomenon of the earliest Cambrian. Both these pieces of evidence
provide critical evidence that the Cambrian explosion was a real event that started off
below the crown-group bilaterian level, contrary to the more extreme molecular results.
One important factor to note arises from revised timescales of the Cambrian. There is a
relatively long period of time between the Ediacaran and Burgess Shale-style
preservations, consisting of most of the Nemakit-Daldynian and the Tommotian, where
this picture of bilaterian evolution predicts that most phylum stem-groups would be
evolving. The test of this model would therefore be to compare any exceptional
preservations found from that interval to the later Burgess Shale-type ones: rather than
consisting of the same sorts of taxa, the assemblages should be considerably less derived.
Some hint of this comes from the many problematic ‘small shelly fossils’ from this era
(e.g. Bengtson et al. 1990): it is reasonable to equate ‘problematic’ with ‘stem-group’ in
many cases!
GRAHAM E.BUDD AND SÖREN JENSEN 183