The broad faunal succession of the Cambrian explosionRecent reassessment of terminal Proterozoic biotas has led to two important results.
First, the trace fossil record is much simpler than previously suspected, and progressively
diversifies through this period into the Cambrian (e.g. Budd and Jensen 2000; Jensen in
press); second, the assignment of Ediacaran taxa to the crown-groups of bilaterian or even
cnidarian phyla (e.g. Glaessner 1984) has become increasingly difficult. Whilst the status
of nearly all Precambrian presumed metazoan fossils is problematic, none of them can be
confidently assigned to the bilaterian crown-group (Budd and Jensen 2000). The earliest
mineralizing taxa, such as Cloudina (Grant 1990), Namapoikia (Wood et al. 2002) and
Namacalathus (Grotzinger et al. 2000) all seem in general to be assignable to cnidarian or
poriferan grades of organization, and their morphology does not demand placement in the
Bilateria. The successive faunas include various tubes that have been assigned to many
groups, including the Annelida, but are most reasonably and conservatively thought of as
being cnidarian grade (e.g. Anabarites, for discussion see Bengtson et al. 1990; Kouchinsky
and Bengtson 2002; Budd in press). Finally, taxa such as halkieriids and helcionellids,
which probably demonstrate stem-group protostome to stem-group mollusc affinities
(Conway Morris and Peel 1995; Holmer et al. 2002; see discussion in Budd and Jensen
2000), enter the record just above the base of the Cambrian. It should thus be stressed
once more that the earliest Cambrian does not in general yield crown-group members of
the phyla, which are much more characteristic of the Upper Cambrian and younger
sediments (Budd and Jensen 2000; Budd in press).
Figure 9.1 A schematic representation of the relationship between order of appearance in the fossil
record and phylogenetic position. The approach adopted is to consider paraphyletic assemblages
bounded by particular nodes, and to consider the order of appearance in the fossil record of the
earliest representatives of that phylogenetic interval (cf. Huelsenbeck 1994). In A, the fossil record
is good, and the first representatives of a paraphyletic phylogenetic interval (marked by the small
circles) appear in the same order in the stratigraphic column (right) as the phylogenetic intervals
they represent are arranged in the phylogram (left). In B, the record, and the gap between the true
time of origin of particular paraphyletic intervals is random but long. As a result, the first
appearance of fossil representatives of each interval is often after the first appearance of a
representative of a more derived paraphyletic interval. The degree of congruence of the fossil
record to phylogeny is therefore potentially useful for assessing the length of time between the time
of first appearance in the fossil record of a paraphyletic interval and its true time of origin.
182 DATING THE ORIGIN OF BILATERIA