considerable uncertainty about basal Cambrian correlation, and the likelihood that some
Cambrian rocks contain ‘Ediacaran’ fossils (Jensen et al. 1998) means that rocks generally
thought to be Precambrian because of their (Ediacaran) fossil content may in actuality be
Cambrian. The result of this revision is that Ediacaran fossils appear temporally to be very
close to the Cambrian faunas, and may have a considerable overlap with them. Secondly,
opinion today is hardening that none of these taxa can be unequivocally assigned to crown-
group Bilateria. The candidate most recently favoured by some as a crown bilaterian,
Kimberella (Fedonkin and Waggoner 1997) merely shows relatively complex morphology
that is coupled with bilateral symmetry, compatible with being a stem-group bilaterian or
even, according to some theoretical reconstructions, a stem-group eumetazoan (Jägersten
1972). New morphological details of Ediacaran fossils are continually emerging (e.g. Dzik
and Ivantsov 2002), but these have not necessarily clarified the affinities of the organisms
in question.
Finally, it should be noted that rocks slightly older than Ediacaran age, the Doushantuo
Formation of South-West China, now dated at around 599 Ma (Barfod et al. 2002) have
been claimed to yield embryos of bilaterian affinity (Chen et al. 2000; Xiao 2002),
although this claim has been subject to searching criticism (e.g. Xiao et al. 2000). Reports
of segmented, annelid-like taxa in the Doushantuo Formation (e.g. Chen and Xiao 1991)
and elsewhere (e.g. Russia: Gnilovskaya et al. 2000) are not compelling, and are more
likely to be the remmants of multicellular algae (Xiao et al. 2000). Nevertheless, the
presence of cnidarian-like taxa does seem possible, especially with the recent publication
of further morphological details (Chen et al. 2002), including taxa of apparent tabula-like
structure; although presumably an algal affinity cannot be definitively ruled out. It is
worth noting in this context the great diversity of algae in these sediments (Steiner 1994;
Xiao et al. 2000). If these taxa are stem- or crown-group cnidarians, then the implication
would be that at least stem-group bilaterians would have evolved at this stage.
Nevertheless, the unmineralized nature of the most prominent tabula-like form gives
pause for thought (Xiao et al. 2000); raising questions about when mineralization in
‘corals’ took place. Tabulates are clearly not crown-group corals and, although of
cnidarian grade (e.g. Copper 1985), could conceivably be not particularly closely related
to extant corals. As Xiao et al. (2000) suggest, these taxa might even be stem-group
eumetazoans.
To summarize: the Precambrian fossil record has, to a certain extent, fulfilled
Darwin’s prediction, although not in a way that he would have imagined. Many fossils are
known, including eukaryotes (e.g. Butterfield 2000). Furthermore, very close to the end
of the terminal Proterozoic, many large body fossils appear, but they do not demonstrate
conclusive evidence of being crown-group bilaterians. The expected Darwinian pattern of
a deep fossil history of the bilaterians, potentially showing their gradual development,
stretching hundreds of millions of years into the Precambrian, has singularly failed to
materialize. Since the body fossil record has been so unyielding, attention has shifted in
recent years to the trace fossil record. Trace fossils have certain advantages over body
fossils: they can be, and often are, preserved in sediments, such as sandstones, in which
body fossils are usually absent or fragmentary; they are not (normally) subject to
transport or to other types of biostratinomic alteration; and their preservation is not
dependent on the generating organism possessing hard parts, for example. Indeed, the low
172 DATING THE ORIGIN OF BILATERIA