The problems of the Cambrian fossil recordThe so-called ‘Cambrian explosion’ is, together with the extinction events at the end of
the Cretaceous, the most well-known biological event in Earth history (cf. Conway
Morris 1998a; Knoll and Carroll 1999; Budd and Jensen 2000; Zhuravlev and Riding
2000; Erwin and Davidson 2002 for recent reviews). Whilst it has been most notable for
the rather sudden appearance of invertebrate macrofossils in the record, it is increasingly
being recognized as a very complex interwoven network of events, really a revolution in
how the biological world was organized (e.g. Butterfield 2000; Zhuravlev and Riding
2000). As a result, research has been expanded to consider the context in which this event
took place, typically the period of 100 million years termed ‘Geon 5’ (600–500 Ma: see
Hofmann 1990). New aspects that have been considered recently include changes in the
flora, represented especially by the enigmatic acritarch record (Butterfield 1997; Vidal
and Moczydlowska-Vidal 1997), and the importance of environmental change (e.g. the
evidence for extensive glaciations that in its extreme constitutes the ‘Snowball Earth’
hypothesis; Kirschvink 1992; Hoffmann et al. 1998).
This broadening perspective has led to the inescapable conclusion that the Cambrian
explosion, considered in its widest sense, is a truly important Earth history event. As
such, there can be no possibility that the events of this time are, overall, an illusion
created by the vagaries of preservation in the rock record. It is therefore considerably ironic
that the macroinvertebrate fossil record, which led researchers as far back as Buckland and
Darwin to worry about the problem, is the one under most scrutiny today. Darwin struggled
with the problem of the Cambrian explosion (which according to his stratigraphic
understanding, took place at the base of the Silurian) and recognized the potentially grave
problems it presented for his theory of evolution by natural selection of tiny incremental
changes (Darwin 1985, ch. 9). Indeed, his discussion of the problem in The Origin of
Species is well worth reading for its amusingly modern tone. For Darwin, there was no
possibility that the fossil record as known could truly represent the history of life. There
simply must have been vast periods of time in which the world ‘swarmed with living
creatures’ (Darwin 1985, p. 313; originally published in 1859). Yet his efforts to explain
why these creatures had not been found were lame, and the consequent weakness
represents a genuine tear in the argumentative fabric of The Origin of Species as a whole.
Almost 150 years later, Darwin’s problem remains as stark as ever. Despite extensive
searching, the rock record of the Precambrian (as now defined) has not once yielded a
single truly convincing body fossil of bilaterian affinity. The only possible exceptions to
this rule are provided by the so-called Ediacaran fossils (the most comprehensive overall
introduction in English, tellingly, remains Glaessner 1984; more recent overviews include
Gehling 1991, Runnegar and Fedonkin 1992, Runnegar 1995, Narbonne 1998, and
Grazdhankin and Seilacher 2002). In recent years, two important facts have become
apparent about these highly enigmatic fossils that Glaessner and many others considered as
straightforward members of extant bilaterian phyla. The first is that they are uniformly
younger than previously thought. Glaessner (1984), for example, estimates the oldest
Ediacaran fossils at 650–660 Ma, but more recent dating shows that the oldest Ediacaran
fossils are not much older than 565 Ma, and that diverse Ediacaran assemblages are
younger than about 555 Ma (Martin et al. 2000). Despite the recent fixing of the
Precambrian-Cambrian boundary stratigraphically (Landing 1994), there remains
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