setting before and after the bilaterian expansion, and the cnidarian/sponge thickets that
would have constituted benthic ecology before this event are unlikely to have offered
much resistance to bilaterian expansion. Indeed, intriguing evidence for different niche
occupancy of lower metazoans and bilaterians, perhaps a relic of the times before the
bilaterian invasion, has recently been presented (Yuan et al. 2002). Nor can one argue that
the lack of bilaterian community structure would have impeded the early bilaterian
pioneers: some of the most productive communities today are monocultures that do not
depend on high diversity to support a high biomass.
The important conclusion that one can draw from the above considerations is that
bilaterian abundance expansion in its early stages is likely to have been exponential, with
both ecological fragility and diversity-dependent invasion resistance being significantly
weaker than might normally apply. In particular, all connected or close shelf areas that did
not cross climatic zones should have been able to be colonized with high abundance
extremely rapidly, certainly faster than the normal resolution of the fossil record. Given a
broad tectonic setting of rifting continents (Gubanov 2002; Smith 2000), one might
expect that cross-continent transfer would be relatively easy in the early stages of the
radiation, with provinciality only becoming established going into the late Early
Cambrian. Biogeographical studies in the Cambrian have often attempted to demonstrate
‘centre of origin’ theories (e.g. Ushatinskaya 1996). However, some authors have
suggested that Cambrian patterns of biogeography reveal a long prehistory (in order to
have time to generate the distinct patterns seen at first appearance; Fortey et al. 1996)
suggesting that they are dependent upon ancient patterns of vicariance (e.g. Lieberman
1997, 2002). In particular, Lieberman (2002) suggested that the break-up of Pannotia,
around 600–550 Ma, gave rise to the principal vicariant patterns shown by basal
trilobites. These arguments are amongst the strongest available for suggesting a deep
origin of metazoan clades.
Nevertheless, the choice of trilobites as a study clade raises the problems associated
with such an approach in their acutest form. The ancestral trilobites were large,
calcareous organisms that, by hypothesis, diversified rapidly and were geographically
widespread. They were no doubt capable of leaving distinctive trace fossils; and are
among the commonest of Cambrian fossils. Why, then, do trace fossils assignable even to
arthropods appear only towards the end of the Nemakit-Daldynian, and body fossils of
trilobites themselves only at the base of the Atdabanian, some 25 myr after the base of the
Cambrian? Based on the modelling of Tavaré et al. (2002), the enormous gap—of perhaps
100 myr or more of implied cryptic trilobite history—would suggest that the fossil record
did a much worse job of recording early trilobites than early primates, which have a gap of
‘only’ approximately 36 myr between their modelled time of origin and the first
appearance in the fossil record. Yet it would be widely agreed that trilobites have an
excellent fossil record when compared with that of primates. One possible way out of this
undoubted dilemma may be to recognize that the continents drifted apart only slowly
after Pannotian rifting; and the vicariant events identified by these authors do indeed
belong in the Early Cambrian rather than latest Proterozoic, as in Gubanov (2002) and, by
implication, Smith (2000).
GRAHAM E.BUDD AND SÖREN JENSEN 179