RECONSTRUCTING THE CAMBRIAN RADIATION

The appearance of diverse fossils in the basal Cambrian is a major evolutionary radiation because of its suddenness, great diversity of body plans, and the first abundant skeletal types. These have been commonly accepted as evidence for rapid evolution of metazoans and other taxa. The event actually consists of the sequential appearance over 10-15 Ma of infaunal bioturbators, small enigmatic sclerites, and body fossils. The alternative longer period of Precambrian evolution is supported by advanced forms in the early Cambrian, some molecular sequence dates, and Neoproterozoic possible bilaterian embryos and a few fossils that resemble later animals. The ancestors of the Cambrian biota are not yet, and perhaps may never be, identified.

However, phylogenetic paleobiogeographic analysis of early Cambrian trilobites indicates vicariant events in their ancestry that were caused by the breakup of continents 20-70 Ma before the Cambrian, indicating a long, hidden evolutionary history. In view of this, we revisit hypotheses for a long bilaterian history without concomitant fossil remains. These include small size, facies differences, marginal habitats, low population density, lack of preservable parts, or some combination of these. Since the Cambrian radiation resembles other incremental taxic radiations (i.e., following the Frasnian-Famennian, Permo-Triassic, and K/T events) special or unusual evolutionary processes need not be invoked. Instead, we suggest that rare ancestral populations were already present in marginal environments and radiated/expanded as environments changed, just as in later radiations. These in fact are the very conditions that encourage evolution in general and speciation in particular, as codified in punctuated equilibrium. The hypothesis can be tested by knowing the typical densities and environmental distributions required for organisms to have a high probability of fossilization and the probability that species can persist at levels below these densities for long periods of time. Data from modern ecology, population structure, other radiations, and the nature of paleontological preservation can prove informative and suggests a process like punctuated equilibrium provides an excellent explanation for the immigration and spread of taxa during the Cambrian radiation.