A test of whether rates of speciation were unusually high during the Cambrian radiation

The Cambrian radiation represents an interval when nearly 20 animal phyla a ppear in the fossil record in a short geological time span; however, whethe r this radiation also represents a period of extremely rapid speciation rem ains unclear. Here, a stochastic framework is used to test the null hypothe sis that diversity changes in one of the dominant Early Cambrian groups, th e olenelloid trilobites, could be produced by tempos of speciation known to have operated during later time periods. Two continuous-time models, the Y ule model and the birth and death process model, and one discrete-time mode l. the Bienayme-Galton-Watson branching process model, were used. No statis tical evidence for uniquely high rates of speciation during the radiation i n these trilobites was found when the continuous-time models were used with low or moderate extinction rates, (lie rates typically associated,vith the Cambrian radiation, although the p values are fairly low or, in one case, significant when high extinction rates were used. However, rates of speciat ion were higher than the average Phanerozoic rates of speciation. The discr ete-time model produced equivocal results: either rates were unusually high or the model is inapplicable during the Cambrian radiation. This suggests that there was nothing unique about evolutionary processes relating to the tempo of speciation during the Cambrian radiation.