ESTIMATING THE TEMPO OF THE CAMBRIAN EXPLOSION

Most previous attempts to reconstruct the timing of fossil appearances in the early Cambrian have relied on the assumption that first appearances are globally synchronous to correlate sections from different regions. However, biostratigraphic methods of correlation are unable to account for the fact that lower Cambrian fossil ranges are strongly facies controlled, or for the significant unconformity in the type Nemakit-Daldynian/Tommotian boundary sections (from Siberia) that creates the appearance of a sudden diversification. Previous work by three of the authors and their colleagues circumvented these problems by instead assuming synchrony of global variation of carbon isotope ratios (δ¹³C) in carbonate rocks, which was used to date a published dataset of fossil occurrences from the Terreneuvian Series. In that absolute timeline, the diversification of skeletal animals appears to have occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca 540–538 Ma), a larger pulse in the mid- to late Nemakit-Daldynian (ca. 534–522 Ma), and a moderate pulse in the Tommotian (ca. 524–522 Ma).

Here we investigate the robustness of these findings after accounting for several sources of uncertainty in dating the origination times of fossil taxa: incorrect correlations between sections, error in radiometric dating, non-uniform sedimentation rates, and incomplete fossil preservation. Using a Monte Carlo simulation algorithm that incorporates multiple potential correlations, inherent margins of error in radiometric dating, several types (linear, exponential, and sinusoidal) of non-uniform sedimentation rates, and confidence intervals for the true time of taxon origination, we are able to produce a statistically rigorous record of animal diversification through the first 25 million years of the Cambrian.