ESTIMATING THE DURATION AND TEMPO OF THE CAMBRIAN EXPLOSION

Despite recent advances in the understanding of the Cambrian explosion, many details of the event remain unclear, including the rate of diversification, the order of appearances of major clades, and the number and timing of pulses of origination. Here we present a statistically rigorous timeline for the appearances of skeletal animals during the first ~20 million years of the Cambrian Period (Terreneuvian Epoch). Our results build on earlier work by Maloof et al. (2010) that used radiometrically calibrated carbon isotope chemostratigraphy to reconstruct the pattern of appearances through this interval at two-million-year resolution. That work suggested that the diversification of skeletal animals began early and extended throughout this interval, with pulses of appearances near 540–538 Ma, 534–530 Ma, and 524–522 Ma. However, it is not clear to what extent preservational biases and uncertainties in correlation and dating have influenced these patterns. 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 the incompleteness of the fossil record. To do this, we use a Monte Carlo simulation algorithm that incorporates multiple potential correlations, margins of error in radiometric dating, several types (linear, exponential, and sinusoidal) of non-uniform sedimentation rates, and confidence intervals for true times of taxon origination. We also use novel statistical methods to estimate the duration (and thus the rate) of the diversification, and to identify the most likely number of pulses of origination and their timing.