Exploring alternative global stratigraphic correlations across the Ediacaran-Cambrian transition
Fred Bowyer
University of Edinburgh

A major increase in animal (metazoan) diversity during the lower Cambrian (ca. 538.8–521 Ma), termed the ‘Cambrian explosion’, represents one of the most profound evolutionary events in Earth’s history. Whilst metazoan body fossils are present in strata of the preceding Ediacaran Period, the evolutionary links between the Ediacaran and Cambrian biotic records have been notoriously difficult to constrain. This is largely due to: 1) uncertain affinities for the majority of the Ediacaran biota, 2) variable stratigraphic completeness across the Ediacaran-Cambrian transition, and 3) taphonomic biases and/or endemism of key boundary marker fossils. Despite these complications, a number of significant advances have been made in recent years through focused efforts by international research groups. These include the development and refinement of regional stratigraphic frameworks that incorporate high resolution δ13C chemostratigraphic and paleontological datasets, and the temporal calibration of regional datasets by radiometric geochronology. An over-arching goal is to integrate these regional datasets into a globally representative and radiometrically calibrated framework, which would permit hypothesis testing of (e.g.) scenarios that relate biotic evolution to environmental change, both spatially and temporally.
Here, we first explore regional composite δ13C chemostratigraphic and biostratigraphic datasets (ca. 543–528 Ma), with a focus on four globally distributed successions. We then present new data from fossiliferous sections of remote southeastern Siberia and correlate these within a refined Siberian Platform composite. We compare each regional composite dataset, and construct (by visual alignment) four updated possible global δ13C age models to calibrate associated biostratigraphy, each of which are anchored by available radiometric ages. Differences between age models result from ongoing uncertainties in stratigraphic correlation and alternative temporal calibrations for the Basal Cambrian negative δ13C excursion (BACE). Each age model differs with respect to the degree of overlap between the Ediacaran and Cambrian fossil records. We discuss intervals of uncertainty that require future targeted research, and several key features that are common to all age models, which may together permit a more nuanced understanding of evolutionary dynamics across the Ediacaran-Cambrian boundary.