Dating the first Siberian trilobites with a Bayesian, stratigraphic age model

Trilobites are the most diverse animal phylum in the Cambrian, and are critical stratigraphic markers. Despite their significance, the exact sequence of trilobite appearances across the globe, and their respective ages, are difficult to reconstruct due to a scarcity of radiometric dates. Using visual correlation of carbon isotope (δ¹³C) excursions and integration of available U-Pb age constraints, Landing et al. (2021) have estimated the age of the first Siberian trilobites at roughly 521 million years ago (Ma), which would make them the world’s oldest trilobites. Here, we aim to provide a more precise date, with uncertainty, using a Bayesian stratigraphic age model that takes the global signature of δ¹³C excursions to correlate proxy records from multiple locations. Our model integrates radiometric dates across sections, using ages from well-dated sections to inform age estimates in sections with little or no age information. The model works by evaluating the fit of Bayesian splines to different alignments of the sections, using Markov chain Monte Carlo methods to obtain the posterior distributions. This approach provides an objective evaluation of different possible alignments, generating a probabilistic age-depth model. We apply this model to the lower Cambrian sections of the Anti-Atlas mountains in Morocco, and to the Sukharikha River section of the northwestern Siberian platform. The Moroccan sections provide a uniquely detailed δ¹³C record and multiple radiometric dates, and this age information is transferred to the Siberian section and the emergence of trilobites in the fossil record. Preliminary results indicate that the first appearance of Siberian trilobites is younger than previously estimated, and consequently closer in time to the first trilobites in Morocco and elsewhere.

Landing, E., Schmitz, M., Geyer, G., Trayler, R., & Bowring, S. (2021). Precise early Cambrian U–Pb zircon dates bracket the oldest trilobites and archaeocyaths in Moroccan West Gondwana. Geological Magazine, 158(2), 219-238.