Precise early Cambrian U–Pb zircon dates bracket the oldest trilobites and archaeocyaths in Moroccan West Gondwana

Abstract

New U–Pb radioisotopic ages on early Cambrian volcanic zircons condition a high-resolution Bayesian age model that constrains the first occurrences and zonations of West Gondwanan archaeocyaths and trilobites in southern Morocco. The oldest archaeocyaths in the Tiout Member of the Igoudine Formation (519.71 + 0.26/− 0.35 Ma) are c. 6 Ma younger than the oldest Siberian archaeocyaths. The oldest Moroccan trilobite fragments, from the lower member of the Igoudine, are constrained to 519.95 + 0.43/− 0.40 Ma. The succeeding Issendalenian Stage (i.e. Hupetina antique – Eofallotaspis tioutensis – Fallotaspis plana – Choubertella – Daguinaspis trilobite zones) spans c. 1.5 Ma (519.78 + 0.26/− 0.37 Ma to 518.43 + 0.25/− 0.69 Ma). Identifiable Moroccan fallotaspidids and bigotinids, among Earth’s oldest trilobites, occur above a positive δ ¹³C excursion dated with our age model at 520.27 + 0.59/− 0.57 Ma, and correlated with the IV excursion peak within the lower range of Siberian Atdabanian Stage trilobites ( Repinaella Zone). This excursion is the best standard for a Cambrian Series 2 base. The oldest West Gondwana trilobite fragments are c. 1 Ma younger than those in Siberia and c. 0.5 Ma older than the oldest Avalonian trilobites ( Callavia Zone). This diachrony means a trilobite first appearance datum is an inappropriate chronostratigraphic base for Cambrian Series 2. Taxonomic differences in the oldest trilobites between Cambrian palaeocontinents are in accordance with trace fossil evidence for the group’s appearance possibly as late as c. 530 Ma in the Cambrian Evolutionary Radiation. Coeval 519–517 Ma dates from Avalonia (cool-water siliciclastic shelf) and West Gondwana (tropical carbonate platform) sections with distinct macrofaunas emphasize these successions were latitudinally separate by the late Ediacaran Period.