TRILOBITE MINERALIZATION, DIACHRONOUS LOWEST OCCURRENCES OF TRILOBITES, AND A GLOBAL LOWER CAMBRIAN SUBDIVISION STANDARD

Appearance of the Cambrian Evolutionary Fauna is marked by a transition between the Terreneuvian Series (without known, or mineralized, trilobites) and Series 2 (commonly with trilobites) and appearance of more complex ecologic communities. Our new dates show the Terreneuvian–Series 2 (ca. 29 Ma) brackets two thirds of the Cambrian, with ca. 13 Ma referable to Series 2.

The Lower Cambrian has severe correlation problems—e.g., provincial tropical vs temperate biotic assemblages. Many reports assume the lowest occurrence (LO) of trilobites defines the Series 2 base, but this reflects immigration, preservation, lithofacies, unconformities, and collecting bias. No geochronologically meaningful “FAD” of trilobites exists, and their LOs are diachronous between paleocontinents based on U-Pb dates, δ¹³C chemozones, and a problematic acritarch biostratigraphy. In addition, conflicting models propose a short vs long early trilobite evolutionary history. Indeed, Terreneuvian trace fossils (Rusophycus, Cruziana) often considered trilobite-produced in higher strata, early redlichioid and olenelloid faunal provinces, and different lowest fallotaspidoid-derived genera on each paleocontinent suggest a long evolutionary history, with trilobite mineralization at ca. 520 Ma in a calcitic ocean.

The LO of trilobite sclerites is at ca. 519 Ma in Morocco and Avalonia (South Wales, New Brunswick [with questionable provenance]). The Morocco material is within a positive δ ¹³C excursion (likely IV). The lowest South China forms appear above an unconformity in the “CARE” excursion at ca. 518 Ma. East Siberian trilobites appear in IV with reappearance of archaeocyath-microbial mounds above unfossiliferous mudstones. Confusingly, the “standard” acritarch zonation places derived Baltic S. mickwitzi Zone trilobites in an S. ornata-Fimbria. Zone while younger Helio.-Skiagia Zone acritarchs occur below the lowest Siberian trilobites, making them (unlikely) equivalent to the Baltic H. kjerulfi Zone. A solution to unreliable biostratigraphic correlations is a global standard for the Series 2 base within an early trilobite sequence at the IV excursion peak—i.e., in the Siberian lower Repinaella Zone (lower Atdabanian) at Zhurinsky Mys. Definition of a GSSP by an isotope excursion is not unprecedented—the Ediacaran GSSP is a strongly negative δ¹³C excursion in a cap carbonate.