Re-evaluating molecular clock maximum age calibrations revives pre-Ediacaran divergence estimates for animals

Abstract

Divergent models persist for the emergence of animals. Lipid biomarkers suggest animals originated before 635 million years ago (Ma), but their body fossil record extends to only 574 Ma. Molecular clocks might differentiate between these records, but we demonstrate how their accuracy and precision hinge on maximum age calibrations. We evaluate the Ediacaran Weng’an Biota (China) as a maximum calibration utilizing microfossils from the Kheseen Biota (Mongolia). These two biotas have similar preservation and microfossil assemblages, containing no definitive animals. The Kheseen Biota, deposited at a time (∼550–526 Ma) when animals were undoubtedly present, questions the validity of the Weng’an calibration. We discuss the uncertainty in maximum calibration strategies and propose multiple pre-Ediacaran deposits with favorable taphonomy, preserving both non-animal body fossils and biomarkers, as currently the most reliable calibrations. Our analyses support a late Tonian to mid-Cryogenian (∼800–700 Ma) origin for Metazoa.