An assemblage of carbonaceous compression macrofossils from the Tonian Changlingzi Formation, Liaonan region, North China

The Tonian Period represents a critical interval for the ecological rise of crown-group eukaryotes. However, our understanding of the evolutionary trajectory of early eukaryotes, particularly regarding morphological and functional innovations, remains incomplete due to limited fossil records. In this study, we describe a new assemblage of carbonaceous compression macrofossils, comprising Chuaria, Tawuia and the ‘worm-like’ annulated tubular fossil Protoarenicola from the Tonian Changlingzi Formation in the Liaonan region of Liaoning Province, North China. Dark discoidal structures, which are interpreted as epibionts, were identified on the surface of these macrofossils. Biometric analysis suggests that Chuaria with a minimum diameter of no less than 1.2 millimeters may share a biological relationship with Tawuia. The Protoarenicola specimens, characterized by their narrow tube width and the continuous distribution of transverse annulation spacing, may represent an ecotype within the genus. This study also highlights the increasing biostratigraphic significance of annulated tubular fossils, including Protoarenicola, Pararenicola and Sinosabellidites, for global Tonian successions. In addition, macroalgae may have played a significant role in promoting primary productivity and the diversification and ecological expansion of small-sized organisms.