Tonian discoidal fossils from North China: Relating discs to worm-like annulated tubes and their paleoecological and evolutionary implications

Abstract

Common members of Proterozoic macrofossil assemblages usually include discs and tubes. Particularly, discoidal forms (e.g., Chuaria and abundant unnamed discs) and tubular forms (e.g., Tawuia and the worm-like annulated taxa Protoarenicola, Pararenicola, and Sinosabellidites) dominate most Tonian macrofossil assemblages. Previous work on Proterozoic materials indicated that some discoidal fossils could be detached holdfasts of co-occurring erect benthic organisms, e.g., some tubular fossils and frondose fossils, but little attempt has been made to quantify the potential relationship between the discoidal fossils and the discoidal holdfasts of tubular fossils from Tonian successions. In this study, we find biometric and chemospatial links between discoidal fossils with a central opening and worm-like annulated tubular fossils from the Tonian Jiuliqiao Formation (∼950–720 Ma) in Huainan region, northern Anhui province, North China, which supports the notion that discoidal fossils represent detached holdfasts of tubular fossils and that tubular fossils with or without a holdfast share a potential biological or ontogenic relationship. The paleoecological implication of possible holdfast coalescence (physical contact and fusion of two or more adjacent holdfasts) for Pre-Cryogenian benthic ecosystem and the evolutionary implication of this study for other Proterozoic enigmatic discoidal fossils, particularly for those with a central opening or circular ring, are discussed. Detached holdfasts and holdfast coalescence may be more common than we have thought in the Pre-Cryogenian, and the paleoecological and geobiological role of benthic algae and their holdfasts during this period is probably underestimated. Our study also highlights the adaptation of holdfast morphology to Precambrian mat-ground substrate, and provides evidence for the co-evolution of substrate and benthic organismic morphology during this period.