PHOTOSYNTHETIC GREEN ALGAE FROM THE EARLY TONIAN NANFEN FORMATION IN NORTH CHINA

The rise of algae in bacterium-dominated oceans marks a critical ecological transitions in Earth’s history. However, little is known about this transition with regard to the causes and timing. This is largely because the early evolution of eukaryotes remains elusive due to the dearth of reliable crown group algal fossils in the Proterozoic Eon. Molecular clocks calibrated by the red algal fossil Bangiomorpha suggest that the divergence between green algae (Viridiplantae) and red algae (Rhodophyta) occurred ~1.25 Ga. However, few unambiguous green algal fossils have been documented until the Ediacaran. Although the putative coenocytic green alga Proterocladus has been reported in the late Tonian (800 to 720 Ma), its biological interpretation as a green alga is compromised by the incomplete preservation of available specimens, and this fossil taxon has been largely ignored in recent molecular clock analyses. To better understand this taxon, which has great potential to unveil the early evolution of green algae and to calibrate molecular clocks, we carried a paleobiological investigation that revealed abundant well-preserved specimens of Proterocladus from the early Tonian (1,000 to 900 Ma) Nanfen Formation in North China. Our study revealed diagnostic cellular characters and general morphologies of Proterocladus, suggesting that the taxon is an erect benthic green algae with close phylogenetic relationship with the extant Chlorophyta. In addition, the large population of Proterocladus from the Nanfen mudstone indicates that crown group green algae may have colonized and flourished in early Tonian oceans. Given the many other occurrences of Tonian benthic algae, including Bangiomorpha, Longfengshanian, and Protoarenicola, photosynthetic eukaryotes may have established a strong basis for their ultimate ecological dominance and their contribution to the rise of atmospheric oxygen levels in the late Neoproterozoic.