POSSIBLE MULTICELLULAR GREEN ALGAE IN EARLY TONIAN MARINE ENVIRONMENTS

Early eukaryotic fossils are crucial to understanding the evolution of the Earth’s biosphere and geosphere. Molecular clock estimates suggest that crown-group members of key algal lineages evolved and diversified by the Tonian Period, but this diversification is poorly reflected in the fossil record. Here we report newly discovered, centimeter-scale macrofossils from outer shelf marine facies of the ca. 950–900 Ma Dolores Creek Formation, Wernecke Mountains (northwestern Canada). These fossils, variably preserved by iron oxides and clay minerals, represent two distinct size classes. The larger cm-scale fossils feature unbranching thalli composed of uniform cells with differentiated cell walls and punitive holdfasts, while the smaller mm-scale specimens display branching. The fossils are likely green macroalgae making them among the oldest yet recognized in the fossil record, thereby filling a critical gap in the timeline of algal evolution. The Dolores Creek macroalgae, together with recently reported ca. 1000 multicellular chlorophyte from North China provide evidence that benthic green algae inhabited shallow marine habitats by the early Tonian Period.