Discovery of branched tubular algae and microscopic tubes with annular-helical thickening from the Mesoproterozoic Ruyang Group of Shanxi, North China

Most previously reported Mesoproterozoic (1600-1000 Ma) fossils are from silicified carbonates, and they are commonly small in size (15–20 μm), ranging from coccoids to filaments in morphology. Many of them can be interpreted as cyanobacteria. Carbonaceous shales represent another source of palaeontological information of the Mesoproterozoic biosphere. Sphaeromorphic and acanthomorphic acritarchs, as well as filamentous fossils, are common in many Mesoproterozoic shales. Here we report an unusual assemblage of tubular fossils from the Meosproterozoic Ruyang Group in Shanxi, North China. Some of these tubes are branched. Others bear annular-helical thickening that might be represented by one extinct algae to adapt to a desiccating environment.