Tubular microfossils from the Sturtian cap carbonates of the Rasthof Formation

The evolution of ecosystems between the two Neoproterozoic low-latitude glaciations (at 716 and ~635 Ma) is poorly understood because few microfossils have been described from rocks of that age. Immediately succeeding the Sturtian low-latitude glacial deposits of the Chuos Formation, cap carbonates of the Rasthof Formation (746-635 Ma) preserve remnants of ancient marine organisms. Here we describe tubular organic-rich microfossils from the Rasthof Formation at the Okaaru locality. Organic-rich and agglutinated tubular microfossils present in the acid-insoluble residues of Rasthof microbialaminites exhibit a range of lengths (125-1300 µm) and widths (15-70 µm). Nearly all tubes have one rounded, closed and sometimes widened end resembling a proloculus and an end with an occasionally visible aperture. The diameters can vary along the lengths of the tubes, sometimes due to the presence of spiraling ridges or transverse constrictions. Angular agglutinated minerals including K-Si-Al, quartz and Ti-rich phases are present on the surfaces of most tubes and their characteristic grain size is 1-3 µm with few grains >5 µm. One specimen consists of a 1-5 µm wide test with an open-ended aperture, and smaller mineral grains are present inside the structure, suggesting that the test was hollow during preservation. Mineral grain sizes vary between samples, with the largest tubes containing the smallest minerals. Tubular microfossils from the Okaaru locality share a number of characteristics with the later Neoproterozoic and Cambrian tubular microfossils, which have been interpreted as examples of the first foraminifera. These characteristics include agglutinated, tubular tests with transverse constrictions, grooves, lineations, or ridges. The deep subtidal environment in which these tubes are found also hosts other agglutinated forms, and thus these fossils are consistent with the paleoenvironment. This finding extends the morphological range of agglutinated tubes into the Cryogenian, suggesting that oceans at this time harbored more representatives of modern eukaryotic lineages than previously thought.