Evolutionary history of chlorophyte algae - organic-walled microbiota from the Neoproterozoic (Tonian) Visingsö Group, Sweden

Omtale

A new assemblage of organic-walled microfossils from the Tonian Period, found in the middle-upper Visingsö Group in Sweden (788-740 Ma), is notably diverse. It includes 34 species of unicellular and colonial microalgae, including new taxa, as well as five species of cyanobacteria. The new taxa are: Leiosphaeridia fibrosa n. sp., L. lacrimaforma n. sp., L. spinopylomifera n. sp., Pterospermella primaria n. sp., Globulus n. gen., Globulus unitus n. sp., Palaeocystis n. gen., Palaeocystis novum n. sp. This assemblage is contemporaneous with the Chuar Group of Grand Canyon, U.S.A. and other Tonian assemblages, and its characteristics help identify evolutionary trends in early eukaryotic microbiotas on a global scale. The affinities of the eukaryotic microbiota are recognized by the phenotypic morphologies and diagnostic reproductive stages (cysts), as known in modern lineages of algae. These fossils are attributed to the classes Pyramimonadales and Chlorophyceae (Chlorophyta), and to representatives of Eustigmatales and Xanthophyceae (Stramenopiles). This biota contributed to the oxygenation of environments and the generation of organic matter, thereby enhancing carbon burial. We evaluated the taxonomy of the microfossils, compiled biochronologic ranges, and reviewed the oldest records from the Palaeo-Mesoproterozoic. This suggests a minimum divergence age for the Phylum Chlorophyta and the Group Stramenopiles at 1744 Ma, and for the primary plastid endosymbiosis and sexual reproduction at a much earlier time. This age is important for testing models of the eukaryotic tree of life and for calibrating molecular clock estimates, which currently appear to be too young. It also helps to reconcile genomic and fossil-based methods for reconstructing phylogeny.