Gloeocapsomorpha prisca Zalessky, 1917: a new study. Part I: Taxonomy, geochemistry, and paleoecology

Gloeocapsomorpha prisca Zalessky, 1917 is emended and redescribed and a neotype is designated from Middle Ordovician kukersites of the Baltic Shale Basin, Estonia. Gloeocapsomorpha was a colonial, probably cyanobacterial organism and its fossil remains are represented by at least three morphotypes. These morphotypes result from growth and life cycle stages which are overprinted by post-mortem changes. Gas chromatography of hydrocarbons from pyrolysed kukersite yields a diagnostic low molecular weight, odd-dominated suite of n-alkanes, maximizing at C19. The emended diagnosis of Gloeocapsomorpha incorporates these biogeochemical criteria. Morphological and biogeochemical characteristics of Gloeocapsomorpha show strong similarities with certain species of the modern Entophysalidaceae cyanobacteria. Entophysalis major, a mat-forming, and sometimes stromatolite-forming, cyanobacterium, is suggested as a modern analogue for G. prisca.