A filamentous cyanobacterium showing structured colonial growth from the Early Devonian Rhynie chert

This paper describes a new aquatic filamentous-colonial fossil cyanobacterium from the Early Devonian Rhynie chert that grows on sediment and submerged plant parts. It is associated with the formation of microbial mats, and occurs in structured colonies, in which the individual filaments are aligned more or less parallel into flat, irregular stands, or united radially into hemispherical aggregates; it may also form elongate, fan-shaped tufts. Individual filaments are ∼ 3 μm in diameter, and consist of uniseriate trichomes composed of barrel-shaped cells enveloped in a thin but distinct sheath. Heterocysts and akinetes have not been observed, which suggests that the cyanobacterium belongs to the cyanobacterial subsection III (Oscillatoriales). This is the first account for sessile, structured colonial growth in cyanobacteria from the Rhynie chert.