R. Radoičić collected the fossiliferous rocks for this study in the Upper Cretaceous outcrop of Mt. Grebnik, Mirdita Zone, Serbia and Montenegro (Fig. 1). Fossil cyanobacterial fragments occur in great numbers in the lens-shaped deposits of fine-grained mudstone, wackestone and packstone immediately above paleokarstic bauxite deposits (Fig. 2). More were obtained from the collection of P. De Castro, University of Naples, Italy. S. Golubic collected modern specimens of Scytonema on the mud flats (with ponds and mangroves) on the west coast of Andros Island, Bahamas, mainly in the area bounded by the tidal creeks Polamasola and Three Creeks. The collected specimens were preserved both air-dried and in solution of 3% formaldehyde in environmental water.
Preparations and Microscopy
Petrographic thin sections were prepared from compact fine-grained Cretaceous limestone cut both perpendicular to and parallel with to the bedding plane. They were examined using both transmitted and cross-polarized light microscopy. Other sections were subsequently polished, briefly etched with dilute HCl, washed in deionized water, dried, coated with gold-palladium and studied using scanning electron microscopy.
Specimens of modern Scytonema were washed in deionized water prior to analysis and mounted on slides for transmitted light microscopy. Selected samples were gradually dehydrated in an ethanol series and critical-point-dried using liquid CO2. Whole and fragmented specimens were observed by SEM.