FIRST REPORT OF MICROBIAL MATS WITH PYRITIZED CYANOBACTERIA ON BRACHIOPOD SHELLS FROM THE CINCINNATIAN GROUP (UPPER ORDOVICIAN, KATIAN)

Pyritized filamentous cyanobacteria have been discovered by scanning electron microscopy of Ordovician brachiopods from 19^th century collections in the Natural History Museum, London. The cyanobacteria form microbial mats on three strophomenid brachiopods (Rafinesquina ponderosa) from the Cincinnatian Group (Upper Ordovician, Katian) near or in Cincinnati, Ohio. There is no additional stratigraphic or locality information. The cyanobacterial mats are found only on the concave exterior surfaces of dorsal valves as individual strands and tightly-bound mats of interwoven strands. The cyanobacteria include uniserial, unbranching strands of cells that range from 5 to 9 microns in length and width. Some of individual strands are up to 700 microns long and many are sinuous. The cells are uniform in size and shape along the strands; there are no apparent heterocysts. Some well-preserved specimens have a reticulate structure on the surfaces of the pyritic structures, with each unit a few hundred nanometers in diameter. The morphotype of these Ordovician cyanobacteria resembles that found in the extant Order Oscillatoriales. Along with the brachiopod shell surface, the cyanobacterial mats also overgrow the encrusting cyclostome bryozoan Corynotrypa inflata, including the walls of broken zooids and terminal diaphragms. We suggest the cyanobacterial mats developed shortly before final burial of the brachiopod shells. Since the cyanobacteria were photosynthetic, the shells are inferred to have rested with their dorsal valve exteriors upwards in the photic zone. That Cincinnatian brachiopod shells were occupied by cyanobacteria has been previously well demonstrated by their microborings but this is the first direct evidence of surface microbial mats on the shells. The mats no doubt played a role in the paleoecology of the sclerobiont communities on the brachiopods, and they may have influenced preservation of the shell surfaces by the “death mask” effect. The pyritized cyanobacteria can be detected with a handlens by dark squiggles on the brachiopod shells, but must be confirmed with SEM. We encourage researchers to examine the surfaces of shells from the Cincinnatian and elsewhere to find additional evidence of fossilized bacterial mats.