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Paleobotanical Section

Tomescu, Alexandru Mihail Florian [1], Rothwell, Gar W. [1], Honegger, Rosmarie [2].

Cyanobacterial fossils of the Early Silurian Passage Creek biota (Virginia).

Early Silurian (440 Myr old) fluvial deposits in Virginia have yielded cyanobacterial fossils that represent a new type of preservation and provide the oldest conclusive evidence for the presence of cyanobacteria on continents. They reveal that cyanobacteria were part of communities of thalloid and crustose organisms well developed in riverine wetlands at the beginning of the Silurian. The fossils occur in floodplain sediments of the Llandoverian lower Massanutten Sandstone. An 8 cm long carbonaceous compression reveals two types of filaments embedded in coaly amorphous matrix. Rare uniseriate filaments are sinuous, unbranched, and consist of spheroidal units 4.9-8.0 μm in diameter. The majority of filaments consist of 2-3 parallel rows of closely spaced spheroids 3.1-8.0 μm in diameter, included in cylindrical structures 10.8-30.0 μm in diameter that are unbranched, usually straight or gently curved. The spheroids represent spherulitic siderite (FeCO3) aggregates. Several types of evidence indicate that the filaments represent cyanobacteria whose cells have been replaced by diagenetic siderite. (1) They occur in abundance within a carbonaceous compression corresponding to cyanobacterial extracellular matrix. (2) Siderite aggregates have consistent ranges of size and shape that fall within those of cyanobacterial cells. (3) Shapes and sizes of filaments are consistent with those of living cyanobacteria. These data ally the uniseriate filaments with nostocaceans, and the multiseriate filaments with multitrichomous oscillatoriaceans. (4) Preservation of the fossils conforms to results of chemical and taphonomic studies, which have stressed the characteristic resistance to degradation of cyanobacterial extracellular polymeric substances (sheath and slime) in contrast to cell contents. (5) Spherulitic siderite is a common product of diagenesis in reducing sediments rich in organic material, and the replacement of cyanobacterial cells can be explained by a simple model for the conditions present in the floodplain deposits of the lower Massanutten Sandstone.

1 - Ohio University, Department of Environmental and Plant Biology, Porter Hall, Richland Avenue, Athens, Ohio, 45701-2979, USA
2 - University of Zürich, Institute of Plant Biology & Zürich-Basel Plant Science Center, Zollikerstrasse 107, Zürich, CH-8008, Switzerland

fossil preservation

Presentation Type: Paper
Session: 42-8
Location: Maybird (Cliff Lodge)
Date: Wednesday, August 4th, 2004
Time: 10:00 AM
Abstract ID:64

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