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Home » Biology Articles » Paleobiology » Micropaleobiology » Decastronema kotori gen. nov., comb. nov.: a mat-forming cyanobacterium on Cretaceous carbonate platforms » State of preservation

State of preservation
- Decastronema kotori gen. nov., comb. nov.: a mat-forming cyanobacterium on Cretaceous carbonate platforms

The SEM images of polished and lightly etched preparations that expose the fossil structures in low relief show morphological properties the same as those visible in light microscopy (Fig. 4Alien.gif). At a higher resolution the fossils appear to be constructed mainly of minerals. At magnifications ranging from 300 to 4000x the SEM images indicate that preservation of wall structure was accomplished in two ways.

In the more common mode of preservation, the fossil structure is expressed entirely by mineral grains (crystallites) with sharp euhedral boundaries (Figs. 4B-Clien.gif). The walls of the tubules are comprised of very fine micrite with isodiametric grains ranging between 0.3 and 0.8 µm in diameter. The layers of the wall are made up of alternating relatively coarser (0.4-0.8 µm) and finer (0.3-0.5 µm) grains of micrite. Each layer is characterized by a different and uniform grain size of micrite with sharp euhedral boundaries (Fig. 4Glien.gif). In cross sections of the filaments the layers are concentric (Fig. 4Blien.gif), in longitudinal sections V-shaped (Fig. 4Clien.gif). The lumen of the tubules is filled with larger grains 10-15 µm in diameter.

The second mode of preservation was found in specimens with walls of a reddish, rust-color when viewed with an incident light microscope. Under SEM, these walls evince divergent layers (Figs. 4A & D-Flien.gif) like those of the other mode. At higher magnifications the walls are not granular, but spongy with a fine, submicron level porosity (Fig. 4Hlien.gif). The reddish color is due to ferric oxides and hydroxides (limonite). The wall structure in these specimens is less affected by acid so it produces deeper relief as, for example, the conical shape of the layers in the three-dimensional display (Fig. 4Ilien.gif).

In both types of preservation, the lumen of the tubules is filled with relatively large micritic and microsparitic grains (cf. Figs. 4B & Dlien.gif), up to 20 µm wide and 40 µm long, commonly conforming to the internal diameter of the tubule (Fig. 4Elien.gif). The clarity of the lumen observed in thin sections by light microscopy (Fig. 3lien.gif) is explained by the greater conductance of light in the relatively large calcitic grains that occupy the lumen (Figs. 4E-Flien.gif). Their large size suggests that the tubes were empty when the grains were deposited. The clear interiors of the tubes are in marked contrast to the dark appearance of the walls which is caused by light diffraction and scattering in the numerous small crystals.

The two modes of preservation are indistinguishable under transmitted light microscopy. Therefore, it is unlikely that the iron accumulated during the lifetime of the organism. The wall may have been enriched in iron postdepositionally by chelation with the organic matter associated with the sheath. Ferrous iron in solution may have leached from the nearby bauxite deposits, then transported to overlying beds in anoxic ground water and later oxidized to limonite.

The carbonate matrix surrounding the fossils is an irregular network of unsorted carbonate grains of different sizes with streaks of finer grains meandering around islets of larger sparitic grains (Figs. 4A & Flien.gif). The size of the grains comprising these islets often show a progressive centripetal increase, a characteristic of pore fills. This suggests that the fossil was deposited in a fine-grained porous sediment (possibly calcareous mud) and later diagenetically altered by cementation and pore filling. The mudstone - wackestone matrix of the fossils presumably was laid down in an oxidized low-energy shallow marine environment on an ancient carbonate platform. This interpretation is consistent with the lack of preserved organic matter and the absence of reduced minerals.

The abundance of these fossils ranges widely. They may have been distributed over a large area of shallow sea or accumulated in minor depressions on a mud flat. The site in the Mirdita zone of Metohija contains high concentration of fossils in a lens-shaped accumulation. Its position immediately above terrigenous bauxite deposits suggests early transgression. We assume that prior to deposition the fossils had been transported by tidal currents.

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