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

The identity of the fossil
- Decastronema kotori gen. nov., comb. nov.: a mat-forming cyanobacterium on Cretaceous carbonate platforms

The fossil was originally described (Radoičić, 1959) as minute thick-walled cylindrical bags closed at one end, ca 500 to 780 µm long, with 32 to 80 µm external and 10 to 24 µm internal diameters. It was ascribed to the genus Aeolisaccus Elliott, 1958, under the name A. kotori Radoičić, 1959. A supplementary description with photomicrographs was published later (Radoičić, 1972) showing calcareous tubes with thick walls composed of a series of inserted conical units. De Castro (1975) examined and measured large populations of this fossil in shallow-water Senonian carbonates from the Apenninic Carbonate Platform. He recognized that the the peculiar architecture of the walls was comparable to that of modern scytonematacean cyanobacteria and deduced that this fossil was an ancient cyanobacterium.

When observed in petrographic thin sections under transmitted light the fossils appear as cylindrical tubules, each characterized by a dark wall and a clear lumen. The walls are cylindrical and smooth internally, but uneven externally (Fig. 3Alien.gif). They are composed of funnels that diverge outward, and often have thin and undulating margins (Fig. 3Blien.gif). This unique architecture can be examined by optical sectioning, or reconstructed from numerous transverse, oblique and longitudinal cuts through the fossils in thin section. The three-dimensional aspect of the series of inserted funnels is deducible from transitions between saggital and tangential sections (Figs. 3C-Elien.gif). The fossil tubules, of different lengths and randomly oriented, are scattered in the sediment in densities ranging up to 100 tubules mm3. Such a distribution suggests that they were deposited as fragments, possibly after some transportation. They were never seen in growth position.

Longer tubules are sometimes branched, that is the clear central core (lumen) along with the inner layers of the wall penetrates laterally through the outer wall to form a branch. Several cases of this unusual 'inside out' branching pattern were seen and illustrated by De Castro (1975, Pl. 5, figs. 1-9), who recognized them as false branching of the type characteristic of the filaments in the family Scytonemataceae.

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