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Biology Articles » Paleobiology » Micropaleobiology » Decastronema kotori gen. nov., comb. nov.: a mat-forming cyanobacterium on Cretaceous carbonate platforms » Stratigraphic vs. paleoenvironmental considerations

Stratigraphic vs. paleoenvironmental considerations
- Decastronema kotori gen. nov., comb. nov.: a mat-forming cyanobacterium on Cretaceous carbonate platforms

Decastronema kotori was found first in the Turonian rocks (Upper Cretaceous) of the external Dinarides (Radoičić, 1959). Subsequently it was found in strata ranging from uppermost Aptian to Paleocene, with a maximum abundance in the Senonian (reviewed by De Castro, 1975). Although the organism survived the massive extinctions at the Cretaceous-Tertiary transition, it has not yet been encountered in earlier or later deposits. This is unusual for a member of cyanobacteria, which as a group diversified early in the Proterozoic and persisted over time while maintaining their conservative phenotypic properties. Heterocystous cyanobacteria, of which Decastronema is one, were reported from Devonian Rhynie chert (Kidston & Lang, 1921), but may have evolved during the Mesoproterozoic, between 1300 and 1500 million years ago, as suggested by the findings of the fossil Archaeoellipsoides, interpreted as akinetes of heterocystous cyanobacteria similar to Anabaena (Golubic et alii, 1995).

The peculiar divergent layering of the sheath of Scytonema and Decastronema along with the false branching of filaments is a stage in the evolution of filamentous cyanobacteria during which the cells differentiate to assume different forms and functions: The 'vegetative' trichome cells fix carbon and grow in localized meristematic zones, the heterocysts fix nitrogen and the akinetes store nutrients and act as resting spores. These properties in modern cyanobacteria serve as phylogenetic markers. The interpretation of the architecture of the Cretaceous fossil is consistent with the presence of these elements in modern heterocystous cyanobacteria.

The time constraints on the range of Decastronema may be environmental as well as evolutionary. If our conjectures based on comparisons with modern Scytonema are correct, the fossil lived on the intertidal flats adjacent to land. The intertidal areas are normally very narrow strips of the coast. Consequently it is possible that the fossil has been overlooked in stratigraphic surveys due to its narrowly defined ecological niche. Environments within the intertidal range are especially vulnerable to frequent changes in sea level that limit recording of the occurrence of intertidal biota to episodes that geologically are very short. Short-term sea level oscillations that follow Milanković cycles, as recorded in microstratigraphic analyses (Longo et alii, 1994; D'Argenio et alii, 1992, 1997; Buonocunto et alii, 2002), may have caused many repeated short exposures and submersions on carbonate platforms as well as significant lateral migration of habitats and their biota. Each such change, although it has an ecologically dramatic impact, is no more than an instant on the geological time scale.

The long-term survival of shallow water biota on carbonate platforms requires that sedimentation be successful in matching subsidence. Their demise may be caused by drowning, as was the case with mid-oceanic platforms (Camoin et alii, 1998), or by major tectonic changes associated with continent collisions, as was the case with the Periadriatic carbonate platforms. Carbonate platforms such as the Bahamas that have existed for a long time are the exception. Their longevity is due to the relative stability of conditions on the trailing side of the continent.

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