Taxonomic revision
- Decastronema kotori gen. nov., comb. nov.: a mat-forming cyanobacterium on Cretaceous carbonate platforms

In 1975 De Castro determined that the microfossil he was studying was misidentified and that its true taxonomic affinity is among the cyanobacteria. This called for a new nomenclatural combination with an assignment to a different genus. However, in view of the remaining uncertainties in the identification of the fossil, De Castro hesitated "to transfer the species Aeolisaccus kotori from a 'genus' to which it certainly does not belong to another to which it might not belong." (...) "For the moment," he concluded, "it seemed preferable to indicate the species under study by the name assigned to it by Radoičić, at least until further information permits to specify the existing doubts".

Here we confirm De Castro's interpretation of Aeolisaccus kotori as fossilized sheaths of an ancient scytonematacean cyanobacterium. Our detailed investigations give us added confidence, so we propose a separate generic identity for this now well-defined microbial fossil, placing it in the Phylum (Division) Cyanobacteria, Order Nostocales, Family Scytonemataceae as:

Decastronema, new genus

Etymology: In honor of Professor Piero De Castro, of the University of Naples, Italy, for his valuable contributions to paleontology.

Diagnosis: Tubular filaments with bright core and dark walls, comprised of divergent and externally tapering layers arranged like a stack of inserted cones. The layers appear concentric in transverse and V-shaped in longitudinal section.

Iconotype: Figs. 3B-C & E

Type species: Decastronema kotori (Radoičić) combinatio nova

Description: Filaments and filament fragments with bright lumen and dark walls (as observed in transmitted light in petrographic thin sections), preserved mostly as tubules with walls comprised of divergent and externally tapering layers (Figs. 3A-F). The remains of the walls are preserved in a distinctive grain arrangement (Figs. 4B-C & G) or by iron-rich, spongy textures (Figs. 4A, D-F & H-I), rust-colored in polished slabs and petrographic thin sections. The tubules of D. kotori have an internal diameter of 14.2 ± 4.05 (330) µm expressed as Mean ± Standard Deviation (n). The walls appear dark and thick in petrographic thin sections, with an external diameter averaging 55.6 ± 12.5 (330) µm (recalculated after De Castro, 1975), characterized by divergent layers that in three dimensions form as a stack of inserted funnels. The outer margins of the divergent layers are often thinned and curved (Figs. 3A-C). Most tubules are short, randomly distributed fragments open at both ends. Larger fragments are often branched, which in well-preserved specimens can be recognized as being one-sided false branches -- the core and inner layers of the wall protrude through a lateral break and perforate the outer layer of the main filament to form a branch.

Decastronema kotori is interpreted as the fossil remains of a large filamentous microorganism, which produced and dwelt in tubular sheaths made of extracellular polymeric substances (EPS, polysaccharides with some polypeptides). The architecture of the sheaths is consistent with the unique mode of growth, development and differentiation in a small group of modern species among the heterocystous cyanobacteria assigned to the genus Scytonema of the section Myochrotes.

A comparison of the fossil with the modern representatives of that genus reveals that both employ a unique and complex mechanism to construct the upwardly divergent sheath of extracellular polymers and false branches. That mechanism involves cellular polarity, differentiation among cells to perform specialized functions and the localization of zones of active growth (meristems). Intercalary N-fixing heterocysts anchor the trichomes in the sheaths, so that their further growth produces false branches. Several fossil and modern taxa of similar morphology exist. Their representative populations range widely in size (Fig. 8).

D. kotori is common in Santonian and lower Campanian sediments, often extraordinary abundant, usually in association with Thaumatoporella parvovesiculifera Rainieri, less commonly with small foraminifera. It has also been reported in rocks ranging in age from Aptian to Paleocene (De Castro, 1975) and found widely distributed in the Apennines (De Castro, 1975), Dinarides (Adriatic Islands, Dalmatian coast, the external Dinaridic domains; Radoičić, 1959; Gušić & Jelaska, 1990) and Helenides (Fleury, 1980).

Comment: The transfer of Aeolisaccus kotori Radoičić to the new genus Decastronema is based on a correction of the attribution of this particular fossil. The legitimacy of the genus Aeolisaccus and its type species A. dunningtoni (Elliott, 1958) is not questioned here. But other species of that genus have been reclassified as foraminifera, e.g. A. tintinniformis Mišík, 1971. A. ampliformis Pantić, 1972, has been assigned tentatively to genus Erlandia, and the systematic position of A. inconstans (Radoičić, 1967) and A. gracilis Pantić, 1972, remains uncertain (see Zaninetti, 1976). We consider the properties of Decastronema kotori unique and defined well enough to merit recognition as a discrete genus regardless of the taxonomic treatment of other species of Aeolisaccus. That Decastronema is related to the modern Scytonema is very probable. However, because of objective limitations imposed by fossilization on the criteria for taxonomic identification, we do not recommend that the names of extant biotaxa be used for fossils.

The proposed revision does include those species of Aeolisaccus that have been identified as cyanobacteria. In addition to Decastronema kotori, this involves only one species: Aeolisaccus barattoloi De Castro, 1989, which is reassigned here as:

Decastronema barattoloi (De Castro) comb. nov.

This fossil shares most morphological characters with D. kotori. It too consists of hollow tubular segments, but is smaller and has thinner walls. The outer diameter is given as less than 33 µm. It ranges from the Maastrichtian, where it is most abundant, to the Danian, across the K/T boundary (De Castro, 1989; Barattolo, 1998).

Comment: The existence of discrete populations of Decastronema with a broad spectrum of sizes is consistent with data on Cretaceous material from various sites and with the speciation of similar forms in comparable modern environments (Fig. 8).

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