Recent cyanobacterial counterparts of Paleozoic Wetheredella and related problematic fossils

Recent and subfossil calcareous structures similar to Paleozoic marine problematic fossils known as Wetheredella Wood, 1948 have been found in a sea-linked, mildly alkaline crater lake on Satonda Island, Indonesia. They are generated by calcifying mats of coccoid cyanobacteria (Pleurocapsa group) growing in crypts and crevices between the foliaceous thalli of calcareous red algae and agglomerations of nubecullinid foraminifers which form small reefs along the lake shore. The Wetheredella-like structures occur at depths ranging from near the water surface down to about 8 m. They are produced by: A) periodic in vivo calcification of surfaces of subglobular cyanobacterial aggregates by Mg calcite, and B) by early post-mortem permineralization of subsurficially located, decaying coccoid cell clusters with microbially mediated aragonite. Periodic in vivo calcification of the mat surface is controlled by seasonal changes in calcium carbonate supersaturation in the epilimnion. Several other problematic fossils such as Aphralysia Garwood, 1914, Asphaltina Mamet, 1972, Asphaltinella Mamet et Roux, 1978, Sphaeroporella Antropov, 1967, and Koskinobullina Cherchi et Schroeder, 1979, which are strikingly similar to the vesicular/tubuloid microbial structures from Satonda Crater Lake, could have, by analogy, also been produced by calcified mats of pleurocapsalean cyanobacteria. They must have thrived in presumably excessively alkaline and calcium carbonate highly supersaturated basins of the paleo-epicontinental seas.