Article: Ultrastructure and early diagenesis of the Viséan alga Koninckopora

The problematical genus Koninckopora is one of a number of fossil dasycladacean algae which possess micritic microstructures. The microstructure of the carbonate which encrusted Koninckopora is compared with microstructures of other plant and invertebrate groups which are known to have transformed from aragonite to calcite. It is concluded that the encrusted parts of Koninckopora were originally composed of calcite not aragonite. Koninckopora thus differs from all living calcareous dasycladaceans and probably most fossil forms. It is suggested that the calcite that encrusted Koninckopora was a stable carbonate, that is, a low-magnesian calcite. Calcitic encrustation has developed on a number of occasions during dasycladacean evolution. The occurrence of an acicular wall in the so-called double-walled Koninckopora has previously been considered to be a primary biogenic structure. It is shown that this acicular layer is an early cement overgrowth. The presence of double-walled forms has been used erroneously as a biostratigraphical indicator in the Visean.