The Paleozoic problematica Wetheredella and Allonema are two aspects of the same organism

Wetheredella is a calcareous encrusting microproblematicum known only from its two-dimensional expression in thin-sections. It occurs in Cambrian through Permian rocks, often in association with Rothpletzella and Girvanella, probable calcifying cyanobacteria. We show that Wetheredella can be produced by sectioning of the Paleozoic incertae sedis sclerobiont Allonema, which has until now been recognized based on its surface, 3D aspect. Therefore, we propose that Wetheredella should be regarded as a junior synonym of Allonema. Structures resembling Wetheredella have also been obtained in thin-sections through another encrusting microproblematicum, Ascodictyon, which, however, differs in the ultrastructure of its wall, which consists of parallel calcite crystals in contrast to radial crystals in Allonema. Allonema specimens used in the study were extracted from the Middle Silurian Mulde Brick-clay Member (Gotland, Sweden), representing an off-platform environment below the photic zone. This, together with the primary calcitic, radial wall structure of Allonema argues against a cyanobacterial or algal affinity, which had been considered for Wetheredella. The taxonomic position of Allonema remains unresolved, but the sclerobiont shares many common characteristics with some Paleozoic encrusting foraminifers. Combining the occurrence data on Allonema in its surface aspect and the “Wetheredella” aspect recognized in thin-sections may shed more light on its affinity and paleoecological functions, particularly its association with cyanobacteria and its role as a framework-builder in Paleozoic reef.