Discovery of a phosphatic helical-looking microstructure in Sphenothallus (Cnidaria) from the Late Ordovician of Estonia: Implications for phosphatic biomineralization

The tube of Sphenothallus cf. longissimus from the Late Ordovician of Estonia features a phosphatic, broadly laminar structure. It consists of four to five major laminae; each of the laminae is homogenous exteriorly and shows remains of fibres at the bottom. The fibres in most external tube parts exhibit a plywood structure akin to that of Carboniferous Sphenothallus specimens from Russia, whilst the microstructure of a fibrous lamella in the internal part of the tube resembles the aragonitic helical structure found in molluscs. The reinterpretation of the plywood structure as an original biomineral structure of Sphenothallus and the discovery of a phosphatic structure resembling the aragonitic helical structure of molluscs suggest that the biomineralization capabilities of Sphenothallus may have been exceptional among Cnidaria. It now appears possible that phosphatic biomineralization was significantly more advanced than carbonate biomineralization within Cnidaria. This enhanced phosphatic biomineralization could have provided Sphenothallus with evolutionary advantages over similar encrusting tubicolous organisms with less developed biomineralization during the Paleozoic era.