Lower Ordovician Siphonia cylindrica Eichwald, 1840 from north-western Russia: a pseudo-sponge and a natural ‘recorder’ of geological history

The composition, structure and texture of hard, pebble-sized mineral bodies resembling rounded cylinders with a through-going axial cavity, ascribed by Eichwald (1840) to fossil Porifera under the name Siphonia cylindrica, have been re-examined. The objects can be found at two sites in the vicinity of St. Petersburg (north-western Russia), where they are located at the base of a weakly cemented glauconitic sandstone of the Leetse Formation (Lower Ordovician, Floian). Specimens of S. cylindrica from the collection of Eichwald and a new collection gathered by the author were imaged using scanning electron microscopy (SEM) imaging, energy-dispersive X-ray microanalysis (EDX), and X-ray microtomography (micro-CT). The new data negate previous erroneous assumptions about the siliceous sponge nature of these bodies and suggest that these are phosphorite pseudofossils of nodular genesis. Host rock composition and condition, as well as the main features of the formation and reworking of the nodules, were recorded inside the nodules and only now are available for recovery and discussion.