Fossilized polyp remains in Silurian Heliolites (Anthozoa, Tabulata) from Nunavut, Arctic Canada

Calices within a substantially silicified corallum of Heliolites garnieri Dixon, 1996, from the lower part of the Barlow Inlet Formation (Ludlow) on south-western Devon Island, contain what appear to be the silicified remains of soft tissues of coral polyps. These remains apparently represent the peristomal portion of an oral disc, incorporating a symmetrical radial array of 12 spicules (intra-polyp sclerites), and a membrane-like, possibly epidermal structure. These remains, and more non-descript material in a few coralla belonging to two other species of Heliolites, occur at local intra-corallum discontinuities where polyps died, but the surrounding colony continued skeletal construction. Their preservation indicates that, before the organic remains were fully destroyed by decomposition, they were rapidly enclosed and sealed by precipitation of microcrystalline quartz or a precursor, and that the failed calices were capped rapidly by local construction of basal epitheca. Clusters of hollow microspheres were also preserved at these discontinuities, suggestive of micro-organisms with a predilection for sites of tissue decay; these microbial decay agents possibly helped to isolate microenvironments conducive to early precipitation of silica.