Article: Spicules in Silurian tabulate corals from Canada, and implications for their affinities
Specimens of Favosites from upper Llandovery strata of Anticosti Island show three types of calcite structures, herein interpreted as spicules, preserved within their calices and on top of the last tabula. This is stratigraphically younger material, some 50 m higher than fossils described two decades earlier, in which calcified polyps, each with 12 retracted tentacles, were noted. These more recently found structures show striking similarities in form and position to point, collaret and capstan spicules found in the soft tissues of modern pipe corals, i.e. the Octocorallia (Alcyonacea). Where preserved in a distinct pattern on top of the calcite tabulae, the spicular sclerites in Favosites occur in a particular sequence. Twelve individual, or sometimes six pairs of, triradiate point spicules have shrunk to a circlet near the middle of the calice (resting on the last, outermost, tabula). Surrounding the point spicules are 3–6 circlets of curved, usually perforated, lenticular collaret spicules; and surrounding these are scattered, much smaller, capstan spicules. The spicules display variability, probably ontogenetic, in their form and relative sizes; and they are more similar in form to calcareous spicules of alcyonacean corals than to those known from calcareous sponges. Structures with 12-fold radial symmetry in Heliolites, originally described by one of us as ‘septal florets’, consist of elements that are considered comparable with the point spicules found in Favosites. They have been recognized in ten species of Heliolites from Silurian (Wenlock–Ludlow) strata in the Canadian Arctic islands.