Atractosella, a Silurian alcyonacean octocoral

Fusiform calcitic spicules occur abundantly in the Lower and Upper Visby Beds and sparsely in the basal Hogklint Beds on the Island of Gotland in the Baltic Sea. The stratigraphic interval spans the uppermost Llandoverian to lowest Wenlockian. The spicules can be referred to the genus Atractosella Hinde, 1888, previously interpreted as a sponge. The new species A. cataractaca is described on the basis of the Gotland material. The spicules are built up of concentric layers, representing stages of growth. The surface is usually covered with granula or small thorns. The calcite shows good alignment of the c axes parallel to the spicule axis, but the direction may deviate up to +-8° between opposing sides of the spicule. Occasional branched spicules are interpreted as fused adjacent normal spicules. Two natural spicule associations on a bedding surface show the organism to have had a rounded outline and a diameter of a few centimeters. In all the investigated characters, Atractosella is indistinguishable from a modern soft coral of the family Alcyoniidae. It is interpreted as the earliest known representative of the octocoral order Alcyonacea, extending the range of this group from the Lower Jurassic to the Lower Silurian. Mirmor andreae Lamont, 1978, described originally as an ostracoderm, is reinterpreted as a species of Atractosella. A. cataractaca is common in fine-grained sediments and may have lived attached to hard objects on soft bottoms.