Rugosphaeraster Gale 2021, gen. nov.

Genus Rugosphaeraster gen. nov.

Type species. Rugosphaeraster ruegenensis sp. nov.

Diagnosis. Robust abactinal ossicles block-like, irregularly polygonal to rhombic, mostly equidimensional, with notches for papulae at the corners. Surface of the abactinal ossicles bearing transverse strips or irregularly radial pustules of imperforate stereom. Adambulacrals bear large, irregularly arranged, bifid spine bases.

Derivation of name. From the Latin ruga, with reference to the rugose and striated surfaces of the abactinals.

Remarks. Other than abactinals, only adambulacrals are known. The affinities of the genus are obscure, but the structure of the large abactinals and adambulacrals indicates that this genus belongs to the Sphaerasteridae. The robust structure of the abactinal plates, with vertical sides and large papular notches, is quite different to that found in Valettaster, in which the plates have the form of truncated cones, the spaces between which are packed with secondary ossicles to form a flat exterior surface (Figs 23H–J, 24F, G). In the very thick form, with large papular notches, the abactinal plates bear a resemblance to those of Stauraster primitivus (Fig. 5D, F, L), but this is probably convergent, and no broad radial plates like those of S. primitivus are present in the material. The morphology of some abactinal plate types of R. rugensis gen. et sp. nov. is closely comparable with those of other sphaerasterids. The rhombic to six-sided large, elongated abactinals are similar to actinal interradial ‘I plates’ (see below), and the small plates with slanting sides, indicating imbrication, are similar to actinally positioned ossicles of other sphaerasterids. The adambulacrals are typical of sphaerasterids, in the presence of tall abradial surfaces, a square actinal surface, proximal and distal:proximal pits for ligamentary attachment, and replacement of muscles by carious ligament pits; however, these characters are plesiomorphic for sphaerasterids and do not suggest generic affinities.

A well-preserved specimen of Rugisphaeraster ruegensis gen. et sp. nov. from the Campanian of Germany is present in the collections of the Berlin Museum, which shows the form of the abactinal surface, but is currently unavailable for study.