The lower Carboniferous blastoid Astrocrinus tetragonus (Austin and Austin), from Grassington, Yorkshire, and the validity of Astrocrinus benniei (Etheridge)

Abstract

Despite surviving longer than any other blastozoans, blastoids were exceptionally conservative in their morphology and usually symmetrically pentaradiate. Astrocrinus is a rare exception that lacked a stem and some thecal plates, although differing interpretations of its morphology and taxonomy have been published. Astrocrinus had a flattened, tetralobate theca covered in minute spines, and with a plane of symmetry through the AB interray and the D ray. Four ambulacra are long, thin, and curved down deep sinuses between the thecal lobes to reach the basal surface. The D ambulacrum is short, broad and horizontal. Astrocrinus tetragonus was first described from the Carboniferous Limestone of Settle, Yorkshire. Here it is recorded for the first time from the Brigantian, near Grassington. The new specimens confirm that A. tetragonus had a single basal plate which is kite-shaped, entirely surrounded by four radials and separated from the D ray radial. Astrocrinus benniei was described from the Scottish Brigantian and its basal plate is elongate pentagonal with a short common suture with the D ray radial. Astrocrinus occurs in the Irish Asbian and Brigantian. To date, only A. tetragonus can be confirmed from Ireland.