Plating and pectinirhombs of the Ordovician rhombiferan Plethoschisma

Abstract

When first described, Plethoschisma grandis was thought to have an unusual plating arrangement compared to other pleurocystitids, although the ventral surface was largely unknown. New specimens showing more of the ventral surface indicate that Plethoschisma has very similar ventral plating to Deltacystis and that four of the original dorsal plate designations of Plethoschisma were incorrect. A new subfamily, Deltacystinae, is erected to receive the two similarly plated genera. The pectinirhombs of Plethoschisma are reinterpreted as confluent and in the B2/IL2 and IL3/L4 positions, and one of the new specimens has the largest pectinirhomb and greatest number of dichopores of any known glyptocystitid. A phylogenetic analysis of the Pleurocystitidae reveals that the Deltacystinae, new subfamily, is the sister group to the Pleurocystitinae, new subfamily, and that pectinirhombs apparently evolved independently in the Pleurocystitidae and the Cheirocrinidae.