Cyathotheca suecica and its bearing on the evolution of the Edrioasteroidea



The European Ordovician edrioasteroid Cyathotheca has a variable tubular theca composed of a single plate. The oral surface is covered by five large inter-radial deltoids fused to biserial ambulaeral cover plates, and a periproct in the C-D inter-radius accompanied by two or three adanal plates. Cyathotheca lacks ambulaeral flooring plates. The American and Baltic Ordovician genus Cyathocystis is identical except that it has a complete ring of small peripheral plates surrounding the deltoids. Both genera are assigned to the family Cyathocystidae. The Devonian genera Timeischytes and Hadrochthus are regarded as neotenous offshoots from the Agelacrinitidae and unrelated to cyathocystids. True cyathocystids arose neotenously from the Cambrian stromatocystitids by attachment of the centro-dorsal and failure to develop further aboral or ambulaeral plates. This branch represents a separate evolutionary line to the main stromatocystitid-edrioasterid-isorophid line.