Biological Reviews of the Cambridge Philosophical Society
41: 368-391
Publication
A phylogenetic chart and classification system modified from that used in 'Treatise on invertebrate paleontology, Pt. U' (Durham and others, 1966) is presented. The Ordovician genera Bothriocidaris, Eothuria, Aulechinus, Ectinechinus are considered true echinoids; the latter two are specialized early echinoids not close to the ancestral type. Eothuria, with no lantern, is placed in monotypic order Megalopoda, new superorder Megalopodacea. Characters of the ancestral echinoid are inferred from primitive characteristics that all or some of these genera have in common. The girdle and external gills of Echinothurioida indicate they are euechinoids allied to the diadematoids and not direct descendants of the Paleozoic Echinocystitoida (lepidocentroids). Principal evolutionary trends and modifications of plates and other features are discussed.