New data on perittocrinids and hybocrinids (Crinoidea, Echinodermata) from the Middle Ordovician of the Baltic region

Age variation of radial plates of perittocrinids and aberrant cups of the hybocrinid genus Hoplocrinus from the Middle Ordovician of the St-Petersburg region are examined. Three patterns for the development of folds on the radial plates of perittocrinids are recognized. The differences between patterns are of species level. It is proposed here that the folded structures of perittocrinids (as well as the pore-structures of other pelmatozoans) were not only used for respiration, but also very likely for feeding. Tetracionocrinus transitor is a young individual of a species of the genus Perittocrinus or a paedomorphic species of this genus. The study of aberrant cups of Hoplocrinus suggests that its ancestor could have had biserial arms. Phylogenetic implications of this interpretation are discussed, and homologies within the phylum Echinodermata, reassessed. The three-armed genus Baerocrinus may be an aberrant form of a species of the genus Hoplocrinus.