Paracrinoidea is an extinct class of blastozoan echinoderms. They lived in shallow seas during the Early Ordovician through the Early Silurian. While blastozoans are usually characterized by types of respiratory structures present, it is not clear what types of respiratory structures paracrinoids likely had. Despite the taxon's name, the paracrinoids are not closely related to crinoids.
Paracrinoids are characterized by a mouth with two to five feeding arms arranged asymmetrically, or somewhat bisymmetrically. They have a U-shaped gut, and their anus is located next to the mouth. They have irregularly shaped bodies (theca), and a stem, superficially similar to crinoids, and may have used the stem to attach themselves to a substrate, although some reconstructions show them partially buried in sediment.
Only 13 to 15 genera are known. See List of echinodermata orders
Taxonomy Order †Comarocystitida ?Family †Heckeritidae †Heckerites Family †Amygdalocystitidae †Achradocystites Volborth, 1870 †Amygdalocystites Billings, 1854 (= Ottawacystis) (?= Billingsocystis) †Oklahomacystis Parsley & Mintz, 1975 (Bassler, 1943) †Ovulocystites Frest et al., 1980 Family †Comarocystitidae Balther, 1899 †Comarocystites Billings, 1854 †Implicaticystis †Sinclairocystis Order †Platycystitida Family †Platycystitidae Subfamily †Canadocystinae Frest, Strimple, & Coney, 1979 †Canadocystis (=Sigmacystis) Subfamily †Platycystitinae Frest, Strimple, & Coney, 1979 †Globulocystites Frest, Strimple, & Coney, 1979 †Globulocystites rotundatus Frest, Strimple, & Coney, 1979 (type) †Globulocystites cristatus (Bassler) (Referred to Globulocystites by Frest, Strimple, & Coney, 1979) †Globulocystites infundus Frest, Strimple, & McGinnis, 1977, p. 215 (Referred to Globulocystites from Platycystites by Frest, Strimple, & Coney, 1979) †Platycystites Miller, 1889 †Platycystites faberi Miller, 1889 (type) †Platycystites ovalis Frest, Strimple, & Coney, 1979 †Platycystites sp. Frest, Strimple, & Coney, 1979 (NMNH 112086, Blackriverian, Bromide Formation, Carter County, Oklahoma) †Family Malocystitidae †Malocystites †Wellerocystis †Family Bistomiacystitidae †Bistomiacystis Order incertae sedis †Family Springerocystidae Bassler, 1950 (suggested as paracrinoids related to Canadocystis by Sprinkle, 1973, but Parsley & Mintz, 1975 regarded them as eocrinoids, as originally described) †Columbocystis †Foerstecystis †Springerocystis
Doubtful paracrinoids: †Allocystites (suggested as a paracrinoid related to Canadocystis by Sprinkle, 1973 but not Parsley & Mintz, 1975) †Paleocystites †Ulrichocystis
Phylogeny Cladogram after Limbeck (2018).
References Fossil Invertebrates, 1987, Boardman, Richard S., Cheetham, Alan H., and Rowell, Albert J., editors, pp. 580-1. Blackwell Scientific Publications.