Paracrinoidea is an extinct
class
Class, Classes, or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used d ...
of
blastozoa
Blastozoa is a subphylum of extinct echinoderms characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. It ranged from the Cambrian to the Permian. Biserial, triradiate, and pentaradiate ambulacra ...
n
echinoderm
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s. They lived in shallow seas during the Early
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
through the Early
Silurian
The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
. 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
crinoid
Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...
s.
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
In biology, a theca (: thecae) is a sheath or a covering.
Botany
In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a common ar ...
), 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.
[Fossil Invertebrates, 1987, Boardman, Richard S., Cheetham, Alan H., and Rowell, Albert J., editors, pp. 580-1. Blackwell Scientific Publications.]
Only 13 to 15 genera are known. See
List of echinodermata orders
Class Paracrinoidea
* 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
or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...
''
** †Family
Springerocystidae Bassler, 1950 (suggested as paracrinoids related to ''Canadacystis'' 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 ''Canadacystis'' by Sprinkle, 1973 but not Parsley & Mintz, 1975)
* †''Paleocystites''
* †''Ulrichocystis''
Phylogeny after Limbeck (2018)
-+-- ''C. laevis'' (''Cryptocrinites laevis''?)
`--+-- ''Columbocystis ovata'' (''Columbocystis ovata''? Eocrinoid)
, -- ''Columbocystis typica''
`--+--+-- ''Bistomiacystis globosa''
, , -- ''Bistomiacystis schrantzi''
, `-- ''Achradocystites schmidti''
`--+--+-- ''Comarocystites punctatus''
, , -- ''Comarocystites tribrachius''
, , -- ''Implicaticystis shumardi''
, `-- ''Implicaticystis symmetricus''
`--+-- ''Canadocystis tennesseensis''
`--+-- ''Canadocystis barrandei''
, -- ''Canadocystis emmonsi''
`--+-- ''Malocystites murchisoni''
`--+-- ''Sinclairocystis praedicta''
, --+-- ''Amygdalocystites florealis''
, , -- ''Amygdalocystites radiatus''
, , -- ''Oklahomacystis bibrachiatus''
, , -- ''Oklahomacystis spissus''
, , -- ''Oklahomacystis tribrachiatus''
, , -- ''Oklahomacystis trigonis''
, `-- ''Wellerocystis kimmswickensis''
`--+-- ''Globulocystites cristatus''
, -- ''Globulocystites rotundatus''
, -- ''Globulocystites'' (originally ''Platycystites'') ''infundus''
, -- ''Platycystites cristatus''
`-- ''Platycystites faberi''
References
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{{Taxonbar, from=Q142224
Ordovician echinoderms
Silurian echinoderms
Early Ordovician first appearances
Silurian extinctions
Prehistoric animal classes
Echinoderm orders