Paracrinoidea is an extinct
class
Class 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 differently ...
of
blastozoa
Blastozoa is a subphylum of extinct animals belonging to Phylum Echinodermata. This subphylum is characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. This subphylum ranged from the Cambrian to ...
n
echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
s. They lived in shallow seas during the Early
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
through the Early
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleoz ...
. 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 animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which 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 (plural 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 comm ...
), 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
This List of echinoderm orders concerns the various classes and orders into which taxonomists categorize the roughly 7000 extant species as well as the extinct species of the exclusively marine phylum Echinodermata.
Subphylum Crinozoa
C ...
Class
Paracrinoid
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 cle ...
ea
* 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 ''problematica'' is a term used for a 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, uncertain ...
''
** †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 deuterostome classes