Ctenocystoidea is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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, which lived during the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

and

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 ...

periods. Unlike other echinoderms, ctenocystoids had

bilateral symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symme ...

, or were only very slightly asymmetrical. They are believed to be one of the earliest-diverging branches of echinoderms, with their bilateral symmetry a trait shared with other

deuterostome Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...

s. Ctenocystoids were once classified in the taxon

Homalozoa Homalozoa is an obsolete extinct subphylum of Paleozoic era echinoderms, prehistoric marine invertebrates. They are also referred to as carpoids. Description The Homalozoa lacked the typical pentamer body form of other echinoderms, but all were ...

, also known as Carpoidea, alongside cinctans,

solutes In chemistry, a solution is defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...

, and stylophorans. Homalozoa is now recognized as a polyphyletic group of echinoderms without radial symmetry. Ctenocystoids were geographically widespread during the

Middle Cambrian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (di ...

, with one species surviving into the Late Ordovician.

Description

Like other echinoderms, ctenocystoids had a skeleton made of stereom plates. Ctenocystoids had near bilateral symmetry, with some species exhibiting slightly asymmetric plate shapes. Like cinctans and '' Ctenoimbricata'', most ctenocystoids had large marginal plates surrounding the sides of their body, but unlike cinctans and ''Ctenoimbricata'', which had only one row of marginal plates, most ctenocystoids had two rows of marginal plates. ''Courtessolea'' had only one row of marginal plates, like cinctans and ''Ctenoimbricata'', whereas ''Conollia'' lost the marginal plates entirely. The anus of ctenocystoids was surrounded by a pyramidal periproct as in other echinoderms. It was located at the posterior end, defining a clear anterior-posterior body axis unlike other echinoderms. All ctenocystoids had a ctenoid apparatus, a comb-like arrangement of movable plates at the anterior end of the animal. Beneath the ctenoid apparatus was a mouth, bordered on each side by a food groove.

Classification

Relationships

Ctenocystoids are likely among the most basal stem-group echinoderms. They have also been interpreted as aberrant

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 ...

ns and as stem-group

hemichordates Hemichordata ( ) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and includ ...

. The presence of stereom plates indicates that they most likely belong to the echinoderm total group, rendering a hemichordate affinity unlikely. ''Courtessolea'' was probably the most basal ctenocystoid, given its anatomical similarities to ''Ctenoimbricata'' and cinctans. ''Conollia'' and ''Jugoszovia'' may be closely related to each other, as both have a reduced marginal frame compared to other ctenocystoids.

Genera

The following genera of ctenocystoids have been named:

Distribution

Ctenocystoids were widespread during the Middle Cambrian, and have been found in the United States, the United Kingdom, Australia, France, the Czech Republic, Poland, Spain, and Morocco. The earliest ctenocystoids date to the beginning of Stage 5 of the Cambrian, now known as the

Wuliuan The Wuliuan stage is the fifth stage of the Cambrian, and the first stage of the Miaolingian Series of the Cambrian. It was formally defined by the International Commission on Stratigraphy in 2018. Its base is defined by the first appearance of t ...

age, or possibly slightly earlier, in late

Cambrian Stage 4 Cambrian Stage 4 is the still unnamed fourth stage of the Cambrian and the upper stage of Cambrian Series 2. It follows Cambrian Stage 3 and lies below the Wuliuan. The lower boundary has not been formally defined by the International Commission o ...

. Most species date to the Wuliuan and

Drumian The Drumian is a stage of the Miaolingian Series of the Cambrian. It succeeds the Wuliuan and precedes the Guzhangian. The base is defined as the first appearance of the trilobite '' Ptychagnostus atavus'' around million years ago. The top is de ...

ages of the

. The geologically youngest ctenocystoid, the only one known from the

, is ''Conollia'', from the

Sandbian The Sandbian is the first stage of the Upper Ordovician. It follows the Darriwilian and is succeeded by the Katian. Its lower boundary is defined as the first appearance datum of the graptolite species '' Nemagraptus gracilis'' around million ...

of the United Kingdom.

History

The class Ctenocystoidea was named in 1969 by Richard A. Robison and James Sprinkle. It originally contained one species, ''Ctenocystis utahensis''. The name comes from the Greek words '' ktenos'', meaning "comb", and '' kystis'', meaning "sac". It was originally assigned to the echinoderm subphylum

References

{{Taxonbar, from=Q5792856 Paleozoic echinoderms Prehistoric animal classes Homalozoa