| Ctenocystoidea | |
|---|---|
 | |
| A specimen ofCtenocystis | |
Scientific classification | |
| Kingdom: | Animalia |
| Superphylum: | Deuterostomia |
| Clade: | Ambulacraria |
| Phylum: | Echinodermata |
| Class: | †Ctenocystoidea Robison & Sprinkle, 1969 |
| Type genus | |
| Ctenocystis Robison & Sprinkle, 1969 | |
| Genera | |
Ctenocystoidea is anextinctclade ofechinoderms, which lived during theCambrian andOrdovician periods. Unlike other echinoderms, ctenocystoids hadbilateral symmetry, 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 otherdeuterostomes. Ctenocystoids were once classified in the taxonHomalozoa, also known as Carpoidea, alongsidecinctans,solutes, andstylophorans.[1] Homalozoa is now recognized as a polyphyletic group of echinoderms without radial symmetry. Ctenocystoids were geographically widespread during theMiddle Cambrian, with one species surviving into theLate Ordovician.
Like other echinoderms, ctenocystoids had a skeleton made ofstereom plates. Ctenocystoids had near bilateral symmetry, with some species exhibiting slightly asymmetric plate shapes. Like cinctans andCtenoimbricata, most ctenocystoids had large marginal plates surrounding the sides of their body, but unlike cinctans andCtenoimbricata, 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 andCtenoimbricata,[2] whereasConollia lost the marginal plates entirely.[3] The anus of ctenocystoids was surrounded by a pyramidalperiproct as in other echinoderms. It was located at the posterior end, defining a clear anterior-posterior body axis unlike other echinoderms.[4]
All ctenocystoids had a ctenoid apparatus, a comb-like arrangement of movable plates at the anterior end of the animal.[5][3] Beneath the ctenoid apparatus was a mouth, bordered on each side by a food groove.[1]
It appears the rightambulacrum was completely lost early on, with the remaining left ambulacrum dominating the body.[6]
| ||||||||||||||||||||||||||||||||||||
| Phylogenetic relationships within Ctenocystoidea[7] |
Ctenocystoids are likely among the most basal stem-group echinoderms.[1] They have also been interpreted as aberrantblastozoans and as stem-grouphemichordates.[8][9] The presence of stereom plates indicates that they most likely belong to the echinoderm total group, rendering a hemichordate affinity unlikely.[10]
Courtessolea was probably the most basal ctenocystoid, given its anatomical similarities toCtenoimbricata and cinctans.[2]Conollia andJugoszovia may be closely related to each other, as both have a reduced marginal frame compared to other ctenocystoids.[3]
The following genera of ctenocystoids have been named:[7][11][12]
| Genus | Discoverers | Named | Age | Location | Notes |
|---|---|---|---|---|---|
| Conollia | Domínguez Alonso | 1999 | Sandbian |  United Kingdom | The geologically youngest genus[3] |
| Courtessolea | Domínguez Alonso | 1999 | Wuliuan |  France | The most basal genus[2] |
| Ctenocystis | Robison & Sprinkle | 1969 | Middle Cambrian |  United States | The first genus discovered[5] |
| Etoctenocystis | Fatka & Kordule | 1985 | Middle Cambrian |  Czech Republic | |
| Gilcidia | Domínguez Alonso | 1999 | Middle Cambrian |  Australia, Denmark, France | |
| Jugoszovia | Džik & Orłowski | 1995 | Middle Cambrian |  Poland | |
| Pembrocystis | Domínguez Alonso | 1999 | Middle Cambrian | United Kingdom |
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.[12] The earliest ctenocystoids date to the beginning of Stage 5 of the Cambrian,[13] now known as theWuliuan age,[14] or possibly slightly earlier, in lateCambrian Stage 4.[15] Most species date to the Wuliuan andDrumian ages of theCambrian.[3] The geologically youngest ctenocystoid, the only one known from theOrdovician, isConollia, from theSandbian of the United Kingdom.[3]
The class Ctenocystoidea was named in 1969 by Richard A. Robison and James Sprinkle.[5] It originally contained one species,Ctenocystis utahensis. The name comes from the Greek wordsktenos, meaning "comb", andkystis, meaning "sac".[citation needed] It was originally assigned to the echinoderm subphylumHomalozoa.