Ctenocystoidea is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

of echinoderms, which lived during the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

and Ordovician 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, take the face of a human being which has a pla ...

, 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 Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...

s. Ctenocystoids were once classified in the taxon Homalozoa, also known as Carpoidea, alongside cinctans, solutes, and stylophorans. Homalozoa is now recognized as a polyphyletic group of echinoderms without radial symmetry. Ctenocystoids were geographically widespread during the Middle Cambrian, with one species surviving into the Late Ordovician.

Description

Like other echinoderms, ctenocystoids had a skeleton made of

stereom Stereom is a calcium carbonate material that makes up the internal skeletons found in all echinoderms, both living and fossilized forms. It is a sponge-like porous structure which, in a sea urchin may be 50% by volume living cells, and the rest b ...

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 blastozoans and as stem-group hemichordates. 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 age, or possibly slightly earlier, in late Cambrian Stage 4. Most species date to the Wuliuan and Drumian ages of the

. The geologically youngest ctenocystoid, the only one known from the Ordovician, is ''Conollia'', from the Sandbian 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 Homalozoa.

References

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