The stylophorans are an extinct, possibly polyphyletic group allied to the echinoderm

Echinoderm

Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

s, comprising the cornutes and mitrate

Mitrate

Mitrates are a group of stem group Echinoderms, which may be closely related to the hemichordates.-Morphology:The organisms were a few millimetres long. Like the echinoderms, they are covered in armour plates, each of which comprises a single crystal of calcite...

s. It is synonymous with the subphylum Calcichordata.

The general stylophoran body plan consists of a flattened theca and a single jointed appendage which extends from it. Stylophoran tests are composed of stereom

Stereom

Stereom is a calcium carbonate material that makes up the internal skeletons found in sea urchins, and all other 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 being a matrix of calcite...

 calcite plates like an echinoderm, which has traditionally been the basis for assigning them to Echinodermata. However, they also lack the radial symmetry characteristic of most other echinoderms, with the earlier members of the group being flattened and asymmetrical, and the later ones closer to bilateral symmetry. In Mitrocystites and perhaps in other forms its stem does not end in an attachment organ, and the stem more likely served the organism as a tail for movement. Cothurnocystis

Cothurnocystis

Cothurnocystis along with all of its relatives in Stylophora, are enigmatic organisms whose relationship with other organisms, namely the Echinodermata and the Chordata, are in a state of flux. It was boot-shaped and has been suspected to have contained a primitive notochord...

is asymmetrical and boot-shaped, and Mitrocystites is bilaterally symmetrical and more streamlined.

It has additionally been suggested that some or all of its members might have had gill slit

Gill slit

Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of Cartilaginous fish such as sharks, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...

s like a chordate

Chordate

Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...

, and that their stems contained a notochord

Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

. This reconstruction leads to the alternative hypothesis that some or all of the stylophorans may have been ancestral to the chordate branch of the deuterostomes, rather than being within the echinoderms.

See also

• Calcichordate Theory
  Calcichordate Theory
  The Calcichordate Theory holds that each separate lineage of chordate evolved from its own lineage of mitrate, and thus the echinoderms and the chordates are sister groups, with the hemichordates as an out-group....
  
  
• Edrioasteroidea
  Edrioasteroidea
  The Edrioasteroids are an extinct class of echinoderm that lived from the Ediacaran to the Permian periods of geologic time, about...
  
  
• Helicoplacoidea
• Blastozoa
  Blastozoa
  Blastozoa is a subphylum of extinct animals belonging to Phylum Echinodermata. This subphylum is characterized by the presence of hydrospires.-External links:*...
  
  

External links

• http://sn2000.taxonomy.nl/Taxonomicon
• http://www.borntraeger-cramer.de/pubs/journals/0031-0220/paper/68/443 A calcichordate interpretation of the new mitrate Eumitrocystella savilli from the Ordovician of Morocco