Mitrates are an extinct group of

stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

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 sea ...

s, which may be closely related to the

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

. Along with the

cornute Cornuta is an extinct order of echinoderms. Along with the mitrate Mitrates are an extinct group of stem group echinoderms, which may be closely related to the hemichordates. Along with the cornuta, cornutes, they form one half of the Stylop ...

s, they form one half of the Stylophora.

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 Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

. However, this is arguably the only feature they share with the latter group; they don't have, for example, fivefold symmetry or a water vascular system. Their heads had two sides; one, flat, was covered with large "pavement-like" plates, the other, convex, bore smaller plates. Their tails were long and segmented, resembling the stalk of a crinoid or the arm of a brittlestar. At the opposite end was a hole which may have been mouth or anus - or both. They also bear features reminiscent of

pharyngeal slit Pharyngeal slits are filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth and out the phary ...

s, a character lost in other echinoderms but present in hemichordates, causing R.P.S. Jefferies to hold them as the ancestor of all chordates.

Behaviour

Mitrates have been found with associated trace fossils. Their interpretation requires an understanding of how the animal was oriented in life; it's not agreed whether the convex side of the head was up or down, or indeed whether the "tail" was at the front or back of the organism. The trace fossils suggest that they pulled themselves through the mud with their "tail", and were flat-side up.

Notes

''Rhenocystis latipedunculata''

References

Homalozoa {{paleo-echinoderm-stub