Mitrate

Mitrates are an extinct group of stem group echinoderms, which may be closely related to the hemichordates.

Morphology

The organisms were a few millimetres long.^[1] Like the echinoderms, they are covered in armour plates, each of which comprises a single crystal of calcite. 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.^[1]

Their heads had two sides; one, flat, was covered with large "pavement-like"^[1] plates, the other, convex, bore smaller plates.^[1] Their tails were long and segmented, resembling the stalk of a crinoid or the arm of a brittlestar.^[1] At the opposite end was a hole which may have been mouth or anus - or both.^[1]

They also bear features reminiscent of pharyngeal slits,^[2] a character lost in other echinoderms but present in hemichordates,^[1] causing R.P.S. Jefferies to hold them as the ancestor of all chordates.

Behaviour

Mitrates^ have been found with associated trace fossils.^[3] 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!^[1] The trace fossils suggest that they pulled themselves through the mud with their "tail", and were flat-side up.^[1]

Notes

^ Rhenocystis latipedunculata

References