Cothurnocystis

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Cothurnocystis is a small, unusual fossil echinoderm from the Ordovician period. Each animal had a flat, boot-shaped body called the theca and a thin rod-like stem that may be a tail or a foot. Fossils have been found in Nevada, Scotland, the Czech Republic, France and Morocco.

Cothurnocystis is a well-known member of the stylophorans, a group whose exact place in the animal family tree is debated. Some scientists once thought stylophorans showed signs of a simple backbone and might be early chordates with a tail, calling them Calcichordata. Others think stylophorans were echinoderms, since their shell is built from materials similar to other echinoderms. Stylophorans are unusual because they are not symmetrically arranged like most echinoderms, nor like most chordates.

The body of Cothurnocystis has two parts: the theca (the shell) and the stem (the tail or foot). The theca is flat, boot-shaped and asymmetrical. Its edges are made of several plates, including three spikes: a toe, a heel, and a lip. One side (the obverse) is covered by a thin skin; the other side (the reverse) shows a supporting strut formed by branches of plates near where the stem attaches. There are windpipe–like slits along part of the theca. The stem attaches with four sets of left and right elements and becomes thinner away from the theca. The lower part of the boot around the opening seems to hold both the mouth and the anus.