Hexedriocystis, an aberrant echinoderm from the Upper Ordovician of Morocco

Hexedriocystis is a very unusual echinoderm with a puzzling combination of morphological features. The two described species H. inexpectatus and H. mimus occur in Upper Ordovician siliciclastic rocks from Morocco, North Africa. They show a striking combination of isorophid edrioasteroid, paedomorphic crinoid and blastozoan characteristics including a domed thecal shape with a peripheral rim-like attachment structure, imbricate thecal plates. However, both species developed feeding appendages covered by large peristomial cover plates and six oral plates. Hexedriocystis lived intimately associated with edrioasteroid colonies and its strange morphology is probably the result of convergence with isorophid edrioasteroid.