The edrioasteroids (seat stars) are a small but diverse group of Paleozoic (Lower Cambrian through Pennsylvanian) echinoderms known to include five morphologically distinct types of thecal construction. Domal and clavate thecae predominate, although some are globoid and others axially elongate. Adult thecal diameters range from 6 – 7 mm to approximately 9 cm, but 15 – 30 mm is the norm. Edrioasteroids are not common fossils, although when proper conditions for preservation existed, entire populations were often fossilized so that hundreds or even thousands of specimens can be recovered through excavation. Apparently, preservation depended upon catastrophic kills, most of which seem to have resulted from storm-deposited mud deeply burying the entire substrate surface area.