Echinoderms (e-ky'-no-derms) are sea bottom animals that have external calcite skeletons. Echinoderms have a fivefold symmetry and an internal fluid-filled system that supports respiration, movement, and feeding. A special characteristic of some is their use of tube feet. The best known echinoderm fossils are the cystoids, blastoids, and crinoids, which are favorites among collectors.
Well known, but not common in the central United States, are starfish (asteroids) and sea urchins (echinoids). These animals arose during Ordovician time, about 490 million years ago, and continue to the present. Common in southern states, their fossils are found in many collections.
Brittle stars (ophiuroids), paracrinoids, and edrioasteroids are not common. Brittle stars appeared during the Ordovician and continued into the Mississippian. Paracrinoids are confined to Middle Ordovician rocks. Edrioasteroids appeared in the Cambrian and continued into the Mississippian.