Ophiocistioidea

fossil echinoderm class

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July 1, 2026

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The Ophiocistioidea were a class of extinct echinoderms that lived from the Lower Ordovician to the Upper Silurian periods, approximately 395 to 500 million years ago. These marine invertebrates had a dome-shaped body that was partially or completely covered by a well-developed test, or shell. Their five ambulacral tracts, which are part of their water-vascular system, carried plated tube feet that were relatively large. The Ophiocistioidea are known only as fossils and are part of the broader phylum Echinodermata, which includes modern-day starfish, sea urchins, and sea cucumbers. Echinoderms are characterized by their calcium carbonate skeletons and, in most living forms, a five-part body symmetry.12

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echinoderm
Annotated classification
†Class Ophiocistioidea Lower Ordovician to Upper Silurian about 395,000,000–500,000,000 years ago; dome-shaped body partly or completely covered by well-developed test; 5 ambulacral tracts carry plated tube feet relatively enormous in size. Class Holothuroidea (sea cucumbers) Fossil and living forms (Ordovician about 460,000,000
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†Class Ophiocistioidea Lower Ordovician to Upper Silurian about 395,000,000–500,000,000 years ago; dome-shaped body partly or completely covered by well-developed test; 5 ambulacral tracts carry plated tube feet relatively enormous in size. Class Holothuroidea (sea cucumbers) Fossil and living forms (Ordovician about 460,000,000
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