Paracrinoidea

fossil echinoderm class

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July 23, 2026

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The Paracrinoidea were an extinct class of echinoderms that existed during the Middle Ordovician period, approximately 460 million years ago. These marine invertebrates possessed a stem, a theca (chamber), and arms that bore barblike structures called pinnules. A unique feature of the Paracrinoidea was a specialized pore system within the plates of their theca.1

Echinoderms, as a phylum, are characterized by a spiny skin or covering and a skeleton made of calcium carbonate. They have a rich fossil history, with many bizarre and now-extinct forms. While the exact evolutionary relationships among echinoderms are still debated, Paracrinoidea are classified within the extinct subphylum Blastozoa.213

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echinoderm
Annotated classification
†Class Paracrinoidea Middle Ordovician about 460,000,000 years ago; with stem, theca, and arms with barblike structures (pinnules); plates of theca with pore system of unique type. †Class Parablastoidea Lower to Middle Ordovician about 460,000,000–500,000,000 years ago; resemble Blastoidea but differ in structure of ambulacra and in…
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†Class Paracrinoidea Middle Ordovician about 460,000,000 years ago; with stem, theca, and arms with barblike structures (pinnules); plates of theca with pore system of unique type. †Class Parablastoidea Lower to Middle Ordovician about 460,000,000–500,000,000 years ago; resemble Blastoidea but differ in structure of ambulacra and in…
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