Parablastoidea

fossil echinoderm class

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July 1, 2026

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The Parablastoidea were an extinct class of echinoderms that existed from the Lower to Middle Ordovician periods, approximately 500 to 460 million years ago. They are known only from fossils.1

Parablastoidea resembled the Blastoidea, another extinct class of echinoderms, but differed in the structure of their ambulacra (grooved structures) and the number of plates in their theca (body chamber). Echinoderms, as a phylum, are marine invertebrates characterized by skeletons made of calcium carbonate and typically exhibit five-part body symmetry. The fossil record of echinoderms is extensive, though many gaps exist, and questions about their early evolution persist.12

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echinoderm
Annotated classification
†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 numbers of thecal plates. †Class Rhombifera Lower Ordovician to Upper Devonian about 350,000,000–500,000,000 years ago; theca globular; respiratory structures rhomboid sets of folds or canals.
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†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 numbers of thecal plates. †Class Rhombifera Lower Ordovician to Upper Devonian about 350,000,000–500,000,000 years ago; theca globular; respiratory structures rhomboid sets of folds or canals.
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