Helicoplacoidea

fossil echinoderm class

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June 29, 2026

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The Helicoplacoidea were an extinct class of echinoderms, known from fossils dating back to the Early Cambrian period, approximately 570 million years ago. These marine invertebrates are distinguished by their pear-shaped or spindle-shaped bodies, which were covered in numerous spirally arranged plates. Unlike most other echinoderms, they lacked the characteristic five-part radial symmetry and did not possess a water-vascular system. Their unique body plan and evolutionary position have led to discussions about their relationship to early chordates.123

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echinoderm
Annotated classification
†Class Helicoplacoidea Lower Cambrian about 570,000,000 years ago; pear-shaped or spindle-shaped body with many plates arranged spirally. †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
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†Class Helicoplacoidea Lower Cambrian about 570,000,000 years ago; pear-shaped or spindle-shaped body with many plates arranged spirally. †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
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