Ctenocystoidea

The Ctenocystoidea were an extinct class of primitive echinoderms that existed from the Middle Ordovician to the Late Devonian periods. These animals had saclike bodies attached to a stem that anchored them to the seafloor and were covered by numerous plates. Some Ctenocystoidea are considered important index fossils, useful for correlating rock units across different locations. The discovery of Ctenocystoidea in the 1960s led to a reassessment of echinoderm classification. It is important to note that "ctenocyst" also refers to a sensory organ found in Ctenophora (comb jellies), which are a different group of marine invertebrates.1234