Marine benthic, and rarely pelagic, vertebrate
animals that have five-fold radial symmetry and an
endoskeleton made of spiny plates of calcite. The living
echinoderms are divided into five classes with about 6,000
species, including common forms such as starfish, brittle
stars, sea urchins, sea cucumbers, and crinoids. A rich fossil
record of echinoderms extends back to the Cambrian with
about 20 classes recognized.
The wide distribution and evolutionary changes in the
echinoderms makes them moderately useful index fossils, but
many disarticulate when they die, limiting their utility. They
show a record characterized by the sudden appearance of new
forms, without any intermediate forms preserved. An initial
radiation occurred from the Early Cambrian to Middle
Ordovician that led to a stable period in which many stemmed
forms, especially crinoids, were prolific until an extinction
event at the Permian-Triassic boundary. Many of the echinoderms,
including the blastoids, became extinct here, though the
sea cucumbers, starfish, sea urchins, and brittle stars survived,
but they did not flourish again until the early Mesozoic.
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