Fossil range: Cambrian (or earlier?) - present
A brittle star resting on a brain coral
Subphyla & Classes
• Homalozoa Gill & Caster, 1960
Ctenocystoidea Robison & Sprinkle, 1969
Paracrinoidea † Regnéll, 1945
Cystoidea †von Buch, 1846
• Pelmatozoa †
• Blastozoa †
Eocrinoidea †Jaekel, 1899
† = extinct
Echinoderms (Phylum Echinodermata) are a phylum of
marine animals (including the sea star and the sand dol-
lar). Echinoderms are found at every ocean depth, from
the intertidal zone to the abyssal zone.
Aside from the problematic Arkarua, the first definit-
ive members of the phylum appeared near the start of
the Cambrian period.
The phylum contains about 7,000 living species,
making it the second-largest grouping of deuterostomes,
after the chordates; they are also the largest phylum
that has no freshwater or terrestrial representatives.
The word derives from the Greek εχινοδέρματα
(echinodermata), plural of
"spiny skin" and that from εχινός (echinos), "sea-urchin",
originally "hedgehog" + δέρμα (derma), "skin".
The Echinoderms are important both biologically
and geologically: biologically because few other group-
ings are so abundant in the biotic desert of the deep sea,
as well as the shallower oceans, and geologically as their
ossified skeletons are major contributors to many lime-
stone formations, and can provide valuable clues as to
the geological environment. Further, it is held by some
that the radiation of echinoderms was responsible for
the Mesozoic revolution of marine life.
Two main subdivisions of Echinoderms are tradition-
ally recognised: the more familiar, motile Eleutherozoa,