Phylum Echinodermata - Echinoderms

Tree of Life

Phylum Echinodermata

Kingdon Animalia

Of related interest:
Crawfordsville Indiana Crinoids

Crinoids and Echinoderms

The Echinodermata, (from the Greek meaning spiny skin), is a phylum containing some 13,000 extinct and 7,000 extant species. Living representatives are only found in marine environment, making the phylum the largest lacking terrestrial and fresh water forms. Echinoderms evolved from bilaterally symmetric animals exhibiting fivefold radial symmetry in portions of their body at some stage of life. This pentameral symmetry is readily apparent in familiar adult starfish and sand dollars. Other echinoderms both extant and extinct lack the five point morphology because it was lost somewhere during development (i.e., during ontogeny).

Echinoderms have a vascular system that carries water and which in some echinoderms end in suckered feet enabling the creature to grip and move objects. Reproduction in Echinoderms reproduction is usually via external fertilization through eggs and sperm discharged into the water, and the majority of echinoderms have several planktonic larval stages before reverting to a sessile existence on the seafloor.

 fOSSIL Blastoid Association Plate from Burlington FormationSince most echinoderms have some type of calcareous support exoskeleton (actually often interlocking plates of calcium carbonate), there exists an extensive fossil record tracing echinoderm evolution. Yet, many aspects of their early evolutionary origins are confounded, such that the classification table below is but one of many interpretations to be found in the literature. Importantly, the Echinodermata phylogenetic relationship to other phyla is poorly understood because they were already well differentiated by the Cambrian, and their unique characteristics are not present in other groups. While echinoderms are known from the Cambrian on, the Vendian period has a few soft-bodied fossils that are putative echinoderms or their ancestors. These include Arkarua and Tribrachidium from the Ediacara Hills of Australia. Homalozoans, from which echinoderm may have descended, and eocrinoids, that are not directly ancestral to the true crinoids, are abundant in the early Cambrian fossil record.

A possible early crinoid is Echmatocrinus from the famous Burgess Shale of the middle Cambrian, though many researchers doubt it was a true crinoid. Cotyledion from the much younger early Cambrian Chengjiang Maotianshan  Bundenbach Crinoids AssociationShale is another potential primitive crinoid. Other Cambrian echinoderms included the unusual helicoplacoids. Asterozoans (starfish and brittle stars) appeared in the Ordovician, as did the earliest echinozoans. The oldest asterozoans (the Somasteroidea) have morphological similarities to both starfish and brittle stars, supporting the theory that starfish and brittle stars probably diverged from a common somasteroid ancestor. After the Ordovician, there is an extensive echinoderm fossil record dominated by crinoids and blastoids; such as this stunning Triacrinus crinoid death assemblage from the Devonian Hunsruck Slates near Bundenbach, Germany. Some Paleozoic limestone formations are comprised of almost nothing other than crinoid and blastoid pieces. All the blastoids and most of the crinoids met extinction at the end of the Permian, leaving only the Asterozoans and echinozoans that remain extant today. The Holothurians, or sea cucumbers, are prevalent echinoderms but are extremely rarely fossilized.

Complete fossil starfish are also very rare, and often are but partial plates or segments of arms. The poor Agaricocrinus Crawfordsville Crinoid Fossilfossilization results because the skeleton is not ridged like echinoids (sea urchins), but comprised of numerous small plates (or ossicles) that quickly fall apart after decay of the soft parts of the animal.

Great fields (so to speak) of crinoid gardens inhabited shallower waters during the Paleozoic, essentially from the Ordovician on, and particularly in the Carboniferous (for example, see the famous Crawfordsville crinoids). However, crinoids suffered a major crisis during the Permian period (the P-T even) when most met extinction, with but few survivors into the Triassic period. The Mesozoic era realized another large crinoid radiation, with more modern forms having flexible arms becoming widespread. After another extinction event at the end of the Cenozoic they again declined, with most remaining species constrained to deep waters until present time.

Phylum Echninodermata Taxonomy
Subphylum Blastozoa Eocrinoidea (Cambrian to Silurian) > 30 genera
Parablastoidea (Ordovician) 3 genera
Cystoidea (Ordovician to Devonian) Rhombifera > 50 genera
Diploporita > 40 genera
Blastoidea (Cambrian to Permian) > 90 genera
Subphylum Asterozoa (starfish and kin, also called Stelleroidea)  Asteroidea (True starfish or sea stars - Lower Ordovician to recent)     Forcipulatida
Ophiuroidea (Brittle stars - Lower Ordovician to recent)  Oegophiurida
Crinozoa Paracrinoidea (Ordovician to Silurian) > 10 genera
Crinoidea (Crinoids / sea lilies - Cambrian to recent) Articulata (Triassic to recent, the only remaining crinoids)
Cladida (extinct)
Flexibilia (Lower Ordovician to Upper Permian)
Camerata (Middle Ordovician to Permian)
Subphylum Echinozoa Echinoidea (Sea Urchins - Ordovician to recent) 2 subclasses containing 13 orders and > 700 genera
Class Holothuroidea (Sea Cucumbers - Ordovician to recent) some 200 genera
Class Edrioasteroidea (Early Cambrian to Carboniferous) 35 genera
Class Edrioblastoidea (Ordovician) single genus
Class Helicoplacoidea (Cambrian) 3 genera
Class Cyclocystoidea (Ordovician to Devonian) 8 genera

Additional notes

  • Parablastoidea are small class of primitive blastozoan echinoderms containing three genera found in the early Middle Ordovician
  • The Ophiuroids or brittle stars are probably the most common extant echinoderms. They occur in large numbers in all parts of the oceans, from the shallowest coastal waters to the deep sea trenches.
  • Blastoids persisted until their extinction at the end of Permian, some 250 million years ago. Although never as diverse as the crinoids, blastoids are prodigious as fossils, especially from the Carboniferous, and are particularly abundant in some U.S. midwestern states.
  • Crinoidea has more than 1000 genera, with some 80 extant species.