Echinoderms were one of the first deuterostomes to appear in the fossil record. Animals more complex than cnidarians are split into two major taxonomic groups based upon their embryonic development, the protostomes and deuterostomes. Protostomes include flatworms, roundworms, segmented worms, rotifers, arthropods, and molluscs. Deuterostomes include invertebrate chordates, echinoderms, and vertebrates. In deuterostomes the blastopore becomes the anus, while in protostomes it becomes the mouth. Echinoderms were the first animals to develop an internal skeleton. Both their embryonic development and their endoskeleton lead scientists to the conclusion that echinoderms and chordates were derived from a common ancestor.

Echinoderm Characteristics

Echinoderms are strictly solitary, marine organisms. Echinoderms exhibit bilateral symmetry in their free-swimming tornaria larvae. As adults most exhibit pentameral or radial symmetry. Some, such as irregular sea urchins and sand dollars, have a secondary bilateral symmetry superimposed over a radial symmetry.

The echinoderm skeleton or test is made of individual plates or ossicles made of single crystals of high magnesium calcite. The test is enclosed by soft tissues and grows by accretion. Spines on the ossicles give the phylum its name. During fossilization the high magnesium calcite transforms to a more resistant low magnesium calcite aiding in preservation (Pinna, 1990, p. 163).

Echinoderms have a water vascular system, which consists of an internal network of fluid-filled canals connected to hundreds of external appendages called tube feet. The water vascular system is analogous to a human made hydraulic system that transmits pressure from one area to another in order to perform work. Although echinoderms have muscles, most of the work is done by the water vascular system. The water vascular system is filled with water of normal marine salinity and is used in feeding, breathing, and moving.

Many echinoderms have skin gills or external projections that grow between spines and aid in respiration and nitrogenous waste removal. Echinoderms have a primitive nervous system with no brain; they do not have a head or tail end. Echinoderms are coelomates and most have a mouth, digestive tract, and anus. Many echinoderms have two sides, the side with the mouth is called the oral surface, and the side with the anus is the aboral surface.

Echinoderms Through Time

Echinoderms make their first appearance in the Cambrian. Echinoderms underwent an adaptive radiation during the Ordovician reaching their peak in diversity.

Crinoids (class Crinoidea) are the most numerous and diverse Paleozoic echinoderms and make their first appearance during the Cambrian. Articulates, the only living subclass of crinoids, made their first appearance during the Triassic. The crinoid Delocrinus missouriensis is the state fossil for Missouri.

Blastoids (class Blastoidea) range from the Middle Ordovician to Late Permian. Blastoids are the second most common Paleozoic stalked echinoderm. Blastoids reached their greatest diversity during the Mississippian.

Cystoids (class Rhombifera and Diploporita) range from Early Ordovician to Late Devonian. The cystoid stalk was long and flexible and was used to attach to substrates and move them along the sea bottom.

Brittle stars (class Ophiuroidea) make their first appearance during the Ordovician. Brittle stars have long slender arms that can move them rapidly, but are not capable of pulling or clinging to surfaces like the arms of sea stars.

Sea stars or starfish (class Asteroidea) make their first appearance during the Ordovician. Sea stars have always been important predators of the sea floor. Bivalves developed interlocking commissures at the same time that sea stars with protrusible stomachs arose. They may have also played a role in increased predation during the Mesozoic that led to the extinction of many brachiopods (Prothero, 1988, pp. 317-318).

Sea cucumbers (class Holothuroidea) are known mostly from their calcareous spicules since their soft body is seldom preserved. Sea cucumbers make their first appearance during the Cambrian.

Sand dollars and Sea Urchins (class Echinoidea) are familiar echinoids. Sea Urchins are regular, round echinoids with radial symmetry. Sea Urchins make their first appearance during the Ordovician and represent the best fossilized echinoderms of the Mesozoic and Cenozoic. The swollen tubercles on the ossicles are fitted with spines articulated with ball and socket joints. The spines are used for defense and sometimes walking. The spines are made of a single calcite crystal and aid in identification. Echinoid spines are also used as Mesozoic index fossils. Regular echinoids forage on the sea bottom and show pentameral symmetry with their mouth and anus on opposite sides. Irregular echinoids make their first appearance during the Jurassic. Irregular echinoids have their anus on the edge and show a secondary bilateral symmetry superimposed on their radial symmetry. Most irregular echinoids burrow in the soft sediment of the sea floor. Sand dollars make their first appearance during the Paleocene and diverisfy greatly during the Cenozoic. Sand dollars are detritus feeders adapted for shallow, rapid burrowing (Prothero, 2004, p. 336).

Echinoderms provide important clues to past ecological conditions. Echinoderms are strictly marine organisms. Their form and function is related to the depth at which they lived and to the structure and conditions of the sea floor.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil list includes crinoids, echinoids, asteroids, ophiuroids, and blastoids. The list includes the stems, columns, and calyxes of crinoids (class Crinoidea). Sea urchins and sand dollars represent the echinoids (class Echinoidea). Sea stars (starfish) represent the asteroids (class Asteroidea) and brittle stars represent the ophiuroids (class Ophiuroidea). The genus Pentremites represents the blastoids (class Blastoidea).

Pinna, G. (1990). The Illustrated Encyclopedia of Fossils. New York: Facts on File.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Prothero, D.R. (2004). Bringing Fossils to Life: An Introduction to Paleobiology [2nd edition]. New York: McGraw-Hill.

Rich P.V., Rich T. H., Fenton, M.A., & Fenton, C.L. (1996). The Fossil Book: A Record of Prehistoric Life. Mineola, NY: Dover Publications, Inc.