(phylum Echinodermata-"spiny skin") are one
of the most numerous of marine phyla. Starfish, brittle
sand dollars, and sea urchins are important members of
most ocean floor communities. Brittle stars and sea cucumbers
are the most common animals on the deep abyssal plains
of the ocean (Prothero, 1998, p. 310). As important
as echinoderms are in today's marine realm, the living classes
of their past diversity. Evolutionarily, echinoderms are
important in yet another way. Among invertebrates echinoderms
may be our closest relatives.
Echinoderms & Chordates
were one of the first deuterostomes to appear in the fossil
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.
are strictly solitary, marine organisms. Echinoderms
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
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
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.
echinoderms have skin gills or external projections that
grow between spines and aid in respiration and nitrogenous
waste removal. Echinoderms have a primitive nervous
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
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
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.
(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).
(class Holothuroidea) are known mostly from their calcareous
spicules since their soft body is seldom preserved.
Sea cucumbers make their first appearance during the
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
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
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.
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
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).
Pentremites represents the blastoids (class Blastoidea).
G. (1990). The Illustrated Encyclopedia of Fossils.
New York: Facts on File.
D.R. (1998). Bringing Fossils to Life: An Introduction
to Paleobiology. New York: McGraw-Hill.
D.R. (2004). Bringing Fossils to Life: An Introduction
to Paleobiology [2nd edition]. New York: McGraw-Hill.
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.