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Left: An internal flint
mould of an echinoid (Echinocorys) from
A small echinoid (Cidaris) from Woodeaton Quarry
Despite their alien appearance, echinoids, or sea-urchins as they are
better known, are very common in the seas and oceans of today and are common
fossils too. Their name derives from the Greek 'echin' ('spiny'), referring
to their protective spines and presumably 'ooid' (egg-like) in reference to
their globular shell, or test as it is known. Echinoids are part of a much
larger group of animals known as the Echinoderms ('spiny-skins'), which also
includes the Asteroids (starfish), Holothurians (sea cucumbers), Crinoids
(sea lilies and feather stars) and the Ophiuroids (brittle stars).
Though their body plans are varied, all echinoderms possess key features
which unite the group:
From left: starfish, sea
cucumber, crinoid, brittle star.
A complex skeleton of calcareous plates, with a unique spongy
structure known as stereom.
Five planes of symmetry, referred to as penta-radial.
An internal hydrostatic (water-vascular) system, external extensions
of which are used for locomotion, respiration and feeding.
All live in marine waters.
Echinoderms first appeared in the fossil record in the Cambrian
around 530 million years ago and quickly diversified into many
groups. Echinoids appeared in the Ordovician (around 450
million years ago (mya) but were not very successful at first
and other groups such as crinoids dominated the Palaeozoic. By
the beginning of Mesozoic (250 mya) many of the earlier
echinoderm groups were extinct or in decline and the Echinoids
rose to abundance. They diversified through the Jurassic
(210-145 mya) and have remained successful ever since.
Why are Echinoids important?
Echinoids are very useful to palaeontologists because of their
functional morphology; basically this means that by studying their
anatomy you can tell a great deal about their mode of life and the
environment in which they lived. They are also very common, and
their robust tests and spines are easily fossilised and collected.
Anyone who has hunted for fossils in the Jurassic and Cretaceous
rocks of the UK will no doubt be very familiar with echinoids.
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Regular or Irregular?
Echinoids fall into two categories; regular and irregular. This
isn't referring to how common they are, rather to what shape they
are. Regular echinoids have no front or back end and can move in
any direction. Irregular echinoids have a definite front and back
and do move in a particular direction. This is because regulars and
irregulars have very different ways of life. Irregulars evolved
from regulars, and their anatomy is therefore a modified version of
the regular anatomy. For this reason we will deal with regulars
As mentioned above, regular echinoids have no front or back end.
Instead, the opening for the mouth (peristome) is on their
underside, and the opening for the anus (periproct) is on top. When
viewed from above or below their profile is circular and radially
symmetrical; hence the term regular ('repeating', 'uniform'). This
is because regular echinoids roam the surface of the sea floor in
search of food and need to be able to move in any direction. This
mode of life leaves them very exposed to predators and they have
evolved elaborate spines both for defence and to act as stilts for
Left: A living
echinoid feeding on the algae growing on a coral. Right:
Fossil echinoid (Phymosoma).
Most echinoids quickly loose their spines after death, and most
fossil echinoids either comprise of isolated tests or solitary
spines. However, if you are very lucky you may find a fossil
echinoid which has been buried alive and still has its spines
A typical regular echinoid spine (Phalacrocidaris
Spines vary greatly between species; some are needle-like, others
club-like, some are poisonous and others possess thorny barbs. It
is often easier to identify a species by its spines as opposed to
its test, as they are much more variable and distinctive. Stripped
of spines, many regular echinoids are superficially very similar.
Left: A living
echinoid with long spines. Right: A fossil regular
echinoid with club shaped spines (Pseudocidaris).
Once a regular echinoid finds food it can grasp it with its tube
feet. These are tiny, fleshy suckers which extend from the
hydrostatic water-vascular system through holes in the test. The
holes through which they extend are organised into five distinct
bands, called ambulacra, which run vertically between the peristome
and the periproct. Regular echinoids are largely scavengers and
have a varied diet of plant matter, animal detritus, sponges,
molluscs and barnacles.
regular echinoid with light-purple spines and dark-pink tube feet.
Right: Living regular echinoid with its tube feet
Regular echinoids possess large, powerful, and highly complex jaws,
known as Aristotle's Lanterns, which extend through the mouth to
collect food or scrape organics from shells or other hard surfaces.
The jaws are complex beaks with five teeth. These leave a
distinctive star-shaped grazing trace called Gnathichnus pentax.
Isolated echinoid jaw-parts are not uncommon fossils, but because
they are unfamiliar to most collectors they are either overlooked of
Left: A fossil
regular echinoid which has broken open, revealing the impressive
beak-like jaws (Aristotle's Lantern) inside.
Right: Star-shaped incisions in a piece of oyster
shell caused by a regular echinoid scraping off organics.
Echinoids are armoured with minute defensive spines called
pedicellariae. These are venomous or possess pincers to deter
parasites and clear detritus. Pedicellariae are only preserved in
High magnification picture of a
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The first echinoids were regulars, and irregulars did not evolve
until the Jurassic. Irregulars are much more common fossils though,
and unlike regulars their tests are typically fossilised complete.
Their spines are almost never found attached. Common forms are sea
potatoes and sand dollars.
irregular echinoid with small hair-like spines. Middle:
Fossil irregular echinoid. Right: A
modern sand dollar.
Irregulars lead a very different lifestyle from that of the
regulars. They burrow along the sea floor and bulk-feed on the
sediment to extract nutrients. For this reason the radial symmetry
inherited from the regulars has been modified; the mouth moving to
the front of the animal to collect food and the anus moving to the
rear to leave waste behind. Hence their form is now irregular, with
only one plane of symmetry.
The spines have lost their defensive role and have become reduced
and hair-like. They now help to form the burrow, move the echinoid
through the sediment, gather food and generate circulatory currents
within the burrow. Many irregulars have lost their jaws as they are
unnecessary to their mode of life. The tube feet are modified into
flanges for respiration and gathering food, and the ambulacra are
often sunken to form a petal shape on top of the echinoid.
Left: An irregular
echinoid (Echinocorys). Middle: Echinoid
(Nucleolites?). Right: Echinoid
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