Echinoderms are a group of entirely marine animals having a skeleton or ‘test’ made up of calcareous plates or spicules. Each plate is a single crystal of calcite. The skeleton differs from that of other invertebrates in that it is secreted by the middle rather than the outer body layer, thus the test is covered in soft tissue. Echinoderms typically have five-fold (pentameral) symmetry. They use a hydraulic water-vascular system for respiration, locomotion and transportation of food and waste products.
Echinoderms include five major classes with living representatives – crinoids (sea lilies and feather stars), echinoids (sea urchins and sand dollars), holothuroids (sea cucumbers), asteroids (starfish) and ophiuroids (brittle stars). Some entirely extinct groups include the blastoids, carpoids and cystoids.
The crinoids have a cup-like calyx, which houses the mouth, surrounded by arms used for feeding. Many crinoids have a stem which is fixed to the sea floor with a holdfast. These stalked forms are commonly known as ‘sea lilies’. The arms may or may not branch, and bear small alternate pinnules which give them a feathery appearance. The stem is made up of individual columnals or ossicles, which often disassociate after death.
Stalked crinoids are relatively rare in the modern ocean, and usually confined to deep waters. They were far more abundant and diverse in the Paleozoic, and some limestones exist which consist almost entirely of crinoid fragments. Clonmacnoise limestone is one such example from Ireland – the polished piece in the image below is full of isolated ossicles and short sections of stems which remained intact.
Range: Middle Cambrian – Recent
One major group within the crinoids are the comatulids or ‘feather stars’. They are a free-living group and lack a stem as adults.
The blastoids, like the stalked crinoids, possessed a stem or stalk, surmounted by a calyx from which arose a crown of brachioles. The brachioles were used for food gathering and various respiratory structures are found in the calyx. In contrast to the crinoids, the plates of the calyx were generally tightly associated in life and are therefore commonly preserved intact (see left). The brachioles, by contrast, were very delicate and rarely preserved. The blastoids, went extinct at the end of the Permian (250 million years ago).
Range: Silurian – Permian
Starfish are mobile organisms which possess multiple arms (typically five, but not always). Each arm bears tube feet on the oral (lower) side which are operated by the hydraulic water-vascular system, and used for feeding, locomotion and respiration. The tube feet are incredibly strong – for example they can be used to pry open shellfish, a common food for this group. The skeleton consists of ossicles which are usually granular, calcareous, of regular shape and may be associated in rows.
Starfish have a poor fossil record, due to their tendency to disarticulate after death. Nevertheless, examples can be found as far back as the Ordovician.
Range: Ordovician – Recent
The ophiuroids include forms commonly known as brittle stars, serpent stars and basket stars. They resemble starfish, but have long thin arms and a more defined central body disk. They are rarely preserved as fossils.
Range: Ordovician – Recent
Echinoids include the sea urchins, heart urchins and sand dollars. Their skeleton is composed of plates, alternating in bands between those with pores for the tube-feet (ambulacaral) and areas without (interambulacaral).
The ‘regular’ echinoids are radially symmetrical, and live on the surface of the sea floor. Spines are used for protection, and in some cases for locomotion along with the tube feet. The mouth is located on the underside of the animal, and they are omnivorous, feeding on algae, other small organisms or scavenging from carcasses. The plates of the test are relatively loosely held together and break up quite easily after death – disarticulated plates and spines are therefore most common in the fossil record.
‘Irregular’ echinoids have lost their radial symmetry, and are adapted to live within the sediment (infaunal). The two most common types are the sand dollars and the heart urchins. Irregular echinoids are somewhat more likely to be preserved whole in the fossil record than the regular echinoids.