Echinoderms include some of the most familiar seashore animals, such as sea stars (asteroids) and sea urchins (echinoids). Others that may be less familiar are brittle stars (ophiuroids), feather stars (crinoids) and sea cucumbers (holothurians).
Echinoderms are all marine animals with a skeleton composed of many small plates. The skeleton encloses the main soft parts of the body and is covered by a thin layer of skin. On the outside of the body are numerous tentacle-like structures called tube-feet, which are connected to a system of water-filled canals inside the body. The pressure of water in these canals enables the animal to extend or retract the tube-feet. The tube-feet have many uses, including moving the animal around, capturing food and passing it to the mouth, extracting oxygen from the seawater, and sensing the surrounding environment. Most echinoderms have radially symmetrical bodies arranged in five sections, but this is not true of some fossil groups and may not be obvious in some living ones.
Where are fossil echinoderms found?
All of the modern kinds of echinoderms mentioned above were present as long ago as the Palaeozoic Era, 545 to 251 million years ago (mya). Also living at that time were a number of other kinds of echinoderms that died out by the end of the era. These extinct forms include the cystoids (Ordovician to Devonian periods, 490–354 mya) and blastoids (Silurian to Permian, 434–251 mya), both of which were attached to the sea floor by a long stem; the edrioasteroids (Cambrian to Carboniferous, 545-298 mya) which were mostly flattened and disc-shaped; and the strange carpoids (Cambrian to Devonian, 545-354 mya) which generally lacked any symmetry at all.
Sandstones and siltstones of middle Palaeozoic age (Late Silurian to Early Devonian, 420-400 my old) in central Victoria contain fossils of many different kinds of echinoderms, including sea stars, brittle stars, crinoids, cystoids, blastoids, edrioasteroids and carpoids. They occur in the Heathcote, Kilmore, Kinglake, Melbourne and Lilydale districts. In much younger rocks of the Cainozoic Era (less than 65 my old), the most abundant echinoderms found in Victoria are the echinoids or sea urchins. They occur mainly in limestones exposed in coastal areas including the Portland, Port Campbell, Torquay, Geelong and Lakes Entrance districts. Fossil crinoids and sea stars also occur in Victorian Cainozoic rocks but are rare, because their skeletons quickly disintegrated after death and so only isolated plates are generally found.