What are echinoderms?
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).
Fossil sea star (Eoactis stachi), Late Silurian (c. 420 million years old), Melbourne, Victoria
Photographer: Rodney Start. Source: Museum Victoria.
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.
Extinct echinoderms of the Palaeozoic Era. A, a cystoid; B, a carpoid; C, an edrioasteroid; D, a blastoid.
Illustrations sourced from: Bather, F. A. 1907. A guide to the fossil invertebrate animals in the Department of Geology and Palaeontology in the British Museum (Natural History). BM(NH), London, and Zittel, K. A. 1883. Traité de paléontologie. Paris.
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.
Fossil crinoid, Helicocrinus plumosus; late Silurian (c. 415 my old), West Brunswick, Victoria
Source: Museum Victoria
The oldest echinoderm known anywhere in the world may be a fossil called Arkarua from late Precambrian sandstones (about 600 my old) in the Flinders Ranges of South Australia. Arkarua is a small, disc-like organism 4-10 mm in diameter, with five radiating grooves on its upper surface and a marginal rim. It seems similar in appearance to an edrioasteroid (see illustration above). However, because detailed structures of Arkarua are not preserved in the relatively coarse-grained sandstones in which it occurs, its interpretation as an echinoderm cannot be confirmed.
Fossil echinoid (sea urchin), Lovenia bagheerae; late Miocene (c. 8 my old), Portland, Victoria
Photographer: Frank Holmes / Source: Museum Victoria
Edgar, G. J. 1997. Australian Marine Life. Reed Books, Kew, Victoria. [Photographs of living echinoderms from southern Australia.]
Sadler, T., Pledge, N. S. & Morris, B. 1983. Fossils of southern Australia. Part 1: Sea urchins of the Murray River cliffs. Quoll Enterprises, Seaton, South Australia.