Welcome to Tornadoropa : Here you find a description about recent and Fossil Echinoids ( Sea Urchins ) including Ecology, Symbiosis and Predation of Echinoids, presented by Christian Neumann .

Sea urchins are spiny sea creatures found in oceans all over the world. (The name sea urchin means sea hedgehog, hedgehog being one meaning of the word urchin). Their shell, which biologists call the test, is globular in shape, and covered with spines. The size of the test in adults is typically in the range of 3 to 10 cm.

Typical sea urchins have spines 1-2 cm in length (e.g. " Sea urchin ", right), a millimeter or two thick, and not terribly sharp. Diadema antillarum , familiar in the Caribbean, has thin spines that can be 10-20 cm long. Sea urchins are usually dull in color, common colors including green, olive, brown, purple, and black.

Sea urchins are echinoderms ( phylum Echinodermata ), which also includes starfish, sea cucumbers, brittle-stars, and crinoids. Like other echinoderms they have five-fold or pentamerous radial symmetry and move by means of hundreds of tiny, transparent, adhesive " tube feet ". The pentamerous symmetry is not obvious at a casual glance, but is easily seen in the dried shell of the urchin.

Within the echinoderms, sea urchins are classified as echinoids (class Echinoidea ). Specifically, the term " sea urchin " refer to the " regular echinoids ", which are symmetrical and globular. The ordinary phrase " sea urchin " actually includes two different taxonomic classes: the Echinoida, and the Cidaroida or " slate-pencil urchins " which have very thick, blunt spines. Besides sea urchins, the Echinodea include three classes of irregular echinoids: flattened sand dollars , sea biscuits , and heart urchins .

At first glance a sea urchin often appears to be an inanimate object, or one which is incapable of moving. Sometimes the most visible sign of life is the spines, which are attached at their bases to ball-and-socket joints and can be pointed in any direction. In most urchins, a light touch elicits a prompt and visible reaction from the spines, which converge toward the point that has been touched. A sea urchin has no visible eyes, legs or means of propulsion, but it can move freely over surfaces by means of its adhesive tube feet, working in conjunction with its spines.

Geological history : The earliest know echinoids are found in the rocks of the upper part of the Ordovician period, and they have survived to the present day where they are a successful and diverse group of organisms. In well preserved specimens the spines may be present, but usually only the test is found. Sometimes isolated spines are common as fossils. Some echinoids (such as Tylocidaris clavigera which is found in the Cretaceous period Chalk Formation of England) had very heavy club-shaped spines that would be difficult for an attacking predator to break through and make the echinoid awkward to handle. Such spines are also good for walking on the soft sea-floor.

Complete fossil echinoids from the Palaeozoic era are generally rare, usually consisting of isolated spines and small clusters of scattered plates from crushed individuals. Most specimens occur in rocks from the Devonian and Carboniferous period . The shallow water limestones from the Ordovician and Silurian periods of Estonia are famous for the echinoids found there. The Palaeozoic echinoids probably inhabited relatively quiet waters. Because of their thin test they would certainly not have survived in the turbulent wave-battered coastal waters inhabited by many modern echinoids today. During the upper part of the Carboniferous period there was a marked decline in echinoid diversity and this trend continued into the Permian period. They neared extinction at the end of the Palaeozoic era, with just six species know from the Permian period.

By the upper part of the Triassic period their numbers began to increase again. The echinoids diversified into many types throughout the Jurassic period and into the Cretaceous period. During the Mesozoic and Caenozoic eras the echinoids flourished. However most echinoid fossils are restricted to certain localities and formations, but where they do occur they are quite often abundant. An example of this is Enallaster which may be collected by the thousand in certain outcrops of limestone from the Cretaceous period in Texas.

Some echinoids (such as Micraster which is found in the Cretaceous period Chalk Formation of England and France) serve as zone (or index) fossils. As they evolved rapidly over time such fossils are useful in enabling geologists to date the rocks in which they are found. However, most echinoids are not abundant enough and may be too limited in their geographic distribution to serves as zone fossils.