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Echinoids
Echinoids, or sea urchins, have five-fold symmetry like other echinoderms, in this case made from five plates that form the bulbous shell. When they are alive, a skin runs over their shells and the numerous spines are attached to it. When echinoids die, the spines fall away from the main shell, so that shells, often look more like knobbly balls.
Many echinoids move across the sea- bed using a combination of their tube feet and their spines. Some echinoids (for example, Micraster and
Hemiaster) burrow in sand, and in these cases their spines are adapted and become almost hair-like.
Echinoids were uncommon in Palaeozoic rocks, but they are frequently found in both Mesozoic and Cenozoic era rocks. Echinoids are used as index fossils in Cretaceous rocks.
(Below) Cross-section through a regular echinoid.
Hard shell and sometimes even spines are preserved.
Tube feet pass through plates. The feet and internal organs are not preserved.
Stomach
Jaws (Aristotle’s lantern)
(Below) The habitats for a some echinoids and starfish.
Starfish or sea stars were found in most environments on the sea floor, but few are preserved as fossils.
Irregular urchins such as sand dollars fed in the top layer of sand and mud on the sea floor.
Brittle stars are rarely preserved as fossils.
Regular echinoids wandered across the sea floor in both rocky and sandy conditions. They could wedge themselves into a rock crevice using their long spines.
Irregular echinoids, such as heart urchins (including Micraster), burrowed through soft sand and mud on the sea floor.
Tube feet could be extended through the burrow for feedng.
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