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(Above) Hemicidaris from above, showing the many ball-shaped protrusions on which the spines rotated. It is a
Jurassic echinoid that lived wedged in the rocks.
(Right) Cidaris
from below. The
large sediment-filled opening is where the jaws were located. It is
a Jurassic echinoid that lived wedged in the rocks.
(Above) Micraster and Hemiaster
are two common heart-shaped Cretaceous echinoids that had hairy spines, and which burrowed in the sand. The heart-shaped shell would have a been a kind of streamlining to help the animals bulldoze their way through the sand. Notice the marking on the top of the fossil, showing the five plates.
(Left) The mouth parts on the underside of a modern echinoid.
(Right) The ‘shell’ of a modern bulbous, rock-dwelling, regular echinoid as seen from above. The many bumps show where the slender spines fitted.
Recognising echinoids
Echinoids are mainly bulbous fossils. It is not always possible to find evidence of the five plates or the spines. However, their shape makes it difficult to mistake them for any other fossil.
You may also find spines separately. They are thin rods of material and are not as readily identified.
(Left) Modern, flat, irregular echinoid, known as a sand dollar. Its flat shape gives it stability on the sand.
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