Identify
a specimen... Sea Urchin (Echinoid) Echinoidia
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Sea urchins are
bizarre creatures, beautiful both in life and as
fossils. The spiny body, or test, usually appears to consist
of five portions, as in most echinoderms. They
first appeared in the seas of the Ordovician period,
about half a billion years ago. Fossil specimens and
modern sea urchins come in many shapes and sizes. So do
the spines; some are fat and club-like, while others are
thin and cactus-like. Tests are usually found without
spines.
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There
are four classic shapes of fossil sea urchin to
recognise, The first is very flat and circular, with a
faint petaloid star-shape on top. This is often called a
sand dollar. Sometimes the star-shape has been abrased
away and the fossil will simply be a rounded, flat stone
- hard to discern from other stones. The genus
Encope, shown left, also has rounded holes at the ends of each
"petal". |
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The
second is the heart-shaped urchin, or heart urchin. They
have bulbous, plated bodies with numerous tiny circular
pores. A particularly common example is the genus
Micraster. |
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The third has a bulbous circular test
with neat pentameral symmetry and a pattern of knobbly
protusions from which spines originally grew. These are
called bosses. There is an intricate pattern of pores
and plates on the surface which makes the find quite a
beautiful fossil. |
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The fourth is similar, but dome-shaped with a flat oval
base. The dome can be very round or almost conical, as
in this example.
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See also: blastoids
See also: brittle stars
See also: sea lilies
See also: starfish
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