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Sea urchins (Echinoidea) belong to the phylum of echinoderms (Echinodermata). Like other echinoderms they have a pentagonal symmetry. The body consists of fuses limestone skeletal parts. The spines were motile and were connected to the chalk plates. Sea urchins live on the bottom of the sea.
Sea urchin shells usually fossilize well because it is made up of Calcite. The spines will usually fall off and are often found loose. Sea urchins occur since the Ordovician period.
Example of regular seau urchins
Sea urchins are divided into regular and irregular sea urchins. The regular sea urchins completely pentagonal symmetrical and have their anus at the top and their mouth at the bottom. Irregular sea urchins have their mouth and anus often both at the bottom. These sea urchins have a less symmetrical shape and usually show bilateral symmetry.
Photos or locations of Echinoidea at this site
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