However, the great majority of species died out during the Mesozoic, as the more advanced euechinoid sea urchins became common.

Today, only a single order, the Cidaroida, survives.

Most fossil forms had multiple columns of ambulacral plates, rather than the two rows found in all living species of sea urchin.

The group is probably a paraphyletic assemblage of stem forms, united only by their lack of more advanced features, rather than a true taxonomic clade.

This article about a sea urchin is a stub.