Echinoids have lived in the seas since the Late Ordovician, about 450 million years ago. Modern relatives of the echinoid are the familiar sea urchins that live in many shallow coastal waters of the world. Echinoids have a hard shell (or test) covered with small knobs (tubercles) to which spines are attached in living echinoids. The test and spines are the parts normally found as fossils.
The spines, some poison-tipped, help protect echinoids from their predators, which include other echinoids, crustaceans, octopuses and fish. Some fossil echinoids made themselves less palatable as prey by having large solid spines. Echinoids also use their spines for moving around the sea bed, and in some groups they are specially adapted for burrowing.