22 Comments - Cast attached to Echinoid, Micraster Coranguinum More to be added. The echinoderms are a marine group known from the Cambrian Period. They have distinctive skeletons made up of interlocking plates or spines of lime. Three more advanced groups of echinoderms first appeared in the Ordovician and can still be found today. Isolated round or star shaped segments of the stems are common in many Palaeozoic and Mesozoic rocks.
Crinoids are now fairly rare.
Starfish are uncommon as fossils. The related brittlestars have long, delicate arms radiating from a central, disc shaped body. The sea-urchins, or Echinoids , are common fossils in, for example, the Cretaceous Chalk. They have compact bodies made up of interlocking plates from which protrude numerous spines. These spines have usually fallen off the fossils.