Sea Urchins: The Echinoids (PA-20)
Sand dollars and sea urchins are echinods, which have neither stalks nor arms and commonly are very spiny. They appeared more than 450 million years ago but did not become common until about 150 million years later.

Modern echinoids have many habitats, which range from reefs, sandy shoals, and shallow bays to deep water that is seldom disturbed by waves. Their food is varied, ranging from small algae and bits of seaweed to small invertebrate life.

The oldest echinoid fossils are spheroidal, but later types vary in shape. Most are flattened on the upper and lower surfaces. The skeleton was composed of many calcareous plates arranged in a five-fold symmetry and, during life, was covered by a tough skin studded with spines. Complete fossil echinoids are rare, for most fell apart after death. Usually only individual plates and spines are found, but they are useful to paleontologists as indicators of the age of the rock. They are commonly found in Mississippian and Pennsylvanian rocks in Indiana.

Our Hoosier State Beneath Us: Paleontology