Echinoids show up in the fossil record from the Ordovician (about 450 million years ago), but their fossils are sparse compared to other organisms like bryozoans and crinoids. They may not fossilize well because the skeleton of the first echinoids was made of thin plates loosely held together by soft flesh (in contrast to modern echinoid plates that interlock to make a rigid skeleton). Many modern echinoids live in places with wave action, but the ancient ones probably lived in calm, sheltered waters. Like modern echinoids, some early echinoids had long spines that they could regrow, and tube feet for feeding or locomotion. Their teeth were weaker than modern echinoid teeth, and they may have lived by scavenging food from the ocean bottom. Echinoids diversified during the Triassic and Jurassic, giving rise to the types we see today, including predatory echinoids.

Echinoderms stand out as the only organisms on Earth with five arms or other elements spaced evenly around a central point. This pentaradial symmetry (penta = “five;” radial = “around a central point") is obvious in some echinoderms, such as sea stars or brittle stars that have five arms. It’s more subtle on others, such as the five rows of tube feet on a sea cucumber or five grooves on a sand dollar. This unusual symmetry of adult echinoderms is not found in juveniles. In fact, echinoderm larvae have two-sided (bilateral) symmetry like humans, and must undergo a metamorphosis to become pentaradial adults. Body plans of animals during their development often say something about their ancestry. The bilateral bodies of developing echinoderms are a reminder that, aside from other back-boned animals (vertebrates), echinoderms may be our closest relatives. Unlike most living echinoderms, many fossil echinoderms did not have pentaradial symmetry.