Brittle stars first show up in the fossil record in the early Ordovician (almost 500 million years ago). The earliest brittle stars had five arms arranged around a central body (pentaradial symmetry), like modern brittle star. Some species looked like they had zippers on their arms because of the two parallel rows of skeletal parts on each arm (modern brittle stars have just one). Brittle star fossils are rare because their skeletons are delicate (brittle) and tend to fall apart. A common brittle star fossil is Ophiopetra lithographica, perhaps because its unusually heavy, spiny arms and stout jaws fossilize well. It may have been a carnivore, eating small organisms on the sea floor. Brittle stars were nearly wiped out in the mass extinction event at the end of the Paleozoic (about 250 million years ago) but became widespread and diverse again during the Cenozoic. Today, there are more than 2,000 species.

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.