Edrioasteroids, relatives of starfish, used to live on Earth during the Paleozoic (until about 300 million years ago). They looked like starfish, with five arms radiating from a round body. Unlike starfish, which move around, edrioasteroids stayed in one place, using a thick stalk to attach to the sea bottom or other surface. The stalk was covered with small, hard plates that made it more rigid. The body was also plated, providing protection and structure. Even the anus, where wastes were expelled on the underside of the animal, was plated. On the top of an edrioasteroid were five feeding grooves. Each groove channeled water into the body where nutrients could be filtered out. The feeding tubes tended to curve in the same direction, creating a pattern of spirals. Edriosteroids were most diverse in the Ordovician. Like many other organisms, they went extinct in the mass extinction event at the end of the Permian.

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.

Echinoderms appear in the fossil record more than 500 million years ago, during the early Cambrian. What are usually left behind are hard mouthparts or parts of their skeletons, made of calcite plates. Rarely, an entire skeleton fossilizes, for example in a situation where it was quickly buried in sand. Even a piece of skeleton can provide information, because echinoderms have specific patterns in their skeletons. The echinoderms you see today have five-point (pentaradial) symmetry, often noticeable in five arms. While some of the earliest echinoderms were pentaradial, others had unusual body shapes. The “helioplacoids” had long, oval-shaped bodies with no arms, and a spiral pattern on the surface from tube feet wrapped around a central core. Helioplacoids went extinct even before the end of the Cambrian, as did a variety of other echinoderms, including the star-shaped Somasteroidea. Some echinoderms survived and diversified, becoming dominant in the oceans of the Paleozoic era.