Like other echinoderms, crinoids have internal skeletons made of hard “ossicles.” The stalk that anchors a crinoid to a reef or other hard surface is strengthened by stacked, disk-shaped ossicles. The delicate rays of a crinoid are supported by smaller, linked ossicles that provide sites of attachment for the muscles that move them. What remains in the fossil record are these calcified, skeletal parts. Large berms of fossilized skeletons of crinoids are testimony to their abundance during the Paleozoic and Mesozoic eras. Scientists believe that the huge volume of calcified skeletal material left behind by crinoids shaped the environment for other species. Which crinoid fossils are present indicates the age of a fossil bed, since many species lived on Earth for short periods (in geologic time). Despite their prehistoric abundance, only one subclass of crinoids is still living today.

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.