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.