Crinoids date from the earliest Ordovician, about 500 million years ago, and a few species still inhabit today's oceans and are cultivated in aquaria by advanced hobbyists. The heyday of crinoids was Carboniferous and Permian times (the Mississippian subperiod of the Carboniferous is sometimes called the Age of Crinoids), and whole beds of limestone may be composed of their fossils. But the great Permian-Triassic extinction nearly wiped them out.
Crinoids
Fossil Picture Gallery
Crinoids are stalked animals that resemble flowers, hence their common name of sea lily. Stem segments like these are especially common in late Paleozoic rocks. (more below)
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
