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.