Many fragmented crinoid fossils are found in limestone beds from around the middle part of the Paleozoic era to the end. From the abundance of crinoid fossils found in limestone beds it is clear that crinoids were much more common in the Paleozoic than they are in modern times. It is thought that the first crinoids came about during the Ordovician and are descendants of more ancient echinoderms called Eocystoids. There is only one crinoid subclass left in modern times out of about 10 subclasses. All other crinoids are now extinct and can only be examined through their unusual fossils. Since the first crinoids evolved, about 5,000 species of crinoid fossils have been discovered. The only subclass of crinoids left is known as the Articulata. The Crinoidea is a class in Articulata with about 600 species living today. These crinoids are commonly found on coral reefs although some live in deeper waters.