Crinoids have graced the oceans for more than 500 million years. Amongst the most attractive fossils, they played a key role in the ecology of marine invertebrates called echinoderms, which include starfish, sand-dollars, sea-urchins and sea cucumbers. Crinoids are unusual-looking animals because they resemble plants more than animals and are commonly known as "sea lillies". Crinoids filter plankton from sea water and evolved a plant-like shape to remain attached to the sea floor.