Fossil crinoid (Taxocrinus undulata). Crinoid, or sea lily, fossil embedded in limestone. Crinoids are marine echinoderms that have been in existence for 500 million years. Thousands of extinct crinoid species have been found as fossils, but only a few hundred species still exist today. They attach themselves to the sea floor with their stem (centre) and use their feathery arms (upper centre) to filter food from the water. This fossil measures 20 centimetres tall and was found near Montgomery city, Indiana, USA.