Crinoids are marine animals that look like flowers. They have an internal skeleton made up of hard plates arranged in a way that allows them to float in water or on the ocean floor with ease. 

These ancient organisms were once very common throughout the world's oceans. However, over time, their populations dwindled drastically due to changing environmental conditions and competition with other species over food sources. Eventually, they became extinct altogether—but not before leaving behind their fossilized remains so we could learn more about what life was like back then!

They appeared in the seas of mid-Cambrian times, hundreds of millions of years before dinosaurs. Crinoids flourished during the Palaeozoic and Mesozoic eras, and some species survive to this day. Although their modern forms are often different from those of their fossil ancestors, living crinoids can still provide clues about how extinct ones must have lived.