What is it? These invertebrates (creatures without backbones) were small brightly colored marine animals. They grew an external, stony skeleton. When did it live? Sometime in a period from 345 to 315 million years ago. Is this type of creature extinct? No. They are numerous in today’s tropical seas. Where did it live? In the warm shallow areas of seas. Some lived in the Arctic and Antarctic. How did it eat? What did it eat? Algae (simple plants) in the water surrounding the creature provided it with nutrition.