What is it? It is a marine invertebrate (a creature with no backbone). When did it live? Sometime in a period from 345 to 315 million years ago. Is this type of creature extinct? There are 200 living types of this creature and about 30,000 fossil forms. Where did it live? Nearly all fossil forms lived in shallow water. How did it eat? What did it eat? This creature had a shell enclosing a pair of tentacled, armlike structures that were used to sweep minute food particles into the mouth.