Fossil trilobite. Fossil trilobites (at top right and bottom left) amongst the fossilised remains of a reef in limestone rock. Trilobites were marine arthropods which were once common throughout the world's oceans for hundreds of millions of years. Most species are thought to have lived on the sea floor in the same way as modern crabs do. Their large eyes helped them see in the dark- ness of the seabed. Their flexible backs allowed them to roll up into a tight ball if attacked by predators. They became extinct 65 million years ago at about the same time as the dinosaurs.