Trilobite. Fossil trilobite, Neoasaphus punctatus, from the Ordovician period (525-440 million years ago). This species represents an intermediate evolutionary stage showing the start of the lengthening of the eye stalks (at centre right). Trilobites were marine arthropods which were once common throughout the world's oceans for 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.