trilobite

Any of a large class (Trilobita) of extinct, marine, invertebrate arthropods of the Palaeozoic era, with a flattened, oval body, 1–65 cm/0.4–26 in long. The hard-shelled body was divided by two deep furrows into three lobes. Some were burrowers, others were swimming and floating forms. Their worldwide distribution, many species, and the immense quantities of their remains make them useful in geological dating.

There were more than 1,500 genera of trilobites, which existed for about 300 million years. They disappeared 250 million years ago, probably due to the evolution of new predators. According to a 1997 study of fossils by a US palaeontologist, the trilobite moulted its body segments individually; this moulting may have taken several days, increasing vulnerability. The largest trilobite found was 70 cm/28 in long, and lived 445 million years ago, in what is now Hudson Bay, Canada. It was discovered by Canadian palaeontologists in 2000.