The Trilobite BookDistant relatives of modern lobsters, horseshoe crabs, and spiders, trilobites swam the planet's prehistoric seas for 300 million years, from the Lower Cambrian to the end of the Permian eras and they did so very capably. Trilobite fossils have been unearthed on every continent, with more than 20,000 species identified by science. One of the most arresting animals of our pre dinosaur world, trilobites are also favorites among the fossil collectors of