Trilobites are hard-shelled, segmented creatures that lived hundreds of millions of years ago ( from the Lower Cambrian Period - 521 million years ago - to the end of the Permian - 240 million years ago) in the Earth's ancient seas and are distant relatives of modern lobsters, horseshoe crabs and spiders.  They are considered to be one of our planet's earliest complex life-forms.  Trilobites went extinct before dinosaurs even existed.