Trilobites are remarkable, hard-shelled creatures that lived some 400 to 500 million years ago in Earth's ancient seas. Trilobites represent the single most diverse class of extinct organisms. Throughout the 270 million years they inhabited the oceans, they continued to evolve into various species, leading to a great deal of diversity in their size, form and function. This means there is a wide variety of trilobite fossils to collect! One of the most appealing factors about trilobite fossils is that you can hold and examine an entire fossil animal as you turn it about in your hand.