Trilobites were arthropods with soft bodies and many legs that were positioned under a heavily calcified, segmented shell. They first appeared in the Early Cambrian Period (544 million years ago) and thrived through the Middle and Late Cambrian times before their extinction in the Ordovician period (495 million years ago). In their 49 million year existence, they diversified quickly and produced a fossil record that geologists refer to often.

Arthropods are a phylum of hard-shelled creatures with multiple body segments and jointed legs. Trilobites are a class of this phylum that includes eight orders: Agnostida, Asaphida, Corynexochida, Lichida, Phacopida, Proetida, Ptychopariida, and Redlichiida. The trilobites were very diverse, especially in size, ranging from less than 1 millimeter to 70 centimeters. That's almost two feet long! Although they ranged in size, they all shared a standard three-part body: The cephalon (head), thorax, and pygidium (tailpiece). The name "Trilobite" refers to the lobes of the...