The name Trilobite means three lobed and refers to the main axial lobe which is flanked by side-ribs on the left and right, not the head, thorax, and tail that one might assume. They were one of the most common creatures of the Paleozoic era and went extinct before dinosaurs ever appeared. . Their exoskeletons lent themselves easily to fossilization due to the high calcite content. Each family has distinct variations- so you can pick out one or a few that match your fossil style.