Picturing dinosaurs and the world in which they lived tickles the imagination. Paleontologists constantly find new information about the Mesozoic Era—the time of the dinosaurs—through new discoveries, research on bones stored away and forgotten, or new experiments that reveal details that scientists centuries ago could not have dreamed. After all, dinosaurs perished about 65 million years ago after surviving through incredible changes in the world for 186 million years. That length of time proves difficult to visualize. Even though dinosaurs were the dominant species during the Mesozoic Era, they were not the only animals on the planet. Reptiles were everywhere, scurrying among leaf litter on forest floors, flying through the skies, and swimming in the waters. By the time the Mesozoic Era wound to a close, dinosaurs had become incredibly diverse and different. Taxonomy is the name for the process of how scientists classify organisms. A taxonomic rank is a list of how these organisms are divided into categories.