The thagomizer is the arrangement of tail spikes on extinct plant-eating dinosaurs, specifically of the family Stegosauridae, which had, depending on the species, between four and fourteen spikes placed in pairs along the tail. The most famous of these was Stegosaurus stenops, a 30-foot animal whose fossilized remains are found in western North America.
Once called simply "tail spikes" or osteoderms scientifically, the word thagomizer had its origins as a mere joke. In 1982 cartoonist Gary Larson published a panel from his Far Side series of comic strips, which showed a lecture hall of caveman scientists, one of which was pointing out the feature of the stegosaur responsible for the demise of "the late Thag Simmons". In fact, so popular was the panel with scientists that the term thagomizer was adopted as an actual anatomical term for the Stegosaurus tail-spike arrangement.