What's with your name and your avatar?
Quoted from
this Wikipedia article:
The thagomizer, or tail spikes, is an arrangement of four to ten spikes on the tails of particular dinosaurs of the clade Stegosauria, of which Stegosaurus stenops is the most familiar. The tail arrangement is believed to have been a defensive weapon against predators.
The term "thagomizer" was coined by Gary Larson in a 1982 Far Side comic strip, in which a group of cavemen in a faux-modern lecture hall are taught by their caveman professor that the spikes were named for "the late Thag Simmons". The term was picked up initially by Ken Carpenter, a palaeontologist at the Denver Museum of Nature and Science, who used the term when describing a fossil at the Society of Vertebrate Paleontology in 1993.
Thagomizer has since been adopted as an informal anatomical term, appearing, for example, on the website of the Smithsonian Institution. The term has been used in displays at Dinosaur National Monument in Utah, in the book The Complete Dinosaur, and in the stegosaur display at the Smithsonian Institution. As of 2007, however, the term does not appear to have been published in a peer-reviewed scientific journal.
Stegosaurus had four thagomizers at the end of the tail, as did most of the Stegosauria (like Huayangosaurus, Tuojiangosaurus, etc.), though there is evidence that at least one (Chungkingosaurus) may have had five or more. So my avatar depicts the end of a Stegosaurus tail. However, it must be kept in mind that while most restorations show the spikes pointing vertically, a recent study has shown that they more likely pointed horizontally, which probably made them deadlier, as in the picture below:
