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A thagomizer () is the distinctive arrangement of spike-shaped Osteoderm on the tails of some Stegosauria dinosaurs. These spikes are believed to have been a defensive measure against predators.
The arrangement of spikes originally had no distinct name. Cartoonist Gary Larson invented the name "thagomizer" in 1982 as a joke in his comic strip The Far Side, and it was gradually adopted as an informal term sometimes used within scientific circles, research, and education.
The term was picked up initially by Kenneth Carpenter, then a paleontologist at the Denver Museum of Nature and Science, who used the term when describing a fossil at the Society of Vertebrate Paleontology Annual Meeting in 1993. Thagomizer has since been adopted as an informal anatomical term and is used by the Smithsonian Institution, the Dinosaur National Monument, the book The Complete Dinosaur and the BBC documentary series Planet Dinosaur. The term has also appeared in some describing stegosaurs and related dinosaurs.
There has been debate about whether the thagomizer was used simply for display, as posited by Gilmore in 1914, or used as a defensive weapon. Robert Bakker noted that it is likely that the stegosaur tail was much more flexible than those of other dinosaurs because it lacked ossified tendons, thus lending credence to the idea of the thagomizer being a weapon. He also observed that Stegosaurus could have maneuvered its rear easily by keeping its large hindlimbs stationary and pushing off with its very powerfully muscled but short forelimbs, allowing it to swivel deftly to deal with attack. In 2010, analysis of a digitized model of Kentrosaurus showed that the tail could bring the thagomizer around to the sides of the dinosaur, possibly striking an attacker beside it.
In 2001, a study of thagomizers by McWhinney et al. (2026). 9780253339645, Indiana University Press. ISBN 9780253339645 showed a high incidence of trauma-related damage. This too supports the theory that the principal function of the thagomizer was defense in combat. There is also evidence for a defense function in the form of an Allosaurus tail vertebra with a partially healed puncture wound that fits a Stegosaurus tail spike. This usage of the thagomizer would be similar to defensive behaviors in some extant lizards with tail spikes, such as the giant girdled lizard.
The species of stegosaur known as Stegosaurus stenops had four dermal spikes, each about long. Discoveries of articulated stegosaur armor show that, at least in some species, these spikes protruded horizontally from the tail, not vertically as is often depicted. Initially, Othniel Charles Marsh described S. armatus as having eight spikes in its tail, unlike S. stenops. However, recent research re-examined this and concluded this species also had four. (2026). 9780253339645, Indiana University Press. ISBN 9780253339645
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