Stegosaurus stenops is probably the best-known dinosaur to the general public that I've posted here, but despite being discovered in 1887, and being represented by multiple fairly intact skeletons, it has puzzled paleontologists in many ways, and continues to do so. Various Stegosaur species were widespread throughout North America and Europe in the Late Jurassic, but S. stenops specifically was from the region represented today by Colorado, Utah, and Wyoming. Since its discovery, the orientation of the back plates, tail spikes, and the animal's entire posture, have been redesigned and reorganized until arriving at what is currently considered an accurate restoration - alternating back plates, rather than in pairs; tail spikes held outward at an angle rather than upright; and an active posture, with head and tail held high.
The purpose of the plates, meanwhile, are what gives this image its title. Too fragile to be armor as originally assumed, the plates are filled with a network of blood vessels. They may have been used for thermoregulation, much like rabbits' ears - by carrying blood out away from the body through an organ with large surface area, they help cool the blood down and lower the body temperature. However, the most recent accepted hypothesis is that Stegosaurus could have used the plates for communication. By flushing high volumes of blood into the capillary network in the plates, they could change their color from a neutral tone to a bright red to intimidate enemies or rivals, or attract females. Birds today such as the Crested Caracaras do this, and are able to change the colors of their faces from gray to yellow to red in order to communicate their emotional state.