Stegosaurs are some of the most instantly recognizable dinosaurs to the general public. They are well known for their distinctive anatomy, especially the large flat plates that covered the neck, back, and tail, the four or more large spikes at the end of the tail, and ocassionally, large spikes arising from their shoulders. New discoveries and investigations are bringing us a more detailed picture of the function of the plates and spikes whose purpose has been debated since their discovery.
Stegosaurus and its closest relatives, appropriately known as stegosaurs, were four-legged plant-eating dinosaurs that had distinctive armor made up of upright flattened plates on the neck and back and spikes on the tail. The different stegosaur species have unique sizes, shapes, and arrangement of armor, but were fairly similar to each other in most other aspects.
The first fragmentary stegosaur fossils were found in Europe and Africa in the early 19th Century, but it wasn't until the discovery of Stegosaurus itself in the United States in 1877, that their true nature was revealed. They were instant media sensations, legitimately alien reptiles that roamed primeval forests on this very planet in the incredibly distant past.
The earliest scientific and artistic reconstructions of Stegosaurus show an animal with truly bizarre proportions. It had very short forelimbs and very long hindlimbs which caused the back to slope steeply to a small head on a stubby neck. The plates were initially placed in a single row down the middle of the back, and some early reconstructions show eight spikes at the end of the tail instead of four.
Other early ideas about stegosaur plate anatomy include the idea that they in fact covered the sides of the animal like a pangolin or armadillo. Another proposal that was published (although probably as a joke) claimed that the plates were oriented horizontally and could have been used as flight surfaces for gliding off cliffs.
More nearly complete remains showed that the plates are actually in two staggered or alternating rows instead of one and that at the end of the tail there were just four spikes. The short forelimbs and long hindlimbs were still reconstructed in a way that created a steeply sloped back and a short, stubby neck.
Examination of the braincase and vertebrae of Stegosaurus revealed further surprises. It was found to have a tiny brain, and an expansion in the hip vertebrae that was commonly, but wrongly, regarded as a second brain. This gave it a reputation of having an IQ so low it needed another brain in its rear-end.
To make matters worse, not only was it nearly brainless, it was thought of as woefully under-armored, the plates offering no protection from predators, and the tail surely too heavy to swing. For much of the 20th Century, Stegosaurus the dullard was a dominant narrative about the animal. Many artistic reconstructions of the time showed carnivorous dinosaurs taking bites of hapless stegosaurs that barely seemed to notice.
