Stegosaurus (Stegg-oh-soar-us) was an armored dinosaur that lived in western North America during the Jurassic Period, approximately 150 million years ago. It was 30 feet in length and stood 10 feet high at the hip (not including the plates extending from the back). Stegosaurus is believed to have walked on all fours, although the shortness of the forelimbs has led some scientists to speculate that Stegosaurus may have been able to raise itself up on its hind legs.
The plates of Stegosaurus are, perhaps, its most outstanding feature. These were roughly diamond-shaped and embedded in the skin from the neck to the tail. They were not directly attached to the skeleton. The plates were relatively small on the neck and toward the end of the tail, and became very large along the back approaching the hip region. An articulated skeleton indicates that the plates were arranged in two, staggered rows (i.e., left and right plates were not immediately parallel to each other, but rather alternated). Further, the plates stood erect along the spine and did not lay flat to cover the back. The impressions of blood vessels along the outer surfaces of the plates suggests that they were covered with skin in the living animal.
The tail of this dinosaur was equipped with four large spikes, arranged as two pairs, which were evidently used as a weapon. There were also small, flattened nodules embedded in the skin elsewhere in the body.
Stegosaurus is sometimes erroneously described as having three brains. This is because, in addition to the actual brain, there were two impressive swellings in the spinal cord: one in the shoulder and the other in the hip region. Such swellings are not uncommon in creatures with a backbone, however, and, in fact, also occur in humans! These swellings represent areas where many nerve cells are exiting the spinal cord to control the arms and legs. Remarkably, these swellings in Stegosaurus were actually larger than the brain itself!