Ankylosaurus was built like a living tank, with extremely thick, bony plates of armor on its body, even including armored eyelids. Its tail also had a second function; it could act as a deadly club. The club was used to swing around and smash into an attacking dinosaur. Its club was so daunting that not even a Tyrannosaurus rex would want to mess with this alarming species.[1] As a defensive creature, Ankylosaurus was one of nature's most perfect designs. It was almost impervious to being bitten from above and its long, strong tail muscles would have been able to swing its club with great force, however, the tail wasn't too flexible and could only swing 45 degrees in each direction.[1] There is very little fossil material from this dinosaur - it was apparently the last and largest of the family of armored dinosaurs. It walked fairly upright on all four legs, carrying its tail off the ground and it is theorized that it would drop to the ground when attacked to use its armor as a shield to cover its legs and underbelly.[1]