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Both Ankylosaurus and Armadillo are covered in armor. Because of their armor, armadillos have few predators, and Ankylosaurus had none.
The name Ankylosaurus means "fused lizard." This refers to the fact that many of the bones in the skeleton of Ankylosaurus were fused together in order to strengthen them enough to support the Ankylosaurus's heavy armor.
Ankylosaurus is not known to have had any armor on its eyelids. However, some of its relatives like Euoplocephalus did have armored eyelids.
Ankylosaurus was covered in bony nodules called scutes. These created armor that made them invulnerable to attack by carnivorous dinosaurs. Ankylosaurus also had a bony tail club that it could have used for defense.
Ankylosaurus was a quadruped. That means they walked on all four legs. They weren't and didn't have to be fast because they were covered in thick, bony armor.
Ankylosaurus's armor was so impenetrable that predators like Tyrannosaurus and dromaeosaurid dinosaurs probably never attacked it. If a predatory dinosaur attacked it, Ankylosaurus could have killed the enemy with a blow from its tail club.
That would depend on the type of bullet. I would say that a bullet from a handgun or a hunting rifle probably wouldn't be able to pass through the armor of an Ankylosaurus, but it would damage the armor. There are more powerful guns, though, such as certain machine guns or anti-tank weapons, which would be capable of penetrating an Ankylosaurus's armor. Of course, this is simply conjecture and one would need a lot of equipment and training to perform an experiment and determine for certain how bulletproof Ankylosaurus's armor was.
Ankylosaurus's armor made it very heavy, and it had very short, sturdy legs. Although this protected it from predators very effectively, Ankylosaurus wasn't very fast.
Like other land animals, Ankylosaurus walked. They were quadrupeds, meaning that they walked on all fours. Because of the weight of their armor and their short legs, they couldn't move very quickly.
Ankylosaurus were covered in bony knobs to protect them from predators. The armor actually made them nearly impossible to attack. Furthermore, the large knobs of bone on the end of the tail, the tail club, could be swung as a lethal weapon to defend the ankylosaurus. As such, ankylosaurus were not hunted even by T-rex. They were too difficult and dangerous to attack.
Ankylosaurus's entire back, tail, neck, and head was covered in bony armor that was impenetrable by Tyrannosaurus. In addition, Ankylosaurus was armed with a bony tail club that would easily have been lethal to an attacking Tyrannosaurus. For these reasons, it is no surprise that Ankylosaurus fossils are never found with Tyrannosaurus bite marks, unlike the fossils of other contemporary herbivores like Edmontosaurus and Triceratops, which are frequently found to have been eaten by Tyrannosaurus. Thus, it is safe to say that Tyrannosaurus rarely, if ever, attacked Ankylosaurus.
A lot of the body had armor, including the back, head, and even the eyebrows! But the tail had no armor, but it did have a club that could have crushed the leg of a Tyrannosaurus rex.