Stegosaurus was one of the earliest dinosaurs.  It was the first of the armored, or plated, dinosaurs.  A row of 20 tough, bony plates grew along this plant eater's back and tail.  The plates helped heat and cool the animal's body by either absorbing the sun's heat or reflecting the sun's heat away from the animal.  At the end of its tail, Stegosaurus had two pairs of sharp spikes to use as weapons.