The back plates on a stegosaurus are usually called scutes or simply plates ; the tail spikes are called the thagomizer.

Quick distinction

  • Back plates: scutes, bony armor-like plates along the spine.
  • Tail spikes: thagomizer, the defensive spikes at the end of the tail.

In plain English

People often call all the pointy-looking parts “spikes,” but technically the big structures on its back are plates , not spikes.

TL;DR: Stegosaurus back “spikes” are usually called plates or scutes ; the actual spikes are on the tail and are called the thagomizer.