Final answer:
The key evolutionary novelty of the thyreophorans is their armor plates, which provided protection, potentially aided in thermoregulation, and were possibly used for display.
Step-by-step explanation:
The key evolutionary novelty of the thyreophorans is their armor plates.
Thyreophorans are a group of armored dinosaurs known for their distinctive body armor. This group includes the famous Stegosaurus and Ankylosaurus, among others. The function of these armor plates was primarily for defense, but they may also have been used in thermoregulation and display behaviors.
The plates and spikes provided protection against predators, making thyreophorans one of the better-protected herbivorous dinosaurs. It's worth noting that while some thyreophorans also had spiked tails, the defining feature that sets this group apart is indeed their body armor, which could come in the form of osteoderms and plates.