Final answer:
Thyreophoran dinosaurs, which includes stegosaurs and ankylosaurs, are principally united by their characteristic body armor, made up of bony plates and other forms of skeletal-derived protection.
Step-by-step explanation:
Thyreophoran dinosaurs are united principally by their body armor. This distinctive characteristic includes the presence of bony plates, spikes, and other forms of skeletal-derived protection. Thyreophora includes stegosaurs and ankylosaurs, both known for their armored bodies. For instance, Stegosaurus had a double row of plates along its back, and Ankylosaurus featured a heavily armored body and tail club.
Although pterosaurs are often discussed alongside dinosaurs, they are not actually dinosaurs but are rather flying reptiles with adaptations for flight, such as hollow bones and wing membranes. Dinosaur classification includes groups like Saurischia ("lizard-hipped") and Ornithischia ("bird-hipped"), the latter being closer to the Thyreophora with features like a complex pelvis. However, the distinct characteristic for Thyreophora is their body armor rather than pelvis structure or other features.