Final answer:
The only feature of Archaeopteryx not possessed by a theropod was its beak. Archaeopteryx had many similarities with theropods but differed by having a beak rather than teeth.
Step-by-step explanation:
The only feature of Archaeopteryx not possessed by a theropod was its beak.
Archaeopteryx is a transition species from dinosaur to bird from the Jurassic period and shares many features with both. While theropods are a group of dinosaurs that include both extinct species and the ancestors of birds, key traits that differ between Archaeopteryx and theropods focus on adaptations related to flight. Theropods typically had sharp teeth instead of beaks, which are common in modern birds, and while Archaeopteryx had feathers similar to those of modern birds, it also possessed some characteristics more aligned with dinosaurs, such as teeth. However, it is the presence of a beak on Archaeopteryx that is a distinguishing feature not found in theropod dinosaurs.