Final answer:
Theropods displayed significant evolutionary adaptations such as upright posture, feathers, efficient lung ventilation, grasping hands with semi-opposable thumbs, skulls with temporal fenestrae, and structures like the wishbone, all indicative of their lineage leading to birds.
Step-by-step explanation:
The key evolutionary novelties of theropods, which were a group of bipedal saurischian dinosaurs, include several adaptations that were crucial in the transition from primitive reptiles to the modern birds. A significant aspect of their adaptation for terrestrial existence was the evolution from a sprawling gait to an upright posture, where the legs were positioned directly beneath the body, increasing their agility and speed. Theropods were also among the early adopters of feathers, a feature closely associated with bird lineage, and displayed various forms from simple filamentous structures to more complex flight feathers.
Furthermore, these dinosaurs evolved a unique respiratory system that involved ventilation of the lungs akin to modern birds, which was more efficient than the bellows-like respiratory systems of other reptiles. Another significant evolutionary development in theropods was their hands, which became more grasping, featuring semi-opposable thumbs that possibly allowed them to manipulate objects and catch prey better.
Additionally, many theropods developed three major divisions in their skull, known as the temporal fenestrae, which allowed enhanced muscle attachments and gave rise to a more powerful bite. Finally, advancements such as the development of a wishbone (furcula) and the modification of the pelvic structure are indicative of an evolutionary trajectory that culminated in the emergence of birds, with the Archaeopteryx being a famous transitional fossil illustrating these changes.