Question

The paired passages discuss competing theories regarding the origins of avian flight.

Passage A
The evolutionary origins of avian flight have sparked intense paleontological debate, but the cursorial, or "ground-up," theory provides the most mechanically sound explanation. This model posits that the ancestors of modern birds were agile, fast-running, bipedal theropod dinosaurs. Abundant skeletal evidence from the fossil record demonstrates that these creatures were highly specialized for terrestrial locomotion, possessing powerful hind limbs and an erect posture. Crucially, paleontological discoveries have revealed that feathers were widespread among theropods long before the advent of flight. It is highly probable that these early plumage structures originally evolved to serve entirely different biological roles, such as thermal insulation in increasingly variable climates or as visual ornamentation for mating displays.

As these feathered bipeds pursued prey or evaded predators, their elongated forelimbs would have been utilized for balance and traction. Over successive generations, these proto-wings were co-opted for aerodynamic stability during rapid running and prolonged leaps. The transitional sequence from running biped to powered flier relies on the premise that running speeds generated sufficient aerodynamic lift to gradually elevate the animal off the substrate. While the energetic cost of launching from the ground is formidable, the biomechanics of terrestrial sprinting provide a continuous gradient of evolutionary advantage, where every incremental increase in leaping distance would be favored by natural selection. Flight, therefore, emerged as an exaptation—a secondary application of forelimb structures and integumentary features that had originally evolved for strictly terrestrial and physiological functions.

Passage B
For decades, the arboreal, or "trees-down," theory has offered the most aerodynamically elegant solution to the puzzle of avian flight. This model argues that the ancestors of birds were arboreal reptiles that navigated their environment by leaping between branches. From a biophysical standpoint, this trajectory is immensely compelling: an animal that utilizes gravity to glide from an elevated position expends a fraction of the energy required by one attempting to fight gravity and launch directly from the ground. Critics of the cursorial model correctly point out the immense physiological hurdles of generating takeoff velocity entirely through hind-limb propulsion.

Arboreal gliders, by contrast, would have followed a well-documented evolutionary pathway from leaping to parachuting, then gliding, and finally active flapping. Early tree-dwelling archosaurs were not equipped with the complex, asymmetrical flight feathers seen in modern birds, but fossil evidence suggests they possessed primitive, filamentous feathers. In the shaded, often cooler environment of the forest canopy, these preliminary structures likely emerged to facilitate temperature regulation. As these animals leaped from tree to tree to capture prey or escape threats, individuals with slightly longer or broader feathers experienced increased drag and reduced descent speeds. Natural selection subsequently refined these insulating structures into aerodynamic surfaces, optimizing them first for gliding efficiency and ultimately for powered flight. Thus, the complex mechanics of active flying were a delayed evolutionary consequence, built upon features that initially arose to serve a completely non-aerodynamic necessity.

Based on the texts, which one of the following perspectives regarding the evolution of avian flight do the authors share?