### Models of the Origin of Avian Flight
How theropod dinosaurs developed the ability to fly is a subject of ongoing debate among paleontologists. Two primary models have been proposed to explain the origin of flight.
Model 1 (Cursorial / "Ground-Up" Model)
Model 1 proposes that flight evolved in small, bipedal, ground-dwelling theropods. These dinosaurs ran along the ground and jumped to catch insect prey or escape predators. According to this model, the proto-feathers on their forelimbs initially functioned to provide stabilization and increase lift during running jumps. Over time, as these dinosaurs ran faster and flapped their forelimbs to maintain balance, the aerodynamic forces increased, gradually transitioning from running jumps to powered, gliding, and eventually flapping flight. Under Model 1, the anatomical modifications for flight, such as powerful pectoral muscles, developed primarily in response to the physical demands of launching upward from flat ground.
Model 2 (Arboreal / "Trees-Down" Model)
Model 2 proposes that flight evolved in tree-dwelling (arboreal) ancestors of birds. These small reptiles or dinosaurs climbed trees using claws on their forelimbs and hindlimbs. Once in the trees, they leaped from branch to branch to avoid predators or search for food. Proto-feathers initially served as parachuting structures to slow falls and control landings. As these animals leaped, the proto-feathers on their limbs allowed them to glide over increasing distances. Over generations, gliding behavior transitioned into active flapping flight to control directional movement and extend range. Under Model 2, gravity provided the initial energy for flight, and anatomical adaptations for powered flight developed after gliding behavior was already well established.
According to the descriptions of the two models, which of the following statements best identifies a point of disagreement between Model 1 and Model 2 regarding the primary behavior that initiated the evolution of flight?
- AModel 1 claims that flight initiated with climbing and leaping from trees, whereas Model 2 claims that flight initiated with running and jumping from the ground.
- Model 1 claims that flight initiated with running and jumping from the ground, whereas Model 2 claims that flight initiated with climbing and leaping from trees.Answer
- CModel 1 claims that proto-feathers initially functioned as parachuting structures, whereas Model 2 claims that proto-feathers initially functioned to provide stabilization during running.
- DModel 1 claims that anatomical modifications for flight developed after gliding was established, whereas Model 2 claims that modifications developed primarily in response to launching from flat ground.