Final answer:
The rotation of larvacean fins allowed for improved locomotion as vertebrates moved from aquatic to terrestrial environments, repurposing their aquatic movements to adapt to land by using the ground for movement.
Step-by-step explanation:
The rotation of larvacean fins aids in locomotion. During the evolution of body design for locomotion on land, vertebrates that transitioned from aquatic to terrestrial environments had to adapt their movement strategies. In the water, the lateral undulations of a fish's body allow for forward movement by pushing against the water. However, as vertebrates began to move onto land, their fins or flippers became contact points with the ground, which they rotated around to facilitate movement. Essentially, the fins that were adapted to aquatic locomotion were repurposed to interact with the ground for land locomotion, continuing the undulatory motion but now using the ground as a surface to push against for movement.