"lamarck," Lamarck.
"Hollow Bone" should be "Hollow Bones".
Completely losing its wings all at once, in one step, is certainly odd. Given the time scales of Generations, and the fact a finger deletion is so simple, this is probably possible, but it does bring up the question of why it would spread. One could argue that the wings were prone to frostbite and had limited value (due to cold conditions) for large parts of the year, and perhaps that flight was difficult for a big, heavy grazing herbivore (something like hoatzins), and perhaps it had no predators that necessitated flight throughout its entire lifespan, but why wouldn't the wings simply become vestigial? Humans themselves have several vestigial or near-vestigial traits, such as third eyelids, simply because there was no evolutionary pressure to lose them entirely.
A drastically shorter wing-finger with a tiny finger-webbing-esque wing could be a good compromise. It would be easy to hide under fluff right next to the body's warmth. Alternatively, you could give them turkey-wings too small relative to body size to let adults fly, but the young could do brief, low-altitude bouts of flight. (Though perhaps not to escape predators: no predators are listed, and the main cause of death in the ancestor's young seems to be starvation and freezing.)
EDIT: On second thought: it's fine. Just go into extensive detail on how it would occur and why it spread.