Being surrounded by sheep, foxes, and quite a few dogs, I would be very surprised if a fox took on anything bigger than a newly born lamb. The foxes I have seen skulk along the edges of the field, watched by the sheep who flock together and keep a very attentive eye on it.
My neighbour had a juvenile GSD, and he was always escaping from the garden to chase sheep around a neighboring field. Unfortunately she had to give him away before there were any casualties.
I have a border collie, but he has no interest in sheep whatsoever, he is well known by the local farmers, and blithely wanders through the fields completely oblivious to sheep, cows or horses. However should he spot a stick, now thats a different matter altogether.
My gut feeling is that the dog was responsible, although a bit of forensics might be required to tie the suspect to the scene of the crime.