In the middle of the movie as they make their escape from the city, the protagonists see what appears to be a decapitated zombie. It approaches, and they are unable to immediately destroy it because the only vulnerable target is missing.

Soon though, it becomes apparent that the head is still attached by a narrow piece of what seems to be skin tissue, and was non-visible because it was hanging down behind the zombie's back.

The spinal column is unambiguously 100% severed (someone correct me if they believe I'm wrong in that regard). What are the implications for how zombies work (within the Romero canon)? This seems to imply zombies that are supernatural in nature, and not the result of some phenomenon that is scientifically explicable. Do any of these implications contradict other events within Romero canon?