@shrike the longer the run of a story, less detailed it is in the mind of the author beforehand... I bet he just had a concept in mind that he wanted make before really reaching this point in the manga. Like the idea that animals already having sigels in this world.
It is even worse if the author never imagine a end to the story, it would mean that the new arcs doesnt talk with older ones, but so far in this manga, i would say that this isn't the case.
A good example of that is the police asking Kaname to go back to school, that talked great with the beginning of the manga with all the missing people fuss that was starting to happen in the school.