Historical facts are not fixed as the laws of nature. It is impossible though to find the truly "oldest" skull of any animal if you are an evoutionist due to the fact that no animal emerged abruptly, they all started as tiny microscopic creatures, and actually as molecules, cells, tissues that evolved into organisms through time continuum.
It is practically impossible to determine when an organism emerged to the level of being perceived as a "skull" because of the unbreakable continuum envolved in the process. You can find the "oldest" skull only in relative terms, hence Tanzanian, Ethiopian then Botswanian skull, etc.
I can assure you that's is not the end; scientists may once again in the future discover the "oldest" skull in Tanzania if further research is done into deeper places that hold historical clues of our earth.