Now, Now, we all know that the skulls were of peaceful, vegan Aztecs who lived as one with nature and their native fellows of other tribes, who were killed by the mean nasty Spainards and then planted by them to make it look like the Aztecs had bloody ritual murder/sacrifices of their enemies and slaves.
(just giving the political correct view, that I totally disagree with)
The truth is often not politically correct, and this is a good example.