What counts as an ability for the purposes of Muraganda Petroglyphs?
Gatherer has the following ruling:


  Animated basic lands have mana abilities, so they won't get the bonus.


So your assumption is correct: any land that has become a creature does not get the bonus from  Muraganda Petroglyphs (except in the rare case where the land has lost its mana ability -- and I'm not aware of any card that actually does this).