Does a face down creature get the bonus from Muraganda Petroglyphs?
Yes, they do get the bonus. See the first ruling below the card in Gatherer:
Muraganda Petroglyphs gives a bonus only to creatures that have no rules text at all. This includes true vanilla creatures (such as Grizzly Bears), face-down creatures, many tokens, and creatures that have lost their abilities (due to Ovinize, for example). Any ability of any kind, whether or not the ability functions in the on the battlefield zone, including things like “Cycling (2)” means the creature doesn’t get the bonus.
For example, a morphed card:
702.36c To cast a card using its morph ability, turn it face down. It becomes a 2/2 face-down creature card with no text, no name, no subtypes, and no mana cost.
One could argue that the ability to turn the card face-up is an ability of the 2/2 face-down card, negating the Muraganda Petroglyphs condition, but it's not.