Enchantment, 3G (4)
Rare
Creatures with no abilities get +2/+2.
Some mages forsake their scrolls and libraries to learn at the feet of ancient trees and sacred stones.
Illus. Scott Altmann

5/1/2007: 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.
5/1/2007: Animated basic lands have mana abilities, so they won't get the bonus.
5/1/2007: Some Auras and Equipment grant abilities to creatures, meaning the affected creature would no longer get the +2/+2 bonus. For example, Flight grants flying to the enchanted creature. Other Auras and Equipment do not, meaning the affected creature would continue to get the +2/+2 bonus. For example, Dehydration states something now true about the enchanted creature, but doesn't give it any abilities. Auras and Equipment that grant abilities will use the words "gains" or "has," and they'll list a keyword ability or an ability in quotation marks.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern