From the Super Mario Wiki
Skeletons are undead enemies in Donkey Kong for the Game Boy. Despite their name, they are only composed of a single human skull. They first appear in Stage 5-2 of the Desert. Skeletons crawl on the ground from side to side and hurl bones at Mario if he approaches them. Touching a Skeleton or getting hit by its bones results in Mario losing a life. Skeletons, as well as their bone projectiles, can be crushed with a hammer.
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