From the Super Mario Wiki
Skeletons are seemingly undead enemies debuting in Donkey Kong for the Game Boy. They are designed to look like modern human skulls and first appear in Stage 5-2 of the Desert. Skeletons wander around on the ground 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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