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The first occasion in which this form is shown is the last chapter of volume 1, as one of the hypothetical future transformations of the Mario series. Mario can transform into a skeleton for three minutes after collecting a Hone, but it has no known advantages or powers.
In volume 2, a Fishbone attacks the heroes by shooting a "Bone Ray", turning Luigi and Yoshi into skeletons. Before Mario can be hit, however, a Super Star grants him invincibility, and he defeats the enemy. Meanwhile, Luigi and Yoshi ask how they will turn back to normal.