The rib protected the fragments.

The specific chemistry and conditions of the site — the wet, anaerobic sediment of the ancient lake or pond — helped preserve the mastodon skeleton, and with it the evidence locked inside the rib.

If the weapon tip had passed between the ribs instead of striking one, the organic material would have been in contact with soft tissue that decomposed quickly, and it would have been lost.

The survival of the Manis evidence is, in a very real sense, an accident of ballistics.