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I cannot believe that grown up scientists would propose that micro blast debris was gouging bones and skin.
They don't propose, they observe that one of the bisons has bone growth over the pieces of extraterrestrial metal. It survived the impact.
(Which, in turn, is consistent with your insistence that the debris was not very fast.)
Well, surely bone/tusks must be harder to penetrate than skin.
Just how much force is needed to penetrate a tusk? I suppose an elephant is a suitable substitute...