Specifically skeletons are a big deal. Lots of games edit them out, WoW had alternate models for the Undead players, who generally have exposed bone joints and other bits of bone protrusions, to cover them all in flesh. I think it is sometimes OK to use skeletons as enemies, but never for player characters, IIRC.

I mean the graph starts in 09, and Chrome launched in 08. I assume that did more to them, but both were probably notable.

Anydice.com can handle this stuff easily. As already pointed out in another comment, it does perfectly match. What it will not tell you is if you grabbed one or 2 arrows, though presumably a roll of 1-x could be used to say you got one, and x+1-15 means you got two.

IIRC it is actually mostly from algea. A small amount from some fern-like plants. By the time trees existed, they were being broken down by bacteria.