i have done the test that mike said to to and it sunk and i have evidence i have got a picture below and i have a picture of the holey part which was requested by mike
Well that's definitely a bone: cellular core and denser margin, plus the overall shape, which I can see better now.
It could sink because it is saturated (ie. the internal holes are full of liquid rather than gas), but I suspect it has begun to mineralize, hence the hardness. ...However, as I wrote on the preceding discussion, I am not an expert on bones. Therefore I can't really comment on it being an aurochs, other than to see it as a possibility.
I hope somebody else can make a firmer comment for you.
Keep the question 'not answered' for a while.