Is it permissible to utilize and claim ownership of an agricultural land, when ancient graves containing decomposed skeletal remains were found on its eastern side?

The land in question is a graveyard, not a part of your farm. The presence of graves and bones therein confirms that it is an endowment (waqf) for those buried in it. It is not permissible for you or anyone else to own it or benefit from it by residing on it, farming it, building on it, or pitching tents on it.