What rights do we have when a new company takes over and changes your job duties and pay?

My company was bought out. A few months later they fired my direct bosses. I kept working. After many others left, I realized someone needed to take charge of the office and asked to consider me for the position of Office Manager w/ additional duties and increase in pay to match what other OM make. They promoted me to OM, but with no salary increase. They disregarded my bonus structure after my bosses left, too. When I questioned it they said they would create a new bonus structure but nothing happened. So I have more responsibility for less pay. I feel discriminated against because I am one of two female managers and we are both severely underpaid compared to our male counterparts. I am still at the assistant salary that I was with my two bosses, less my bonuses, so I'm making even less than I was. Can we do anything about this? It's extremely frustrating and feels like we are being taken advantage of. We're not sure where to go or what to do about it. Any help is appreciated.