If you are an ALPA carrier then seniority is in Section 22 of your contract. Read that section and see if there are any protections. The first order on challenging this would be in the grievance process.
Go to Section 22 and then also to Section 3 or your contract. Section 3 in ALPA deals with pay you need to see if there is clear language that contradicts your present situation. Another thing to remember is that the internal "Statute of Limitations" on filing a grievance on this issue will be in the grievance section, 20. Make sure you don't allow that timeline to go by without taking action on a grievance.
You might consider filing a pay claim for the amount you think you should be owed. If the company refuses then you have a dispute over pay and you can go down the grievance road. Many courts will require you to at least start the grievance process, minor dispute machinery, before they will address the issue. The RLA delegates minor disputes to the system board and grievance process, and if you bypass that it is always the company's first issue on a motion for dismissal.
BTW, what company are we talking about??