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Ok. Nothing in law requires that a supervisor be paid at a salaried or non-hourly rate. The law only works the other way, disallowing people who do not qualify as salaried to be paid as salaried. The Fair Labor Standards Act only has rules about who is allowed to be paid a salary, not who must be paid a salary. This is because being paid at an hourly rate is considered the most just form of payment, because you receive overtime if you work more than 40 hours and your pay rate is tied directly to how many hours you work, which is fine.
Being paid lower than others, even others doing the exact same job as you, is also not illegal, in and of itself. By that, I mean that the mere fact of being paid less is not automatically a violation of any law. If the employer has a legitimate, and non-discriminatory basis for paying differently, then it is not illegal. Being paid differently can be a potential indication though of discrimination. If you have some evidence which suggests that the only reason you are being paid differently is your race, religion, gender, age, disability or FMLA use, THEN it could be considered an indication of illegal discrimination.
If, however, you can't allege discrimination based on one of those factors, it would not be illegal to pay you at a lower hourly rate.