Is it legal to pay women significantly less than men for the same work?

Even though I have more responsibility than any other team member, male or female, I am paid the least ($24k less than the lowest paid male). My boss says it's because I don't have a degree, even though we were all hired with zero experience in the work we currently perform. My lack of degree hasn't hindered my work, or stopped me from assuming greater responsibility. I have approx. 15 years of work experience more than all but one of my team members. There are 8 of us - 4 women and 4 men. All women are paid at least $10k less than the lowest paid man. We all have the same job title and do similar work with the same level of responsibility, except for me, I have more responsibility and perform in two roles.