yes, the reason they have that extra joint exists for them where the femurs are, is so their tail splits into legs when changing into human form. :3 it then fuses back together when going back to their aquatic form to allow for better swimming. hope that makes sense!
Half of it were, so the femur were elastic and flexible when in aquatic form?
yeah, the bottom half of them past their hips are more malleable in their aquatic form than their land form it hardens to support weight when it splits!