Orca (also called killer whales) are an excellent example of convergent evolution. While whales are mammals and mosasaurs are reptiles, they share many characteristics. Both breath air, gave live birth, must surface periodically to breathe and descended from terrestrial ancestors.
As whale and mosasaur ancestors adapted to life in the ocean, their limbs were not required to support their weight. Both mosasaurs and whales no longer needed to have their legs connected to their backbones. While mosasuars retained hind limbs, whales’ hind limbs atrophied to just a pelvic girdle and in some species, the remnants of a femur.