It was. But the infusion of breeds such as the smooth collie have changed the skull shape. The dingos skull shape is apparently it's only differentiating feature. It even differentiates it from it's closest relatives like the Papuan Singing Dog and the Basenji from Africa.
The dingo is classed as a variety of the species "Canis Familiaris" like all the other known breeds so is easily crossbred with other familiaris types (ie all other breeds of dog).