Star Trek Discontinunity - The Romulans

In The Original Series, the Romulans look more or less like Vulcans with pointed ears and a smooth forehead. However in subaquent series the Romulans seem to have developed two very prominent forehead ridges. I am very much at a loss to explain this, beyond “the makeup department screwed up.” Obviously the 80 to90 years between TOS and TNG is to enough time to facilitate an evolutionary shift, since Romulans are descended from a small group of Vulcans who left Vulcainus during the time of the Awakening about 1500 years before TOS. This is also not enough time for an evolutionary shift of that magnitude, therefore precluding the possibility of explaining this discrepancy by hypothesizing that there are two different races of Romulus, when Romulans did not in reality did not have enough time to evolve into two different races. Because of the fact that the Romulans did not have enough time to evolve forehead ridges, this means that none of them should have forehead ridges in the first place. However, except for those in TOS, all of the Romulans do. Once again I am at a loss to explain this because of the fact that Romulans descended form Vulcans only 1500 years ago, a blink of an eye in the evolutionary timescape, precluding the possibility of two Romulan sub-races. The only remaining explanations are preposterous. Therefore this will just have to go on being one of the unsolved mysteries of the Star Trek Universe.