Do any of the novels explain why some Romulans have forehead ridges? Is it because of the Augment virus? If so, how did it manage to find its way into the Romulan genome? Did it happen to them the same way it happened to the Trill?
Because of Spock, the general assumption is that Romulans come both ridged and un-. It's mentioned somewhere in (IIRC) the Vulcan's Noun books, but I think that it's just written off a case of genetic drift over the centuries.
What was the explanation for the Trill again? I don't remember, even though I probably read that book a year ago ....
i don't know what the in-canon story is, but the behind the scenes story is that when they switched to prosthetic foreheads for the romulans it was easier to get guest actors to portray romulans b/c it meant that the actors wouldn't have to shave any parts of their eyebrows. seems rather superficial, but that's the scuttlebutt.
Spoiler: Forged in Fire The Augment virus manages to infect a Trill colony, causing an epidemic of forehead ridges among the hosts who lived there. It explains how the Trill initially looked ridged in "The Host" from TNG.
scuttlebutt's wrong. they added the ridges to make them 'mean' looking next to ridged Klingons. IIRC, Westmore did it.
IMHO, at this point they should write off the ridges and make them all as smooth headed as the Vulcans. I forget exactly how long ago the Sundering was supposed to be, but having Vulcans and Romulans being almost exactly the same in appearance while having vastly different mindsets, seems like the way to go. They did it in the original series and in the new movie, and it makes the split between the two groups seem more like a family squabble as opposed to having two seperate species, which the ridges make you think is the case. It is something that can help bind them together closer, and make them seem like opposite sides of the same coin.