Random thought of the day…in ST: Picard for the first time there was Romulan with curly hair, indicating that the Romulan straight hair/bowl cut look is perhaps manufactured through styling.
Perhaps this is also true of Cardassians?
Imo it stands to reason that in both Romulan and Cardassian societies - both of which are oppressive and dislike individuality - that any sort of physical difference would be suppressed to the extent possible.
I mean - bc of racism and similar attitudes toward physical difference, this is thing in human society now. Meghan Markle’s hair is prime example of this - she has pretty natural curls, but was forced to straighten them to get acting work (and has kept it straight since).
So I stand by headcanon that some Romulans and Cardassians have curly hair, but straighten it.
