Outside of a statement on a certain race of now extinct snake-peoples, not really. They were never really explored within cannon due to how small of a force they were. also, they're from really old print, which no one really considers cannon any longer
Alrighty then, tribe-world shaman? they're basically psychers from worlds reduced to tribal states within the imperium. If catachan didn't receive all the help it did, it would have likely become one.
I say its fine as the Black Dragons could easily claim to be from the Salamanders, other chapters certainly do and now there is no one to tell them to fuck off
And in reply, I am not sure how Chapters at all write up how they are from where. I just assume some First Chapter Master wrote it as such and everybody just went along. Timespans are in the millennias here.