Not wanting to derail this, just wondering about boundaries, but isn't there argument for totally rigid mountain bikes? Surely all proper, contemporary, mountain bikes (not hybrid) have at least a boingy front end, for quite some time?
This isn't leading anywhere, it's not a leading question to try and get something to qualify, just really musing on the criteria.
Rigid doesn't seem to have survived as an off-road aspect, perhaps there's an odd exception, but as a generalism, whenever I've seen line-ups of modern bike ranges, I don't recall rigid being apparent.
Though all that said, which I think is fair comment, I don't perceive it as the spirit or intent of this month's BOTM, so my suggestion would be to disallow it as criteria, just that for the sake of discussion, it does seem to have evolved out of proper mountain bikes.
Don't think rigid mountain bikes were evolutionary.
And actually, fillet brazed rigid mountain bikes did survive, but now are called "boutique", so I guess they could be deemed endangered, but never extinct.
So, let's see those rare birds
Birds are dinosaurs, right?