Well despite the fact that this is very stressful, I would start with the maxim that one should employ Hanlon's Razor[0] and not ascribe to malice what can adequately be explained by stupidity.
It's quite possible, nee probable, that they are just disorganized and genuinely thought you already knew, or that it didn't matter, or otherwise were clueless or inexperienced enough to not really understand what had happened.
I think it would be a mistake to get into it with them on a war footing. Early stage companies like this make mistakes of this kind all the time, and it's not necessarily a sign that the whole ship is doomed. If they have the ability to get revenue and investment and pay salaries and whatnot it's possible that there's a positive outcome in the future.
I would schedule an initial consultation with a lawyer who deals with these kinds of things. Get a referral and schedule an hour or so to go over it with them. They'll probably charge you a few hundred bucks, or even waive it if you have a close friend or existing client of theirs refer you.
Sit down with a lawyer, bring the docs, and explain what happened and get some sense of the implications. Then approach the founders with the attitude that it must be an administrative mistake or misunderstanding. If they are evasive or otherwise shady that should become clear quickly. If they are genuinely concerned and want to help fix the problem then great. But personally I wouldn't blow up your relationship with everyone until you have a better idea which of those two scenarios is more descriptive.
[0] http://en.wikipedia.org/wiki/Hanlon's_razor