In over your head? Possibly, but you can make the best of it. Take an instrument your comfortable on - if that's a fretted bass, then go for it (although if it's a pointy BC Rich, gore spattered creation, you'd better hope the rest of the cats play with their eyes closed).
For each song you put in front of you, take a couple of seconds to pick out important points. For example, what's the key signature, does it change, and are there any passages with a significant number of accidentals? What's the time signature, and does that change as well? How many different rhythms are involved? A lot of jazz tunes involve some kind of 4/4 walking line, maybe with a few twists thrown in; even a jazz fusion number with some scary looking rhythms may turn out to consist of only two or three main "rhythmic riffs" with minor variations.
There's also the people skills aspect. Turn up on time or even a little early. Set up quickly and make sure you're in tune. Talk with the rest of the band and sound out their approach - are they sticklers for accuracy, demanding that every note be in the right place or is it more of a listening, responding sort of gig, where the written score is only the starting point?
I'd also take along some way of making notes - at most rehearsals and auditions, I'll have a pencil, manuscript paper and probably my MD recorder. I might make a lot of mistakes but this shows that I'm a diligent learner. If I get invited back, I'm confident of showing definite improvement, which will probably count for a lot.
Finally, even if you end up train-wrecking every song, see what you can learn from the situation. The consensus might be that you're not ready for that gig but they might be able to give you a few pointers in case they're looking again a little bit further down the line.
Wulf