Hmm. If anything I'd expect them to have evolved a lot of interesting deep-sea adaptations that should render them rather unlike our 500 MYA ancestors... remember they've had just as long to evolve to their peculiar environment as we have.

Still, you've got me thinking about this question. I would think the present-day fish that most resembles our ancestor of that time ought to be something less exotic. Perhaps something like a hagfish.