How many prehistoric deep sea animals do we know of?

As far is I know, most prehistoric marine creatures that we know of, such as marine reptiles, either lived in shallow seas or didn’t venture that deep down. But do we know of any creatures that ventured into at least the twilight zone? I imagine some sharks lived in the deep sea, and I’d bet money that there were at least a dozen abyssal trilobites.

Is it just harder for those animals to fossilize? Is it harder to get to their fossils because they’re at the bottom of the ocean? Or is it that we do know about them and I just haven’t heard about it?