Imagine you’re a Plesiosaur looking for something delicious to eat then you see this un-appetizing fish swimming around. So instead, you do the logical thing. You bite but not kill it so it sinks down at the bottom of the ocean laying there wishing it was dead, but again it’s so unappetizing that nothing wants to eat it. Even if it’s the last fish in the vast ocean, the bottom feeders that will consume anything don’t go near it. Corpse for its whole time alive, nothing touching it so it is left an untouched fossil, preserved perfectly. The fish itself isn’t unique, really basic, and nothing special to it.