An intermediate form of flounder?
Flounders and other flatfish are bizarre enough: one eye migrates to the other side of the head. What's weirder is that no one knew how this started - there were no forms in the fossil record with one eye partially migrated. It's hard to swallow the such a mutation could have arisen in a single leap. Well, not we have an apparent intermediate form, found, as so many important fossils are, in a forgotten museum drawer. "The left eye of this single specimen, a 50-million-year-old fish called Heteronectes, has migrated toward the top of the skull, but not all the way over."
How weird is that? Still unclear - how useful an eye in that position would be.