The term "living fossil" gets bandied about a lot, but this eel found in an underwater cave in the Pacific Ocean is unlike any living relative, and has features that are only found in the fossilized remains of its ancient brethren.
Found in a cave 35 meters underwater in the Republic of Palau, Protoanguilla palau is so unlike other Anguilliformes (eel-like critters) that it has prompted scientists to create a whole new family to describe it: Protoanguillidae. P. palau has features that are unknown among the 19 families and 800 species of recorded eels, comparable only to ancient ancestors.
A mitochondrial analysis of the newly discovered fish has shown 200 million years of independent evolution, resulting in it retaining features like a large head, short body, and collar-like gill openings. In fact, some of the laundry list of different features are so primitive that they're deemed older than the most ancient fossil remains — such as having fewer than 90 vertebrae.
We await the inevitable Syfy movie, in which P. palautopus fights a giant ammonite.