Warm Blooded Fish

Opah, a kind of fish also known as Moonfish (scientific name Lampris guttatus), are the first species of warm blooded fish every discovered.  Their warm blood gives them an advantage as a predator in deep, cold waters, where other fish, because they are cold blooded, are sluggish.

Sharks and tuna, which are also predators, have a much diminished form of warm bloodedness specific to only a few muscle regions that don't operate at great water depths, but nothing approaching the warm-blooded adaptions of the Opah.

Note that the study reporting the finding does not reach the conclusion that warm bloodedness in the Opah is ancestral to warm bloodedness in mammals and birds.  Indeed, this is almost surely not the case, because birds and mammals are descended from cold blooded reptiles (although some dinosaurs were probably also warm blooded).  Instead, warm bloodedness in the Opah is an independent case of convergent evolution.