The coelacanth has been one of my favorite fishes after reading A Fish Caught in Time as a young girl. Over ten years later, I've finally seen a specimen in person at Trieste's Natural History Museum (I somehow missed the one in the Smithsonian). I'd read about how large these fish get (up to 7 feet) but I was still shocked to see it for myself! Called a "living fossil," these fish, who evolved their current form ~400 million years with relatively few changes since, were thought to have gone extinct with the dinosaurs until a live one was netted in 1938 (I was very pleased as a young girl that a woman scientist, for whom its genus Latimeria is named after, played a large role in the discovery). The oldest known living lobe-finned fish, they're more closely related to mammals than ray-finned fish!
Additional reading:
