Anguillidae

Anguillidae, the freshwater eels, are a family of eels. "Anguilla anguilla" is the type species. Most of them are extant. Aside from the living species, there are also prehistoric eels. 
They spend their adult lives in freshwater, but migrate to the ocean to spawn. Eels are an important food fish. Some species are now farm-raised, but not bred in captivity. Many populations in the wild are now threatened. Seafood Watch recommends people avoid eating anguillid eels.
The first fossils of eels is found at the start of the 
Palaeocene, just after the Cretaceous–Paleogene extinction event 66 mya. If prehistoric eels like "Urosphen" were still alive, they would have replaced the extant eels.