The origin of the salmonid fishes: marine, freshwater... or neither?

The evolutionary origins of the salmonidfishes, whether in freshwater or the sea, havebeen debated for centuries. Early viewsfavoured a group of marine ancestry invadingfreshwaters; more recently, there was a shifttowards a freshwater ancestry, on grounds thata return to freshwater to spawn indicates theancestral biome. Salmonids are widely believedto share an ancient common ancestry with thenorthern hemisphere Osmeridae and southernhemisphere Retropinnidae and Galaxiidae. Salmonidae are diadromous, as are Osmeridae,Retropinnidae and Galaxiidae. This suggeststhat diadromy is an ancient behavioralphenomenon across all these groups, that theshared common ancestry of these groups was alsodiadromous, and that the ancestry of Salmonidaewas neither marine nor freshwater, but wasamongst diadromous fishes. This begs thequestion of whether this common ancestor wasmarine or freshwater, a question for which ananswer seems likely to be elusive.