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

The evolutionary origins of the salmonid fishes, whether in freshwater or the sea, have been debated for centuries. Early views favoured a group of marine ancestry invading freshwaters; more recently, there was a shift towards a freshwater ancestry, on grounds that a return to freshwater to spawn indicates the ancestral biome. Salmonids are widely believed to share an ancient common ancestry with the northern hemisphere Osmeridae and southern hemisphere Retropirmidae and Galaxiidae. Salmonidae are diadromous, as are Osmeridae, Retropirmidae and Galaxiidae. This suggests that diadromy is an ancient behavioral phenomenon across all these groups, that the shared common ancestry of these groups was also diadromous, and that the ancestry of Salmonidae was neither marine nor freshwater, but was amongst diadromous fishes. This begs the question of whether this common ancestor was marine or freshwater, a question for which an answer seems likely to be elusive.