River dolphins are all in the superfamily Platanistoidea, while other dolphins are in superfamily Delphinoidea. Clearly river dolphins are anatomically similar to each other, with short dorsal fins and thin jaws, and have been classified by their anatomy.
But to me it seems implausible that all river dolphins are related to each other. Since their rivers are obviously not connected, they must have entered them independently, and then convergently evolved their riverine characteristics. But there are no extant marine Platanistoids.
This suggests that Platanistoidea is not a monophyletic taxon, and the river dolphins are not more closely related to each other than to marine dolphins. My question is: has anyone made any molecular studies to determine the phylogeny of Platanistoidea in relation to Delphinoidea? This would really help with my understanding of cetacean taxonomy.