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QI said (roll podcast title) There is no such thing as a fish, since that group is unbelievably diverse.

Strange that birds are dinosaurs, while Pterosaurs are not. Where is the bipedal fish that looks like a reptile or mamal, but is secretely a fish, too?

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I think it's not about diversity, but lineage. The phenotype for "fish" is so tight and well defined; a salmon is closer related to a human in the tree of life than to a coelacanth even though both are categorised as "fish".

I think you got that comparison backwards.

A coelacanth is a lobe-finned fish which is the group from which tetrapods, and thus humans, evolved.

A salmon is a ray-finned fish which is a very different group. These groups diverged sometime around 300MYA.

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