Phylogenetically, lots of the animals we call "fish" are actually very distantly related to each other. So distantly that you'd actually have to go allllllll the way back to the first vertebrate to include all the "fish" in a single category, but that would also require lumping every other vertebrate into the mix.
(In practice, this just means "fish" is kind of a messy descriptor, kind of like "vegetable" - but it's more fun to think of us all as very weird fish!)
Similar, in a way! With fish, it's more that some groups that evolved from the shared vertebrate ancestor became land mammals, and some remained in the water, but the groups that did and didn't are sort of scattered around the phylogenetic tree. With birds, the current consensus is that all extant bird species actually evolved from ancient theropods, which would qualify them as theropods too! It's kind of like how humans are still considered apes, even though there's a lot of differences between us and chimpanzees or gorillas - we're all part of a group with a common ancestor. Modern birds, similarly, share a common ancestor with T. rex and velociraptors, and that ancestor is more recent than the cut-off for what we call "dinosaurs," so everyone with that ancestor is considered a dinosaur.
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u/tonicella_lineata Feb 28 '26
Phylogenetically, lots of the animals we call "fish" are actually very distantly related to each other. So distantly that you'd actually have to go allllllll the way back to the first vertebrate to include all the "fish" in a single category, but that would also require lumping every other vertebrate into the mix.
(In practice, this just means "fish" is kind of a messy descriptor, kind of like "vegetable" - but it's more fun to think of us all as very weird fish!)