obviously, this is focused on terrestrial animals; i'm mostly trying to unfuck my head from the linnaean "fish-birds-reptiles-amphibians-mammals" system of my youth by putting each of those groups where they actually belong on the tree of life. please note that i am an enthusiast, not an expert; this is what i, personally, have gleaned from trawling several wikipedia pages on modern evolutionary phylogeny, which is itself a science that's still in flux.
the shape of this diagram, i think, gestures at why you might've seen memes to the effect of "okay, but is there really such thing as a fish?" you can have an informal concept of a fish, sure, and it's a useful one for casual conversation, but there's no evolutionary history that ties all fish together but excludes land animals.
the other thing i think is interesting about this diagram is it conveys just how common convergent evolution really is. dinosaurs and birds are warm-blooded the same as mammals are in spite of having branched off from separate family trees; self-managed thermoregulation is just that useful a strategy for survival. you can see a similar effect in the fact that bats invented flight completely independently of birds, or, for that matter, the fact that birds did it well after pterosaurs had developed flight themselves. at the end of the day, physics constrains the number of useful shapes there are for specific tasks. it really makes me feel what a shame it is that we've never made contact with an alien species, because god i'd love to know how much of this has to do with earth and how much has to do with the rules of the cosmos writ large.
diagram notes:
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i truly do mean "most kinds." ray-finned fish are basically almost every species of bony fish you can ever imagine (wikipedia cites a figure of 95%.) the lobe-finned section of this diagram happens to be exhaustive; it's just lungfish, coelacanths, and tetrapods. if you can think of a fish with bones, it's probably a ray-finned one.
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"reptile-like" here is a weird way to try and capture something sort of complicated. the earliest synapsids probably looked and worked like modern reptiles, in that they were cold-blooded, layed eggs, etc., but slowly began to evolve mammalian traits over time, like hair, breasts, a tendency towards live births, and so on. modern mammals are the only survivors of the synapsid line; all of those middle-stage species have since died out.
