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We've all heard it: birds descend from dinosaurs, so they're dinosaurs too. But this got me thinking: doesn't this mean that, for instance, all terrestrial vertebrates – including humans – are technically fish? A recent video by MinuteEarth and the Wikipedia article for "Fish" confirmed my shower thought hypothesis.

Interesting. But... all amniotes, i.e. reptiles (and, by extension, birds) and mammals, descend from amphibians, right? If so, then why aren't they considered amphibians too?

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    $\begingroup$ It's for logical clarity. Languages aren't perfect and there can be only 1 word when 2-3 words would be useful. Amphi means both and bios is life. It means double life technically it doesn't mean water and air. the combination of AM AN and IBI in the word amphibian make it a word that has stood the test of time. $\endgroup$
    – bandybabboon
    Mar 24, 2019 at 2:32

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Mammals and reptiles aren't considered amphibians, because amniotes are not hypothesized to descend from Amphibia. That is to say that Amphibia did not evolve into Amniota. They are sister clades (actually Reptiliomorpha in the Tree of Life tree below).

Tree

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  • $\begingroup$ Ah yes, this makes more sense. I must've misread something and concluded that amphibians are the ancestors of mammals and reptiles. Still though, doesn't Mammalia descend from Reptilia? If so, what about the fact that Mammalia isn't classified within Reptilia? $\endgroup$
    – Lucca
    Mar 24, 2019 at 1:19
  • $\begingroup$ Follow the Amniota link on the page linked above. You will see sister clades Synapsida and Reptilia. I'm not sure what wikipedia says, but I wouldn't trust it in general. $\endgroup$
    – kmm
    Mar 24, 2019 at 13:58
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Some named groups are not monophyletic (see this post for definition if needed). Fishes do not represent a monophyletic group. Groups like "fishes" are completely awful to define but everyone would agree that no, humans are not fish!

Now, it is important to understand for many terms, there might have a scientific and a popular definition of the term. For example, when people talk about reptiles, they talk about turtles, snakes, lizards, squamates but they don't talk about mammals. Hence by "reptile" they don't mean "Reptilia". Have a look at the post If dinosaurs could have feathers, would they still be reptiles?).

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  • $\begingroup$ Huh. I thought the Scientific Classification of life was 100% cladistic, but since the reptilia and mammalia classes aren't contained within amphibia, I guess middle-school Biology kinda lied to me... $\endgroup$
    – Lucca
    Mar 23, 2019 at 23:50
  • $\begingroup$ I think my answer was a little unclear. I edited it. Please read it again. I did not mean that in cladistic, we name groups that aren't clades (clade = monophyletic group). I mean that the terms "Reptilia" and the term "reptiles" don't mean the same thing. $\endgroup$
    – Remi.b
    Mar 24, 2019 at 0:01
  • $\begingroup$ Thanks, but my question remains, though: if – as you've stated yourself – cats are Amphibia, why isn't Felix catus (or any of the other amniotes, for that matter) taxonomically classified within Amphibia? $\endgroup$
    – Lucca
    Mar 24, 2019 at 1:09
  • $\begingroup$ Amphibia do not contains cats though. Amphibia only contains Anura (the frogs and toads), Urodela (the salamanders), and Apoda (the caecilians) (note I corrected a mistake in my above post). Reptilia do contains cats though but you would never state that a cat is a reptile. I think you have a false belief that mammals are part of Amphibia. Amphibia are a sister lineage to Reptiliomorpha (see here) and cats are part of the Reptiliomorpha $\endgroup$
    – Remi.b
    Mar 24, 2019 at 2:18
  • $\begingroup$ The real - and IMHO only - reason is that some people just think it's "cool" to claim that birds are really dinosaurs. $\endgroup$
    – jamesqf
    Mar 24, 2019 at 4:38

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