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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?

Lucca
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    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. – bandybabboon Mar 24 '19 at 02:32

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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?).

Remi.b
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  • 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... – Lucca Mar 23 '19 at 23:50
  • 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. – Remi.b Mar 24 '19 at 00:01
  • 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? – Lucca Mar 24 '19 at 01:09
  • 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 – Remi.b Mar 24 '19 at 02:18
  • The real - and IMHO only - reason is that some people just think it's "cool" to claim that birds are really dinosaurs. – jamesqf Mar 24 '19 at 04:38
  • Whether mammals are reptiles is one of those circular pointless classification arguments you sometimes get in science, that people debate endlessly. It is based on using an archaic classification in a modern system. – John Mar 24 '19 at 16:52
  • @jamesqf birds are dinosaurs, the clade dinosauria is defined using birds. Dinosauria itself is defined as the last common ancestor of Triceratops horridus, Passer domesticus, Diplodocus carnegii, and all of its descendants. Passer domesticus wouldbe the house sparrow. – John Mar 24 '19 at 16:59
  • @John: No, birds EVOLVED FROM dinosaurs (which went on being dinosaurs for many millions of years afterwards). It is the same sort of logical confusion that would insist on saying that all vertebrates are really just fish :-) And if your modern classification system insists on such things, perhaps the problem is with the system? – jamesqf Mar 25 '19 at 17:06
  • It all depends whether by dinosaurs you mean what most people mean (which exclude birds) or whether you mean Dinosauria. While it somehow makes more sense to state birds are dinosaurs, one must highlight that if you make such statement, you should also say "mammals are reptile" (or that vertebrates are fish) which is not something we would want to say. Personally I am fine with both "birds are dinosaurs" or "birds evolved from dinosaurs". – Remi.b Mar 25 '19 at 17:32
  • @jamesqf by that argument human are not mammals, because humans are primates, it doesn't make any sense. Imagine the only surviving mammals were bats, would bats stop being mammals? – John Mar 25 '19 at 21:51
  • @John Following your logic, are humans reptiles? Just accept that in the common language, people can group species in a non-phylogenetic way. I think it is as silly to attack someone that say "birds are dinosaurs" than it is to attack someone that says "birds evolved from dinosaurs". – Remi.b Mar 25 '19 at 23:00
  • @Remi.b what makes you think I had a problem with someone saying birds evolved from dinosaurs? I just have a problem with someone saying birds are not dinosaurs, there is no scientific definition of dinosaurs that does not include birds. biological classification is nested, which is why terms like non-avian dinosaurs exists, or non-human animals. I accept that reptiles is a bit weird because the word is so old so the lay term carries a lot of baggage, but even the original use of the word dinosaurs would include birds given what we know now. – John Mar 25 '19 at 23:45
  • reptiles don't work no matter what you do, crocodiles are more closely related to birds than they are snakes. It is hard to say mammals are reptiles because synapsids are likely not in crown reptilia, that is the mammal clade is not in the reptile clade, they are squarely in amniota however. going by lay usage mammoths and ichthyosaurs are dinosaurs, lay usage is generally useless. – John Mar 25 '19 at 23:50
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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

kmm
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  • 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? – Lucca Mar 24 '19 at 01:19
  • 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. – kmm Mar 24 '19 at 13:58