My question arises from a recent discussion about bird taxonomy. Birds used to be described as “descended from reptiles,” but in recent decades there seems to have been a shift toward formally including birds among the reptiles. As I understand the argument, since birds are descended from archosaurian reptiles, it makes more sense to classify them as reptiles.
Obviously I’m neither an herpetologist nor an ornithologist, so all this is really no skin off my nose. However, based on my wafer-thin apprehension of the taxonomic principles at hand, it occurred to me that, by the same token, human beings (and indeed all tetrapods) could be legitimately described as fish. Since I’ve often heard the “birds are reptiles” argument, but I’ve never heard “humans are fish,” I suspect my thought is in error, but I’m not sure in what way. Have I been a fish all these years and just didn’t know it?
Actaully, mammals could be called reptiles, too. And reptiles could be fish… or amphibians.
Mammals form a clade, as do birds. A clade is a group which includes the ancestral form and all it’s descendants. Thus, “reptile” is not a clade-- it’s a paraphyletic group that is based on morphology, not common descent. But biologists have not, as a whole, switch over to a completely cladistic methodology for classifying organisms yet, so you can still use “reptile”, “bird” and “human”.
Cladistically speaking, we’re all members of the Teleostomi, Osteichthyes, and Sarcopterygii clades - the bony and lobe-finned fishes are all also members of these groups. You could use that classification, and I don’t think many would bat an eye. Not everyone’s going to use it though, and you’ll still find a lot of references using older classification schemes.
To add some clarity (or perhaps some confusion) to the OP, birds are now thought to be descendants of dinosaurs, which are no longer thought to be true reptiles. So I think if you replace “reptile” with “dinosaur” in your OP, it would be, in that regard, more accurate.
But by the logic of the OP, aren’t we all blue-green algae? I myself am a big bang.
No bragging about your sexual prowess, mister! You think you’re a Republican Congressman or something?
Seriously, the main problem with cladistics as a discipline is that it seeks to displace the coherent hierarchy of traditional taxonomy with a system capable of increasingly fine distinctions, but demanding dichotomous splits (no three- or four-way groupings!), and without any terminology. To say that two organisms are in the same clade is a masterpiece of obfuscation – they are related at some level, but no clarity as to what level, and with “clade” having the maddeningly varied meaning of “more closely related than some randomly chosen outgroup.”
Lissener, of course, like all the rest of us, is just a highly developed flatworm.
It’s more a landslide than a trend. Here’s one cite.
Poly: Why do you hate cladistics? Seriously, though, it does make lost more sense than traditional taxonomic methods. It’s just that our everday language has already used up all the the “good” words, and you end up calling the clades amniotes or some other odd sounding term.
The traditional taxonomy is pretty much what you’d expect a creationist to come up with. Not surprising, since Linnaeus preceded Darwin by about a century. Once the concept of common descent was understood and the fossil record started to fill in, the data itself demanded that the traditional methodology by updated. Add to that what the DNA analysis is telling us, and it really makes little sense to call the traditional system “coherent”. Yes, any grouping is going to be arbitrary, but why does that matter? The older system actually obscures important relationships between species, so why is that better? Or maybe I’m misunderstanding what you mean by “traditional taxonomy”…
I’m just confused now. I know birds are generally thought to be descended from dinosaurs. I don’t understand why this should mean birds are dinosaurs. The article you cited seems to simply presuppose that it works that way–that birds are dinosaurs because they are descended from dinosaurs. But I don’t see why anyone should think, much less presuppose, that this principle is a good one to use.
Are humans descended from apes or are humans apes? Would your answer change if all non-human apes went extinct? It’s basically a claidstic arguement.
If X and Y are in the category A, and X is more closely related to Z than it is to Y, then mustn’t Z also be in category A? Which is really what the OP is about in the first place. There are things that we call dinosaurs that are more closely related to what we call birds than they are to other things that we call dinosaurs. Perhaps if a few non-avian theropod dinosaurs were extant today, it would be no more confusing to call birds a type of dinosaur than it is to call bats a type of mammal.
The reason why birds are considered dinosuars is because they are far more closely related to some dinosaurs than a great many other things that we all consider to be dinos.
Put it this way: you agree that a tyrannosaur is a dinosaur, right? And you agree that a Triceratops is a dinsoaur, right? Yet a tyrannosaur is far more closely related to a bird than it is to a Triceratops. It doesn’t make any real sense to include two distantly related organisms in a single grouping while excluding a third group that is much more closely related.
If “dinosaur” is to have any biological meaning it pretty much needs to include the birds. You can’t really draw a circle around all the animals that we call dinosaurs without also drawing it around at least some birds.
We get the same problem when we try to exclude birds from the reptile group. Nobody disputes that crocodiles and turtles are reptiles, yet crocodiles are far more closely related to birds than they are to turtles.
To make the term “dinosaur” mean anything beyond “What I am pointing at when I say dinsosaur” it has to be based on some sort of objective features, yet any objective features you apply to produce a dinsoaur category will also mean you encompass some birds. The same goes for the term “reptile”.
From a non-cladistic point of view we can then go on and split the birds out again by defining a group of dinosaurs “of common descent with a full body cover of feathers that are capable of flight or secondarily flightless”. That allows us to use the classification of ‘bird’ quite freely and retain utility. But the point to note is that we still have to concede that they are dinosaurs because they will inevitably be netted by any objective standards that we use to gather in all the other organisms traditionally called dinosaurs.
Here’s another way of thinking about it: If Jack is your 2nd cousin and you consider him part of your family, wouldn’t it be odd if you didn’t consider Jill, your 1st cousin, to be part of your family, too? After all, you are more closely related to Jill than your are to Jack, and unless you simply don’t like Jill, there is no reason to exclude her.
No, because “fish” isn’t a scientific term*, it’s a vernacular term. But, you can logically construct a clade that includes humans and some fish (the lobe finned ones) but that excludes other fish.
*to the extent that it is used scientifically, it’s acknowldeged to be a paraphyletic group, meaning it doesn’t include all the descendants of the common ancestor of all the members in that group.
So humans aren’t “fish”. Cladistically, humans and what we commonly call fish are both “chordates with calcified skeletons”. Within that grouping you can be more specific and have the group of “chordates with calcified skeletons with appendicular development” (lobe-finned fish, etc.). On one level this makes sense. However, some lines of descent are so morphologically different from their founding ancestor that we instinctively draw a line between them. Birds are lot different from any of their archosaurian ancestors for example. In many cases this reflects the reality that the descendent group lives so differently from it’s founders that the ancestral link became extinct in a very short period of time: virtually as soon as descendents better adapted to the new way of life evolved. The Linnaeun grouping labled “Class” reflects these jumps across a very broad phenotypical gap. Because the transitional types no longer are extant, lobe-finned fish and lung fish were classically classified as “fish” rather than “vertebrates showing adaption to living out of water”.
Well that was kind of the point of me prefacing my OP with the bit about birds and reptiles (or dinosaurs). As far as I can see, the only reason that “Fish” would be considered a paraphyletic group is because it includes the Tetrapods, which have not been considered fish by definition, even though they are all descended from fish (Sarcopterygia, I believe). But if birds must be considered a type of dinosaur because they are descended from dinosaurs, then it seems that it must follow that humans must be considered fish for the same reason. After all, you can also logically construct a clade that includes the birds and some dinosaurs but excludes other dinosaurs, correct?
If you wanted to lump humans and lobe finned fished together, you’d call both “Sarcopterygii”, right? And I think hard core cladists do. Kinda illustrates why cladistic classification hasn’t taken root in everyday language.
I would have answered that humans are descended from apes, and are not apes, whether or not there still existed any apes. I am not sure what point I should have understood from your comments here.
I can see a sensible classification system working this way, and I can see a sensible classification system working in some other way.
Now yer talkin’. I get it now. This is the single piece of information I was missing which brings it all together.
I do wonder what makes the decision between calling “birds” “dinosaurs,” or instead, calling those “dino-birds” you mentioned “birds” instead of “dinosaurs.”
Sorry. I thought it was generally understood that we are a type of ape.
I think it has to do with all the other dinos being extinct. That was why I asked you the two “ape” questions earlier. If all the non-human apes had died off millions of years ago, we might not call humans apes.
Seriously, though, it does make lost more sense than traditional taxonomic methods. It’s just that our everday language has already used up all the the “good” words, and you end up calling the clades amniotes or some other odd sounding term.