Cladistically, I believe humans could be classified as a type of banana, as we share 50% of our DNA with said fruit.
Not really, and not in the same sense that we could be classified as fish. If you take all the organisms that are called “banana” and grouped them with their common ancestor, that common ancestor wouldn’t necessarily be the common ancestor of humans. That would only happen if you went much further back and included the common ancestors of other things that are not called “banana”, too.
And I think it’s more like 30% rather than 50%.
It’s the same argument that makes us apes. Humans, gorillas, and chimpanzees are more closely related, mutually, than any of them are to orangutans, so using “ape” to mean an advanced nonhuman anthopoid, while correct in casual usage, isn’t scientifically tenable. To paraphrase Pogo, “We have me the apes we’re descended from and he is us.”
However, it’s important to keep in mind that cladistic theory does not suggest that we look more like a chimp than an orangutan does; it merely presents a structure based on common descent.
I confess that I’m not any less confused.
Birds are classified as dinosaurs because otherwise the Dinosauria would be a paraphyletic group, and this would be bad.
Humans are classified as apes because otherwise the Pongidae would be a paraphyletic group, and this would be bad.
However, humans are evidently not classified as fish, even though we and every other tetrapod species share a common ancestor with lungfish (which are firmly ensconced within the Osteicthyes, so far as I know). This would seem to clearly establish the Osteicthyes and Sarcopterygii as paraphyletic groups, since they do not encompass all descendants of their last common ancestor.
Osteichthyes are fish. Sarcopterygii are fish. Why are we not fish?
I do not have a major problem with cladistics as a methodology for defining evolutionary and taxonomic propinquity. (Minor challenge to its arrant presumption that it can always identify the proper sequence of distinguishing characters.) What I do have an issue with is the tendency of cladists to throw out the baby with the bathwater, casting out King Philip’s Transatlantic journey and its ease of sorting closeness/distance of relationship in favor of terms so flexible one has no clue whether one is distinguishing subspecific or phyletic differences. We’re in the same “clade” with the chimpanzee, OK – and we’re in the same “clade” as the coelacanth, also OK, but wouldn’t it be nice to be able to distinguish, without several paragraphs of explanatory prose, which is considered closer?
For the record, the sequence of clades here:
Vertebrata, AKA Craniata: Characterized by a clear head region including sense organs and a brain protected by a skull or related structure. Casts out Urochordates and the lancelet (along with all invertebrates, for which we need to go several layers back).
Gnathostomata: Bearing jaws. Casts out lamprey, hagfish, conodonts, and assorted armored jawless fish of the Silurian
Osteichthyes: Bony skeleton, various osteological characters not quickly spelled out in my references. Casts out chondrichthyes (sharks) and extinct placoderms and acanthodians
Sarcopterygii: Paired fleshy appendages supported by multibone structures connected to the spine by the pectoral and pelvic girdles.
Rhipidistia: In origin, multiple bones in lower jaw, enamel and dentine folded in teeth. Casts out coelacanths and lungfish.
Tetrapoda: Fleshy appendages developed to limbs, habit includes land dwelling. Casts out rhipidistians sensu stricto, the extinct lobefin fish like Eusthenepteron most closely related to land vertebrates.
Pentadactyla: Five digits per limb. Casts out Ichthyostegalia, the most primitive amphibians
Amniota: Embryo contained within a fluid-filled amnion (which is in turn within an egg or retained within mother). Casts out other amphibia
Eureptilia: Distinction is described in arcane osteological features, but simply stated, draws a line between “reptiles” in the classic sense plus birds, included, and mammals and “mammal-like reptiles”, excluded.
Diapsida: In origin, skull has two fenestra on each side of head, one at temple level and one higher – though a wide range of variance from this basic structure exists. Casts out turtles and extinct cotylosaurs.
Archosauromorpha: Again arcane skeletal distinctions, but distinguishes between robust and gracile forms. Casts out lepidosaurs (snakes, lizards, tuatara, and extinct allies).
Archosauria: As above, but eliminating rhynchosaurs, trilophosaurs, and prolacertiforms (extinct Triassic reptiles)
Ornithodira: Ankle joint simplified to hinge-like structure, metatarsals elongated to produce digitigrade stance. Casts out crocodilians and some extinct relatives.
Dinosauria: Femur and ankle structured for fully erect stance, other skeletal distinctions. Casts out pterosaurs.
Saurischia: Pubis extends forward in hip, teeth on frontal part of jaws, other distinct skeletal features. Casts out ornithischian dinosaurs.
Theropoda: Bipedal carnivory, assorted adaptations. Casts out prosauropods and sauropods.
Tetanurae: Maxillary fenestra, pleurocoelous dorsal vertebrae, grooved astragalus. Casts out Herrerasaurs and Ceratosaurs
Maniraptoriformes: Elongated, slender metacarpal III, reduced caudal vertebrae, pubis directed downward. Eliminates Megalosaurs and Allosaurs.
Aves: Feathers, furcula (“wishbone”), forelimbs adapted to support wings. Eliminates raptors, tyrannosaurids, and ornithomimids.
Well, that’s because the debate isn’t over yet. The cladists haven’t won all the battles, just some of them. But if and when they do, humans still won’t be called “fish”, they’ll be called Sarcopterygii (or something like that), but laymen will still call the Sarcopterygii in the water “fish” and the ones out of the water something else. BTW, I don’t know that in the most commonly used classification scheme that birds are literally classified in with dinos, but most biologists recognize that they could or should be. AFAIK, there is still the class “aves” that birds belong to all by their lonseomes.
I also seem to recall **Colibri **mentioning in an earlier thread that there isn’t any official classification scheme above the species level. So, there isn’t some “Board of Directors” of Biologists who are going to make this decision-- it’ll grow or die as the consensus forms (or doesn’t).
You just did, right there. All it took was one sentence.
Sorry, I missed this post earlier. That last sentence is correct-- you could construct several claddes as you suggest.
I agree, it would be nice. Linnaean taxonomy at least tried to distinguish among degrees of “relatedness” with its families, orders, classes, and phyla. As knowledge advanced, however, one had to add “sub-families” and “super-orders” and “sub-sub-phyla” and whatnot, until the terminology became a mess. And, there was no objective benchmark for what made one clade a “family” and another an “order”.
My modest proposal for restoring “relatedness”, without returning to subs and supers, is to preface each clade with our best estimate of the number of millions of years since its common ancestor lived. Hence humans and chimps form (roughly, and obviously subject to change) a 7 clade, whereas sarcopterygii, the smallest clade uniting humans and coelacanths, is perhaps a 420 clade. We see at once how much closer we are to the former than the latter.
Of course (as if you needed to be told) I’m not a biologist, so I don’t expect anybody to listen.
Humans could be classified as Sarcopterygians. “Fish” as commonly used, is a paraphyletic term (meaning the group described includes a grouping of organisms that does not also include all the descendants of that group). Sarcopterygians are often referred to as “fleshy-finned fishes”, so depending on how you choose to define “fish”, humans may or may not be fish. But if we are, keep in mind, so are all other tetrapods.
Actually, no they couldn’t. The so called “mammal-like reptiles” (more properly nown as “early Synapsids”) where neither reptiles nor mammals. Mammalia and Reptilia are two legitimately separate clades, which, while separate, share a common ancestor.
Dinosaurs absolutely are considered reptiles. For those not used to seeing that sort of taxonomic scheme, the clade labelled Sauropsida is effectively a synonym for “Reptile”, as it is commonly used. Eureptilia - the True Reptiles, contains, in addition to traditional lizards and snakes, the clade Archosauromorpha. And Archosauromorpha, in addition to including crocodiles and their relatives, all the Ornithodires, which include dinosaurs and pterosaurs. Dinosauria < Archosauromorpha < Eureptilia. Ergo, dinosaurs are reptiles.
And this is worse than the meaningless obfuscation provided by “Class” or “Order”? Linnaean rankings are utterly meaningless as they provide zero taxonomic information about the group in question. What is required for a given taxon to be described as an “Order”? Or a “Family”? Birds are given an equivalent ranking to reptiles, despite clear evidence that they arose from within that group. Ranking tosses out evolutionary relationships in favor of a gradistic approach, rather than a cladistic one. Things are grouped together because they “look” similar.
I guess as you go further back in time one can argue about whether something is a reptile or something else. From talkorigins:
Your modest proposal is that of a phylogenetic tree, not a cladogram. Cladograms are, technically, a graphical representation of hypothesised relatedness, not graphs of evolutionary history. We can use cladograms to hypothesize that X is more closely related to Y than is Z (and by “more closely related”, I mean “shares a more recent common ancestor”), but we cannot map out actual evolutionary history with them. Multiple cladograms are often tested for a given relationship, and the one which best matches the hypothesis is typically chosen as the “correct” one. Numerous issues can, of course, cloud any given arrangement; determining whether two structures are truly homologus or merely analogous, for example.
Only if using an informal definition of “reptile”. Although, a correction to my previous post: Sauropsida is not synonymous with Reptilia; it actually contains the clade Reptilia, plus assorted “parareptiles” (e.g., paraiasaurs). Uniting characters of Reptilia include:
[ul]
[li]Small Tabular bone[/li][li] Presence of suborbital foramen[/li][li] Presence of supraoccipital anterior crista[/li][li] Narrow supraoccipital plate[/li][/ul]
That’s just a partial list, but those were recognized by Romer, and continue to be diagnostic of Reptilia. The problem is not that Reptilia is ill-defined, the problem has been that many authors have used Reptilia as a dumping ground for “reptile-like organisms”, and the term has been diluted over several decades.
This common-usage clause seems kind of counterintuitive, which may be what’s throwing me here. Even though the two terms “Sarcopterygians” and “fleshy-finned fishes” encompass the exact same group, all Sarcopterygians aren’t automatically fish? That seems… well, fishy. If all Sarcopterygians are fleshy-finned fishes, and I’m a Sarcopterygian, then surely that means I’m a fish?
Or would it be equally accurate to say that, while birds are classified as “Dinosaurians,” they may or may not be “dinosaurs” (since “dinosaur,” as commonly used, is a paraphyletic term that doesn’t include birds)? Or that humans may or may not be “apes?” This might well be the source of my confusion, since my OP was inspired by the “birds are dinosaurs” argument. If birds aren’t automatically considered dinosaurs (even though classed in Dinosauria), then that would also explain why humans aren’t automatically considered fish.
QUOTE=Terrifel]This common-usage clause seems kind of counterintuitive, which may be what’s throwing me here. Even though the two terms “Sarcopterygians” and “fleshy-finned fishes” encompass the exact same group, all Sarcopterygians aren’t automatically fish? That seems… well, fishy. If all Sarcopterygians are fleshy-finned fishes, and I’m a Sarcopterygian, then surely that means I’m a fish?
Or would it be equally accurate to say that, while birds are classified as “Dinosaurians,” they may or may not be “dinosaurs” (since “dinosaur,” as commonly used, is a paraphyletic term that doesn’t include birds)? Or that humans may or may not be “apes?” This might well be the source of my confusion, since my OP was inspired by the “birds are dinosaurs” argument. If birds aren’t automatically considered dinosaurs (even though classed in Dinosauria), then that would also explain why humans aren’t automatically considered fish.
[/QUOTE]
Under the old grade-style classification, Sarcopterygii was the Subclass within the bony fish, Class Ichthyes or Osteichthyes, that contained the fleshy-finned fishes: the lungfish, the coelacanth, and the rhipidistians. And Tetrapoda was the Superclass that contained the Classes Amphibia, Reptilia, Aves (birds), and Mammalia. (Reptilia was a catch-all-- anything that laid an amniote egg, or bore live young in an amnion instead, that was not a bird or mammal.)
Under cladistic groupings, everything descended from the first sarcopterygian needs to be in one taxon/clade. That means all lungfish, all crossopterygians (coelacanths and their allies), all rhipidistians, and all tetrapods. Using Sarcopterygii for the grouping is convenient – the word means “fleshy finned” or “fleshy winged,” more or less. Nobody has yet hit on a good way of saying “this group of critters and all the stuff that radiated from them, no matter how far the latter changed” except for the catchall suffix -morpha tacked onto what may already be a sesquipedalian taxonomic term. “Sarcopterygiomorpha” just doesn’t make it as euphonious or mnemonic in nature. 
(Though it does work well as a line from an Irish Jig. :D)
Likewise, Archaeopteryx’s great-granddaddy was almost certainly a “reptile” (in the old broad sense) closely akin to Compsognathus (the “chicken dinosaur”). Its closest relatives, in turn, are among the theropods, the bipedal carnivorous dinosaurs – with the raptors, ornithomimids, and Tyrannosaurs closest and the other theropods next most distant. And their closest relatives, based on hip and leg structure and jaw/teeth arrangements, are the big sauropods like Diplodocus and Brachiosaurus, and the prosauropods that evolved parallel to them at an earlier time.
But note that the reverse is true as well. Cladistically, a tyrannosaurus is closer to a crow or a robin than it is to a Diplodocus or a Triceratops, and far closer than to a crocodile, lizard, or turtle. Odd though the Maniraptoriformes may seem to us, they include creatures with a clear group of common derived characteristics that are obviously more closely related to each other than to anything else. Wrens, loons, velociraptors, and tyrannosaurids all have the specific set of common characters that define the maniraptoriform group.
I think I wasn’t clear in the point I was trying to make. If we traced modern mammals and modern reptiles back to a common ancestor, would that ancestor be classified as a reptile, a mammal or “something else”? Now, that’s partially subjective, but I read the cite I listed as essentially saying: Most people would look at the common ancestor and say “reptile”. But I do appreciate that any insistance on placing any animal in a particular box is problematic, and as we go back further in time that becomes even more problematic since we have developed terms like “reptile” and “mammal” in the vernacular to describe extant species. It’s easy to get hung up on what we call something rather than just explaining what the relations are between organism, so whether we call it a reptile or a mammal or something else is largely besides the point.
The problem is that there are an awful lot of early vertebrates who looked fishy. If we define “fish” to include all of them, then “fish” practically means the same thing as “vertebrate”. Or, if we limit it to later groups, then it becomes the same thing as “Gnathostomata”. Basically, “fish” suffers from the same sort of taxonomic fuzziness that Reptilia used to – only, Reptilia has been cleaned up. There still really is no technical taxonomic definition for “Fish”, wherein you can say, “All these, and no others, are Fishes”, and be certain everyone knows what you are talking about. So, to the extent that Sarcopteryigians are at referred to as “fleshy-finned fishes”, then yes, we are fishes. If fishes are really only Chondrichthyes, then we aren’t fishes…but then neither are placoderms. And if Sarcopterygians are fishes, then so are we. But if Sarcopterygians and Actinopterygians and Placoderms are fishes, but we aren’t, then “fish” becomes a paraphyletic term, which takes us back to the “jumble of organisms” style of definition.
Personally, cladist that I am, I have no problem with describing humans as “fish”. It’s just not a very technical term, since it either applies to a diverse group of critters which are not necessarily more closely related to one another than to anything else, or it includes so many other critters that the average Joe doesn’t think of as “fish” as to be nearly meaningless.
Also, keep in mind that early on, the Sarcopterygians that were described were pretty fish-like, thus they received the common moniker of “fleshy-finned fishes”. It was only later, once evolutionary affinites became clearer, that decidedly non-fishlike (in appearance) critters were included within Sarcopterygii, thereby calling into question, “What, if anything, is a fish?”
Among those paleontologists who accept that birds evolved from dinosaurs, “dinosaur” is pretty much assumed to include birds as well. In fact, there is nothing you can say about ALL dinosaurs that doesn’t also apply to birds. Whatever features unite Tyrannosaurus, Apatosaurus, Ankylosaurus, Triceratops, Hadrosaurus, Pachycephalosaurus, Gallimimus, etc., together ALSO unites birds along with them. And among those listed groups, birds have the greatest affinity with Tyrannosaurus.
That is really the key here: what, if anything, can be said about ALL members of a given group that does not ALSO apply to the taxon in question? If we define fish to include, say, all members of Placodermi, Chondrichthyes and the fishiest members of Sarcopterygii, then we find those same features that unite those groups exist in other groups that are also included within Sarcoptyergii, but that are not superficially “fishy”. Any suite of characters which serves to unite all apes together in a group is ALSO found in humans. Any suite of characters which unites all dinosaurs is ALSO found in birds. And, because of the nested nature of taxonomies, each of those subgroups is, by definition, a member of the larger group.
What about “ectotherm”?
Note that I’m genuinely confused, not trying to be snarky… as a more general question, does ectotherm vs. endotherm enter into the discussion as a means of classification? Because it seems that, in the case of Reptilia, you would say something like “They’re not all ectotherms, because of the birds,” but you can’t use that statement to justify including birds in the group, right?
If you wish to be accurate, it would necessarily be a “something else”, as the common ancestor of two distinct clades is neither. The common ancestor of Mammalia and Reptilia is to be found somewhere within the Amniotes, but at the point at which that group split, it was not yet either one (though, obviously, it would have shared similiarites with both).
I agree up to a point: the name (or label, if you prefer) ought to be a shorthand for describing a group. It’s a lot easier to say “reptile” than “critter with Traits A, B, C, D,…and Z”. Thus, for the sake of taxonomic accuracy, I don’t think it’s unreasonable to try to stick to a formal definition for taxonomic labels. Just as there can be numerous common names for a specific critter (or even a common name which is made up of several critters), the common terms for many taxonomic groups tend not to be terribly accurate, which, in my opinion, hinders, rather than facilitates, discussion. If one uses the term “reptile” to describe something that is not a member of clade Reptilia, it tends to muddy the proverbial waters. That’s why I try to stick to clade names; our early ancestors were Synapsids, but not reptiles, even if they did have some reptilian characters (said characters being more like ancestral tetrapod characters than truly reptilian ones…).
“Ectotherm” wouldn’t work because it cannot currently be shown that non-avian dinosaurs were either ectotherms or endotherms. There is evidence both for and against both possibilities. Generally speaking, since metabolism isn’t something that can readily be identified from the fossil record, it doesn’t factor into classification.