In Straight Dope Classic recently put on the welcome page, Is it true the brontosaurus never really existed?, the ever wize, all-knowing Cecil referred to the group of animals called Dinosaurs as lizards not once, but twice. I realize that he knows that Dinosaurs are not, in fact, lizards but archosaurs–but I wanted to point out that he had not yet corrected that column. I would hate any newcomer to the Straight Dope to think that Cecil was behind the times.
While I really know little about what dinosaurs actually are compared to lizards and other animals, to my knowledge the word “dinosaur” itself actually means “terrible” or “monstrous” (deinos) “lizard” (sauros). And “archosaur”, I believe, actually means “ruler” (arkhos) “lizard”. Of course, this is all just etymology, which very often contradicts actual definitions. So, Little Bird, if what people have long known as dinosaurs are not actually dinosaurs, then what are dinosaurs? Is it now just a word without meaning? I’m sure most people around the world, when they hear the word dinosaur, don’t think of anything else besides prehistoric beasts like tyrannosaurs and stegosaurs.
“Me Grimlock, leader of Archobots! Me Grimlock, king!”
Anyhoo…
Dinosaurs are now generally regarded as a class (in the technical biological sense of “class”) in their own right, which broke off from the reptiles at about the same time the mammals did. There was actually a proposal a while back to demote Aves (birds) to a subclass of Dinosauria, because the birds broke off from the Dinosaurs much more recently, though it was rejected, only partially for reasons of tradition.
Anyone who has read even a sample of Cecil’s work realizes that one of his most, uh, endearing characteristics is his sardonic wit. Just as I doubt strongly that he believes one of his correspondence actually is or could become cornflakes, even if he turned a witty line applying that moniker to him, he is quite well aware of the difference between the subclass of Lepidosaurs that are technically “lizards” and dinosaurs – he was simply using a common everyday word to reference them.
Just as I am not implying that the OP is a member of Order Galliformes in terming his nitpick of this column to show him to be a pedantic ruffled* grouse.
- And yes, I’m well aware that the bird is a Christmassy** “ruffed grouse”
** i.e., “no L”
I addressed both what “dinosaur” means (and why Owen coined that name for them), as well as what “dinosaur” means in phylogenetic terms here. If you aren’t interested in the long version, the summary version is a follows:
- Richard Owen, the fellow who originally coined the name Dinosaur, really did think they were just big lizards, and he even classified them originally within Lacertilia.
- The word now definitely has meaning (despite the fact that the animals it applies to aren’t lizards; but then, there are many names within Dinosauria which were named based on initial perceptions, only to find out later that those perceptions were way off base), and names a valid phylogenetic group.
It’s just a basic fact of life that many common words have one everyday meaning and a somewhat different scientific meaning. E.g., “fruit” means one thing to regular people and something else to a biologist. (Many “vegetables” we eat are really fruits.)
“Quantum leap/step” makes perfect sense to regular people (and to Latin scholars) but offends a particular branch of Physicists (even though their new meaning actually contradicts the older meaning).
So you can refer to lizards in an everyday way or in a Scientific way and neither should get really upset about the other.
There are 4,728,265,109 more important things to worry about.
Darwins Finch posted (in his link) "In 1993, Kevin Padian and Julian(?) May formally defined the various groups using phylogenetic systematics (aka cladistics). Jacques Gauthier had given Archosauria (the group containing dinosaurs, pterosaurs, crocodilians, and, eventually, birds) a cladistic overhaul around 1986, and had confirmed, based on his analyses of numerous characters, that a) dinosaurs were, as suspected, monophyletic – that is, they represented a true group of animals descended from a common ancestor – and b) that under strict cladistic defintions, Aves belonged within Dinosauria, as they, too, were descended from the same common ancestor as all other non-avian dinosaurs. (As a result, one encounters informal mention of “non-avian dinosaurs” in some literature.)
Anyway, Padian’s & May’s formal definitions for the groups were as follows:
Dinosauria = all descendants of the most recent common ancestor of Passer (Swallows) and Triceratops.
Ornithischia = all dinosaurs closer to Triceratops than to Passer (that is, eveything sharing a more recent common ancestor with Triceratops than with Passer).
Saurischia = all dinosaurs closer to Passer than to Ornithischia (that is, eveything sharing a more recent common ancestor with Passer than with Triceratops)."
The problem with this, is that this is not universally accepted. Also, Padian & May started with what they wanted “birds & dinos are the same” then made up the group “Dinosauria” (and used a perfectly good name that did NOT include birds when it was coined) to foward their agenda. Dinosaurs are not birds, birds are not dinosaurs. Dinosaurs can include Pterodactlys & Plesiosaurs if one uses it to mean “Ancient reptile-like creature”. If you are a strict constrcutionist, you’d say that Dinosaur= Ornithischia & Saurischia. But really- there is no good reason to lump those two together- (other than it is traditional).
Sure, indeed if you go back far enough= Dino & Donald have common ancestors- but Donald is a duck, not a dinosaur. So do humans & ameba’s. Humans are not Amebas. Now, indeed, if you want to make up a name of a group that includes everything decended from the common ancestor of humans & amebas you can. But call it “primates” is just going to confuse the matter. :rolleyes: :dubious:
The thing is- if you go back far enough- every living thing is decended from an earlier life form. Thus with Cladistics, you can (and Padian & May did ) pick a point in time that they want to use to prove a point, then use a name which has common meaning well outside what you are using it for- just to forward their agenda. It’d be like going back to the common ancestor of humans & chimps (not all that far back, note) starting there with a new Cladistic “group” and calling it “Humans”. :rolleyes: :dubious: Sure- humans & chimps do have a common ancestor, and yes we are closely related. But playing with “groups” amd using names that have a commonly accepted meaningout of context of that meanin- just to forward your agenda is wrong.
A clade is defined as all the organisms that are descended from a common ancestor. The examples you chose do not accurately parallel the valid point made by Padian and May, that dinosaurs and birds (if they are descended from a common ancestor) are a valid taxonomic clade, and that dinosaurs (excluding birds) are not.
You argue that the name dinosauria should not be used for the clade that includes dinosaurs and birds. You may not be correct on this point, though I am not sufficiently familiar with naming conventions to be sure. If it was discovered (for example) that the most recent common ancestor of the order Carnivora (dogs, cats, etc.) was also ancestral to the walrus (currently classed in the order Pinnipeda), would you expect that:
a) The order Carnivora (without a name change) would now be considered to include the walrus, OR
b) A new name would have to be invented?
What about when cetaceans were discovered to be mammals rather than fish? Should the name mammals have been abandoned in favour of a new name that included whales?
You assert (without evidence) that Padian and May were trying to “further an agenda”, where the evidence presented appears to show that they were correctly applying the standard rules for classification.
The problems I have with cladistics are two:
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It is so dependent on getting the proper order for defining clades within clades, etc. Consider seven species: A, B, C, D, E, F, and G. D, E, F, and G have common characters not associated with A, B, and C, and definitely constitute a natural taxon. A, B, and C are related more loosely to that taxon, and would be part of a higher taxon. Say they’re all part of the same family, with D-G constituting a larger subfamily. Now A is the earliest member of the family, and all the characteristics shared by the other six are also present in A. But B and C, specialized forms later in time than A, each have a character in common with D-G but not with the other. Let’s define those two characters as lowercase b and c respectively. Obviously either b or c evolved first and either b was lost in the specialization of C or c was lost in the specialization of B. In the traditional King Philip classification you don’t have to define which character came first, and thus is the outgroup for the other, for which the evidence is too scanty to decide. But in cladistics you are forced to.
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There is nothing wrong with an allopatric taxon in King Philip taxonomy – if I have the term correct here. By allopatric I mean a group with a common ancestry but which does not include all descendants from that common ancestry. For example, Reptilia traditionally includes all amniotes which did not evolve into birds or mammals. It has clear modern-biological and paleontological bounds and makes sense as a group, even allowing that extinct forms specialized in a wide variety of different ways. And it excludes two large groups of highly successful descendants, who deserve class-level taxonomic classification by themselves. But this form of classification, sensible though it may be, is verboten in cladistics. Rather, you get groups including pelycosaurs and their relatives with mammals, dromaeosaurs and birds, and similar genetically sound but contrary to common-sense-in-grouping clades. I don’t need a taxon of mammals-plus-ictidosaurs – whatever the ictidosaurs’ characteristics may have been, they’re much more closely grouped to cynodonts in my mind than with whales or elephants or hamsters.
Padian and May did not “make up” the group Dinosauria. And one of the main reasons Dinosauria did not initially include birds was the fact that Owen was an anti-Darwinian in the first place, and he did not accept the idea of “common ancestors”. He was simply following the Linnaean tradition of grouping like with like.
That is, of course, an entirely outdated mode of thinking. Sure, not all dinosaurs are birds, but all birds are dinosaurs under a cladistic definition. All current evidence points to birds being descended not from a common ancestor with dinosaurs, not from some generic archosaur, but from true dinosaurian ancestors. Going up through their lineage, we find saurischians, theropods, tetanurans, coelurosaurians, maniraptorans, avialaens, ornithonurans, ornithothoracens, carinateans, and finally, Aves proper. The lineage is unbroken, and it makes no more sense to exclude Aves from Dinosauria than it does to exclude humans from primates.
No good reason besides the fact that they are both descended from a common ancestor, you mean? What better reason could there be?
Well, yeah. I guess it’s a good thing primates don’t include everything between humans and amebas, then. Of course, you neglect the fact that humans are included within primates for a good reason. Perhaps you can tell us what that is. Then tell us why birds should not be considered dinosaurs using the same logic.
No, you can’t. I’m not sure you understand cladistics if you think that’s how it works. Cladistics operates from the assumption that everything is related, and that one can construct nested hierarchies of those relationships. When one looks at the common ancestor of all birds, one notices that that ancestor was undeniably a dinosaur. One also comes to realize that there is nothing one can say about all dinosaurs that cannot also be said about birds. One cannot say anything that applies to all theropods, and only to theropods, that doesn’t also apply to birds.
Uh, no. It wouldn’t. It would be like going back to the common ancestor of humans and chimps and calling that new cladistic group “the Homo-Pan clade”, rather like it is now. The genus Pan is not included with the genus Homo, but they share a common ancestor. Thus, they are part of the same clade.
The “commonly accepted meaning” of Dinosauria is no different now than it was when Owen coined the term. Whether one uses Padian’s and May’s definition, or Owen’s own, in terms of relationships, that group still includes birds. Systematics has grown beyond the desire to simply group like with like. Aves as a separate Linnaean class is evolutionarily meaningless - not surprising, as evolution was the furthest thing from Linnaeas’s mind when he began his classification project.
Part of your problem is that you are trying to understand cladistics in the context of Linnaean taxonomy, which is just plain the wrong way to go about it. Families, orders and suborders are too arbitrary to be of any use.
Of course, no one says that cladistic examinations just fall into place all nice and neat. The problem of homology and loss of characters is a major one, but that is true no matter what system of classification you choose. How does one justify snakes being tetrapods, for example, when they very clearly have no feet? Why are sharks considered vertberates, despite having cartilaginous, rather than boney, skeletons?
And, why shouldn’t you want to determine which came first? Clearly, one character or the other did, correct? Cladistics is an attempt to sort out which came first. Linnaean taxonomy simply sweeps the whole issue under the rug, assuming it even acknowledges the issue at all. And, in some cases, the order of certain characters appearing is not decided beforehand, but is discovered once the phylogeny is hammered out. In this way, it becomes clear that the cartilaginous skeletons of sharks are, in fact, secondarily derived, and they evolved from boney ancestors. This fact need not have been known beforehand.
You are thinking of a paraphyletic group. And there’s not necessarily anything wrong with them in cladistics, either. It’s just they don’t really tell us anything useful about that particular group. As I said before, once you’ve got a clade, there’s nothing you can really say about that group that cannot likewise be applied to the left-out descendants.
Right, because, again, it’s a non-useful distinction. What can you say about all Reptilia that does not also apply to Dinosauria or even Aves? What can you say about all Dinosauria that doesn’t apply to Aves? What can you say about all Amniota that doesn’t include both Mammalia and Reptilia? And so on.
And who decides that Mammalia and Aves “deserve” class-level distinctions, anyway? Ever hear of “taxonomic inflation”? Look at what happened to the brachiopod group Cranioidea. Initially, it was given the rank of Superfamily within Phylum Brachiopoda. Later, they were elevated to the rank of Order. Even more recently, they have been elevated to their own class - the Craniata, and some even advocate the creation of a sub-phlyum for them! So which is their “proper” rank? Cladistics does away with the nonsense of coming up with ranks for the names, and just deals with the names of clades themselves. The definitions of these names can change when new material or evidence comes to light, but that doesn’t wind up screwing up the whole classification scheme.
You may not need those names, but if you have any desire to understand the phylogeny of those groups, they make a lot more sense, and make discussing those clades a lot easier. I’d rather talk about maniraptorans than write out “Dromaesauridae + Avialae (= Archaeopteryx + Ornithurae [= Monoykus + Ornithothoraces {= etc.}])” all the time! And, of course, if one wishes to discuss the characteristics of Maniraptorans sans birds, one simply uses the term Dromaeosauridae.
Besides, the group that includes both mammals and ictidosaurs is Probainognathia. Ictidosaurs are related to mammals, but the two are hardly sister groups, so it’s not like you would ever need to discuss mammals + ictidosaurs in the first place, unless you were talking about all probainognaths (which means you’re also talking about the likes of multituberculates and morganucodontids, because both are more closely related to Mammalia proper than are ictidosaurs).
Simply put, if common descent is fact, then there is but one Tree of Life, correct? Why should the classification of those organisms which comprise the Tree not mimic the actual relationships of those organisms? The Tree branches because that’s how we understand speciation to work. Ancestor-descendent relationships are preserved, even though we may not be able to actually pinpoint a specific ancestor in the fossil record. All of this makes much more evolutionarily-intuitive sense than the arbitrary pigeon-holing of everything into a class or order or superfamily or whatever.
They’re all cold-blooded? So far as I know, all reptiles are cold-blooded, but no mammals or birds are (I’m not sure about the dinosaurs).
The problem I have with cladistics is illustrated by the fact that there is no clade for prokaryotes. Any clade which included all prokaryotes would include all life, period. But there are contexts where it is very useful to talk about “all living things which are not eukaryotes” (and eukaryotes are, if I’m not mistaken, a proper clade).
I suspect that cladistics also has problems in places where the “tree” of life has closed loops. For instance, there’s a species of wolf which is suspected to be a hybrid of the gray wolf and the coyote. Is it a member of both of those clades? And if one asks for the latest common ancestor of that wolf and some other canine, along which lineage ought one to look for that common ancestor?
I’ve been thinking about this, and the above characterization by myself is actually incorrect.
First, however, a brief diversion…
Strictly speaking, a cladogram is simly a representation of a hierarchical pattern. Linnaean taxonomy recognizes (and, indeed, is based on) such hierarchies, so any Linnaean classification must be translatable to a cladogram, and vice versa. Given the original problem noted by Polycarp, a Linnaean classification might look like this:
Taxon ??
Taxon A
Taxon B
Taxon C
Taxon DEFG
Taxon D
Taxon E
Taxon F
Taxon G
Taxon B possesses one synapomorphy (a shared-derived character), b, of Taxon DEFG, while Taxon C possesses c, a different synapomorphy which is also found in Taxon DEFG.
Ideally, any Linnaean hierarchical scheme can be converted to a cladogram, because both are representations of the same hierarchy:
G F E D C B A
\/ / / / / /
\/ / / / /
\/ / / /
\ / / /
\/ / /
\/ /
\ /
\/
\
\
(There is no implied meaning behind the spacing of the taxa…it’s just how things turned out)
Now, the introduction of characters b and c simply imply that the positions of B and C are more or less unresolved. The branches could just as easily be G F E D B C A. So, really, it is not a major problem of cladistics that such situations exist. Neither a cladogram nor a Linnaean classification really address the issue of ancestry to begin with (phylogenetic trees do that job).
That said, that’s not really the point of this post, per my opening statement. What is the point is that there is another issue with Linnaean taxonomy that does cause problems.
Creating Linnaean taxonomies consists of two steps: grouping, and ranking. Grouping – the creation of nested hierarchies – is a non-issue, as the same step is involved in creating a cladogram. The issue is with ranking. First, note again my example of taxanomic inflation, given previously. The bigger problem is that ranking, in essence, has the potential to destroy the intended hierarchical similarities between groups. As Exhibit B, I give you the classification of Vertebrates:
Subphylum Vertebrata
Class Cyclostomata
Class Chondrichthyes
Class Osteichthyes
Class Amphibia
Class Reptilia
Class Aves
Class Mammalia
(The above is from an old zoology text, so there may be different classes in use today; I’m to lazy to look the current taxa used.)
The above class distinctions may make a bit of sense were we to only consider extant species, but even then, we run into some possible problems. If we consider fossil species as well, we run into some very definite problems. Case in point is the position of dinosaurs and birds.
In a strictly hierarchical sense, Dinosauria must lie within Reptilia, as dinosaurs share all the characteristics that make reptiles reptiles. However, many fossils have been uncovered which also point unambiguously (except to a few assorted holdouts) to the inclusion of Aves within Dinosauria. And here, again, this is only speaking in terms of hierarchical similarities, which Linnaean classifications themselves are based upon. So we now have the problem that Reptilia > Dinosauria > Aves. Without resorting to a fair degree of taxonomic inflation for Reptilia, or inventing a whole slew of rankings, there is little hope of shoe-horning all of the assorted relationships from Reptilia to Aves within Reptilia alone. By breaking Aves off into its own subgroup, we lose the affiliation with Reptilia entirely, and the whole classification becomes useless, as the hierarchical relationships which it purports to represent become hopelessly scrambled; one can no longer create a meaningful cladogram from the Linnaean classification.
Ultimately, then, the issue is not really one of who is related to whom and how best that information can be presented, but the fact that the very thing that Linnaean taxonomy purports to display – the information that one is supposed to be able to glean from the classification – is lost from the start. Similar problems exist with the hierarchical similarities of all those classes, and hierarchy is violated seemingly at random in order to “fit” a pre-existing classification that was not prepared for the volume of fossil finds and the implications those finds had for the perceived relationships between groups.
Linnaean taxonomy, then, could be “fixed” easily enough by simply abandoning the ranking labels. “Class” provides no real useful information relative to organization of a group, nor does “Family”, nor many of the other labels (genus, species, domain, and possibly phylum can still be useful, to one degree or another).
And therein lies one of the problems. If dinosaurs are reptiles (which anyone who studies either will tell you they most emphatically are), and some dinosaurs may have been warm-blooded (which there is evidence to support at least some were), then one cannot reasonably say that “all reptiles are cold-blooded”. Besides which, ‘cold-bloodedness’ is not a synapomorphy for reptiles, as the same applies to amphibians, most fish, insects, and all other invertebrates. This is not even considering the fact that warm-bloodedness in birds had to come from somewhere, and the evidence likwise supports its origins well before Archaeopteryx, which is typically branded as “the first bird”.
I don’t see the problem. Look up the Three Domains classification sometime. Prokaryotes are either considered to be just the Monerans (in which case it is synonymous with Monera), or consists of Eubacteria and Archaea. In both cases, it excludes Eukaryotes, which means you most certainly can discuss “all life, sans Eukaryotes”.
That’s not a cladistic problem, that’s an overall classification problem. Both cladistics and Linnaean taxonomies treat species as discreet entities; however, hybrids will necessarily cause problems for any species-oriented classification scheme.
So, instead are you calling him a turkey? 