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Natural Science Forum / Biology / Paleontology / April 2006Who Once Were Called, Thecodonts
From my searching on the Web I do know that the paraphyletic group called Thecodonts have been rearranged into (at least) 4 (more or less) monophyletic groups. However, information found on the Web is infamously disorganized, haphazardly updated, and contradictory in a way that makes Soviet history texts seem models of agreement. We'll leave disagreements, pet theories, ideology, and recalcitrance well enough alone. (And you thought creationists loved to move the goalposts.) Then you have the mystery thecodontoid groups who are just not important enough to be named. Like they had no impact on the world when they were around. Having no descendents of your own doesn't mean you aren't relative when you're alive. Does anybody have leads to all known thecodontoid groups? Taxonomy, anatomy, geological era, ancestors, descendents, close and distant relatives? I'm especially interested in learning about the groups that did not lead to crocodilians, dinosaurs, or pterosaurs. Small thecodontoids that could be or were forced into the background with the appearance of the large suchians and dinosaurs. This is for a project. But the project is not strictly scientific. The philosophy of science enters into the picture, if you are willing to allow that a fantasy setting can have science while, at the same time, containing phenomena and lifeforms that real world conditions do not allow for. It's all for a small, but still important part of a "counterfactual"* Earth. The history, paleontology, and evolution of a creature that can not exist in the real world. --- *Are you aware of how ignorant "counterfactual" sounds to someone who deals with possible alternate realities on a quasi professional basis? We know our proposed alternatives didn't happen. We're not dealing with what did occur, but with what could have occured if things had happened differently. What did happen in 1759 has no relevance to what might have happened if a 1953 Buick had landed on King George III of Britain in 1758. (Sorry about that, but I just had to vent) [snip] > Does anybody have leads to all known thecodontoid groups? Taxonomy, > anatomy, geological era, ancestors, descendents, close and distant > relatives? I'm especially interested in learning about the groups that > did not lead to crocodilians, dinosaurs, or pterosaurs. Small > thecodontoids that could be or were forced into the background with the > appearance of the large suchians and dinosaurs. Start here: http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/270.000.html "Thecodontia" is more or less equal to Archosauromorpha minus the three groups you mentioned above. And now to clean up some terminology: Monophyletic groups (the only kind we like) don't have descendants. They have members. All descendants of any member of the group are also members of the group. That's what makes them monophyletic. Likewise, groups don't lead to other groups, though they include other groups. But you wouldn't say that mammals led to rodents, would you? We can't trace any real pathway of ancestry and descent. All we can do is determine cladistic relationships. You just can't tell an ancestor from the ancestor's cousin. The tree in that link assumes a particular topology; that's why it's a tree. But that topology may be wrong. Particularly for your purposes, there may be archosaurs that aren't included because the creator of the site thinks they're something else. It's distinctly possible that turtles are archosaurs, in which case they would be "thecodonts" and would fit your request. [snip] John Harshman, thanks for the lead. Where monophyletic groups are concerned, does that mean that to be considered a monophyletic group reptiles must incorporate mammals? :) And what about amphibians? My proposal is, a monophyletic group need not include all descendents. That specimens descended from members of a particular monophyletic group can be placed in a separate, daughter group that qualifies as a unique monophyletic group based on traits and features found to be specific to members of that group. Thus Aves would be a monophyletic group based on features unique to birds and not found among Archosaurs. The fact is, matters are not so clear cut as we would wish in the field of biology. Instead of clear cut lines we have fuzzy zones. You get right down to it, Homo habilis is an Australopithecine with upgraded hips and knees. >lan Kellogg wrote: > John Harshman, thanks for the lead. > > Where monophyletic groups are concerned, does that mean that to be > considered a monophyletic group reptiles must incorporate mammals? :) Some think "Reptilia" should be dropped as a formal term. Others, following Gauthier's lead, use it for the final common ancestor of turtles, squamates, tuataras, and crocodylians, plus all descendants thereof. This definition, Clade _Reptilia_, excludes all synapsids (including mammals) and includes birds. Linnaeus' original usage of Classis Reptilia included not only Gauthier's anchors, but also frogs, salamanders, caecilians, and cartilaginous fish(!) Over the years, Class Reptilia has come to popularly signify _Amniota_ with the birds (_Aves_) and mammals (_Mammalia_) arbitrarily removed. (Some remove all dinosaurs and/or pterosaurs as well.) (Personally, I'm inclined to drop it, for what it's worth.) > And what about amphibians? Amphibia has been converted to a clade in two ways: - crown clade: the final common ancestor of frogs, salamanders, and caecilians, plus all descendants thereof. - total (or panstem) clade: the initial common ancestor of frogs, salamanders, and caecilians which was not also ancestral to amniotes, plus all descendants thereof. In either case, it no longer includes certain tetrapods once considered amphibian (e.g. _Ichthyostega_, _Diadectes_). > My proposal is, a monophyletic group need not include all descendents. By the definition of "monophyletic", it does. > That specimens descended from members of a particular monophyletic group > can be placed in a separate, daughter group that qualifies as a unique > monophyletic group based on traits and features found to be specific to > members of that group. What you are describing is a paraphyletic group. > Thus Aves would be a monophyletic group based on > features unique to birds and not found among Archosaurs. _Aves_ is monophyletic, and it is included by a monophyletic _Archosauria_. There are very, very few characters birds share that some other archosaur does not also share. In the modern world, birds are distinguished by warm-bloodedness, hollow bones, three- or four-toed feet, three-fingered hands, stiffened tails, feathers, flexible wrists, wings, keeled breastbones, stumpy tails, "bastard wings" (alulae), fused tarsometarsi, toothlessness, etc. But these did not arise all at once--they arose in the order listed over dozens of millions of years, as the fossil record shows. And they are not hallmarks of _Aves_, they are hallmarks of (respectively), _Dinosauria_ (or a wider clade--crocs may be secondarily cold-blooded), _Saurischia_, _Theropoda_ (or _Avepoda_), _Tetanurae_, _Tetanurae_ (again), _Avifilopluma_ (nearly equivalent to _Coelurosauria_), _Maniraptora_, _Aviremigia_ (or _Avialae_ sensu lato), _Carinatae_ (sensu lato), _Pygostylia_ (or _Avebrevicauda_ or _Ornithurae_ sensu lato), _Ornithothoraces_, _Ornithurae_ (sensu stricto), and _Neornithes_ (or _Aves_ sensu stricto). These clades (monophyletic taxa) spell out a long line of internested descent, and reflect the evolutionary history far more accurately than arbitrary splicing and dicing. > The fact is, matters are not so clear cut as we would wish in the field > of biology. Instead of clear cut lines we have fuzzy zones. You get > right down to it, Homo habilis is an Australopithecine with upgraded > hips and knees. Species are still used as fuzzy categories; clades are not. > > My proposal is, a monophyletic group need not include all descendents. > > By the definition of "monophyletic", it does. And the definition of 'monophyletic' can't change? > > That specimens descended from members of a particular monophyletic group > > can be placed in a separate, daughter group that qualifies as a unique > > monophyletic group based on traits and features found to be specific to > > members of that group. > > What you are describing is a paraphyletic group. How so? >>> My proposal is, a monophyletic group need not include all descendents. >> By the definition of "monophyletic", it does. > > And the definition of 'monophyletic' can't change? It has, but sharpening up definitions requires some underlying justification. Hennig's definition is not complete, but it is precise, which other definitions were not. If you want to replace it with a redefinition, you'd better have some good argument on your side. ...  SignatureJohn S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project University of Queensland - Blog: evolvethought.blogspot.com Who are you going to believe? Me, or your own eyes? > >>> My proposal is, a monophyletic group need not include all descendents. > >> By the definition of "monophyletic", it does. [quoted text clipped - 6 lines] > better have some good argument on your side. > ... And formulating a new definition requires more background than I currently have. Another beautiful idea ruined by the lack of facts. Getting back to the original topic... Basically what I'm looking for is a small, lizard-like crutusari. Something that split off from the line that lead to crocodilians soon after the crutusari first arose. Lasting as a group until the appearance of the first dinosaurs would be a big plus, but not entirely necessary. Most important of all, it has to be a quadraped. No bipedal tendency thank you. Yes, I do plan to commit evil, rotten, (technically speaking) horridly unscientific things to the beast, and his (factually speaking, non-existent) descendents. I think you mean "crurotarsan" (member of _Crurotarsi_). There's nothing terribly lizard-like. Modern crurotarsans (crocodylians) have a sluggish lifestyle, but this seems to be secondarily so. Many other crurotarsans had more upright gaits, and some were even bipedal. Some major crurotarsan groups: - parasuchians (a.k.a. phytosaurus), convergently crocodylian-like, but not closely related - stagonolepidids (a.k.a. aetosaurs), armored plant-eaters - _Postosuchus_, a large, big-headed, bipedal? predator - _Effigia_, _Lotosaurus_, etc. - weird, convergently dinosaur-like, beaked forms - sphenosuchids, small and quick runners (possibly your best bet) - metriorhynchids, very adapted to a marine lifestyle Other "thecodont" groups include dinosaur and ?pterosaur relatives (non-dinosaurian, non-pterosaurian panavians/avemetatarsalians) like _Scleromochlus_, _Lagosuchus_, _Marasuchus_, _Pseudolagosuchus_, and _Silesaurus_. Then there are the non-archosaurian archosauriforms _Euparkeria_, proterosuchids, and erythrosuchids. Finally, there is a great diversity of non-archosauriform archosauromorphs: rhynchosaurs, prolacertiforms/protorosaurs (including _Tanystropheus_), _Trilophosaurus_, etc. Drepanosaurids (a.k.a. megalancosaurids) were once placed here, but are no longer generally thought to belong to _Archosauromorpha_. The most lizard-like of these would probably be something like _Prolacerta_ (its very name means "first lizard", although it is not generally thought to be related to lizards anymore). Within _Archosauromorpha_ it's not very closely related to the crown clade (Clade _Archosauria_, a.k.a. _Avesuchia_), though. > I think you mean "crurotarsan" (member of _Crurotarsi_). -That's_ how you spell it! Thanks. Prolacerta sounds like it would work. How long did it survive into the Triassic and what was its usual habitat and diet? Also, does anyone know of a thecodont or thecodont like animal with a tendency to a serpentine form? When I have enough you'll see what I'm working on. _Prolacerta_ lived during the Induan Age. I don't really know much about--I would hazard that it was a small insectivore. There are no legless archosauromorphs. Squamates are the only amniotes that have evolved leglessness (multiple times, in fact). --TMK > _Prolacerta_ lived during the Induan Age. I don't really know much > about--I would hazard that it was a small insectivore. [quoted text clipped - 3 lines] > > --TMK It doesn't have to be legless, it could have a tendency for short legs and long body and still work. I can't think of anything. Crocodylians are about as close as archosaurs come to that body plan. Maybe hesperornitheans, which still had plumpish bodies and webbed feet, but did have long necks and nearly no forelimbs. Oh, I know -- there's a prolacertiform archosauromorph called _Tanystropheus_. Also had a plumpish body, but the legs are short and the neck is nightmarishly long. > I can't think of anything. Crocodylians are about as close as > archosaurs come to that body plan. Maybe hesperornitheans, which still [quoted text clipped - 4 lines] > _Tanystropheus_. Also had a plumpish body, but the legs are short and > the neck is nightmarishly long. I am looking for an archosauromorph with medium length legs for its size, and a short neck. It will be altered to a great extent in the alternate history I'm working on. (Such as an error in the expression of the genes governing the development of the forelimbs. In the form of a pair of extra forelimbs half way down the body. But that doesn't occur until the late Triassic. That's right, it's one of them there pernicious what-ifs. :shudder: :) ) In short, all this is part of my research for a book on dragons. But not the sad sack gas bags that Discovery Channel documentary came up with. > > > My proposal is, a monophyletic group need not include all descendents. > > > > By the definition of "monophyletic", it does. > > And the definition of 'monophyletic' can't change? It could, but it shouldn't unless you've got some really compelling reasons. > > > That specimens descended from members of a particular monophyletic group > > > can be placed in a separate, daughter group that qualifies as a unique [quoted text clipped - 4 lines] > > How so? A paraphyletic group is, by definition, a monophyletic group minus one or more monophyletic daughter groups. Eg. the traditional Reptilia was Amniota (monoph.) minus Mammalia (monoph.) and Aves (monoph.), and thus a paraphyletic group. You seem to want to redefine a well-defined term ("monophyletic") to mean the same as another well-defined term ("paraphyletic"). This sort of redefinitions are *highly* confusing, so you better have some *really* good reasons for it. So, if you want to speak of groups that, like Thecodontia, do not contain all their descendants, why not just speak of them as paraphyletic? The Cladists may snarl, but everyone will understand what you're saying. > > > > My proposal is, a monophyletic group need not include all descendents. > > > [quoted text clipped - 28 lines] > paraphyletic? The Cladists may snarl, but everyone will understand what > you're saying. What of descendent groups that are so altered from their ancestral group they might as well be considered a different sort of beast? >>>>>My proposal is, a monophyletic group need not include all descendents. >>>> [quoted text clipped - 31 lines] > What of descendent groups that are so altered from their ancestral group > they might as well be considered a different sort of beast? This argument has already been had in biology, about 30 years ago. Ernst Mayr liked the definition you favor, and had an able defender in John Ashlock. (Ashlock defined "monophyletic" as including "paraphyletic", and coined a new term "holophyletic", for what we generally mean by "monophyletic".) But the idea collapsed under the weight of its arbitrariness. How do you define "so altered" rigorously? Where exactly do you put the cutoff point between groups, when everything grades more or less gradually into everything else? Why, for example, are birds so different from other dinosaurs that they deserve their own group, while bats and whales are just submerged within mammals? Why, if you do have such a cutoff for birds as non-dinosaurs, do you put it at Archaeopteryx instead of Microraptor, or Oviraptor, or Sinosauropteryx; or at Sinornis or Ichthyornis? Clades, on the other hand, have objective existence. We may argue about what names to give them or which ones to give particular names to, but the clades themselves are real, historical entities. Evolutionarily speaking, it's also way cooler to think of humans as highly modified fish, and to include us within Osteichthyes where we belong. > Evolutionarily speaking, it's also way cooler to think of humans as > highly modified fish, and to include us within Osteichthyes where we belong. That's why I think that Craniata should be called Pisces instead. > >>>>>My proposal is, a monophyletic group need not include all descendents. > >>>> [quoted text clipped - 52 lines] > Evolutionarily speaking, it's also way cooler to think of humans as > highly modified fish, and to include us within Osteichthyes where we belong. It gladdens my heart to know that I am but a coelocanth with birth defects. :) >>Evolutionarily speaking, it's also way cooler to think of humans as >>highly modified fish, and to include us within Osteichthyes where we belong. > > It gladdens my heart to know that I am but a coelocanth with birth > defects. :) It's just as valid/invalid to call a coelacanth you with birth defects. Leaving aside the equation of birth defects with mutations, you are both equally distant from your common ancestor. >>>Evolutionarily speaking, it's also way cooler to think of humans as >>>highly modified fish, and to include us within Osteichthyes where we belong. >> It gladdens my heart to know that I am but a coelocanth with birth >> defects. :) >It's just as valid/invalid to call a coelacanth you with birth defects. >Leaving aside the equation of birth defects with mutations, you are both >equally distant from your common ancestor. But the last common ancestor probably looked a lot more like a modern coelacanth -- (I know, that should be lungfish!) -- than like Mr. Kellogg. Shouldn't that count for something somewhere in the grand scheme of things? Cladistic classification totally ignores such considerations. In particular, logically (if not in practice) it would be possible for a modern coelacanth to be the exact same *species* as the last common ancestor! In that case the symmetry you suggest would not exist: a coelocanth would *not* be "a human with birth defects" in the same sense that a human is "a coelocanth with birth defects". My point is that cladistic classification, by reducing everything to nodes on a graph, looses some very interesting information, i.e., the amount of *change* between any two nodes. I think that classification methodologies that speak in terms of "grades" are trying to represent that information, and I don't think that is a totally useless endeavor. SignatureJohn Brock jbrock@panix.com > >>>Evolutionarily speaking, it's also way cooler to think of humans as > >>>highly modified fish, and to include us within Osteichthyes where we belong. [quoted text clipped - 24 lines] > trying to represent that information, and I don't think that is a > totally useless endeavor. Quantifying that amount of change becomes a hopelessly arbitrary exercise in practice; this is the basic reason for the decline of evolutionary systematics in favour of cladistics. >>>> Evolutionarily speaking, it's also way cooler to think of humans as >>>> highly modified fish, and to include us within Osteichthyes where we belong. [quoted text clipped - 11 lines] > the grand scheme of things? Cladistic classification totally > ignores such considerations. "Looks like" is a subjective criterion. To a suitably trained anatomist, or a molecular biologist, they may not look too different. If all that you can bring into justifying that sort of classification is gross morphology based on naive similarity, then it is hardly good reason to adopt it. > In particular, logically (if not in practice) it would be possible > for a modern coelacanth to be the exact same *species* as the last [quoted text clipped - 8 lines] > trying to represent that information, and I don't think that is a > totally useless endeavor. Cladistics in the sense of a single cladogram may, but a tree that has character state changes indicated by branch lengths represents the change being measured. And the key term here is "change being measured". Pick a character and change the branch lengths. For example, to take a silly illustration, the DNA code hasn't changed at *all*. So in this case each branch length would be equally long (and the cladogram entirely unresolved). You need to realise that the amount of change is a measure of the state being used. SignatureJohn S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project University of Queensland - Blog: evolvethought.blogspot.com Who are you going to believe? Me, or your own eyes? >>>>Evolutionarily speaking, it's also way cooler to think of humans as >>>>highly modified fish, and to include us within Osteichthyes where we belong. [quoted text clipped - 8 lines] > But the last common ancestor probably looked a lot more like a > modern coelacanth -- (I know, that should be lungfish!) -- Either works. There is some argument about which is closer to us. > than > like Mr. Kellogg. Shouldn't that count for something somewhere in > the grand scheme of things? Cladistic classification totally > ignores such considerations. No, it shouldn't count. Trying to make it count leads to all manner of difficulties and arbitrary decisions. You can try to represent "evolutionary distance" in a classification, or you can try to represent cladistic relationships. Trying to do both at once only leads to the inability to recover either. (And coming up with an objective measure of "evolutionary distance" is not easy either. Consider the bird/theropod vs. whale/mammal question.) > In particular, logically (if not in practice) it would be possible > for a modern coelacanth to be the exact same *species* as the last > common ancestor! Yes, to the same degree that it would be possible for a modern human to be the exact same species as the last common ancestor. Meaning that it contradicts everything we know about evolutionary history, but would be possible in some alternate world. > In that case the symmetry you suggest would not > exist: a coelocanth would *not* be "a human with birth defects" in > the same sense that a human is "a coelocanth with birth defects". True, but moot since the case doesn't exist and will never exist in a world anything like the one we have. > My point is that cladistic classification, by reducing everything > to nodes on a graph, looses some very interesting information, > i.e., the amount of *change* between any two nodes. I think that > classification methodologies that speak in terms of "grades" are > trying to represent that information, and I don't think that is a > totally useless endeavor. I do, for the reasons outlined, which I will repeat: if you try to represent two kinds of information, you end up representing neither. And you are faced with a host of arbitrary decisions involving which groups deserve elevation in rank (again, why birds but not whales?) and where to place the dividing line in a continuum of divergence. > > > > > My proposal is, a monophyletic group need not include all descendents. > > > > [quoted text clipped - 31 lines] > What of descendent groups that are so altered from their ancestral group > they might as well be considered a different sort of beast? What of them? Nothing in particular. They still have to be included if the overall group is to be monophyletic. You seem to be hung up on the word "monophyletic". You want to speak of thecodonts, so just do it. It's out of academic fashion, but there's nothing actually *wrong* with it, as long as you accept they're a paraphyletic bunch. > John Harshman, thanks for the lead. > > Where monophyletic groups are concerned, does that mean that to be > considered a monophyletic group reptiles must incorporate mammals? :) That depends on what you want to call "reptiles". The most common recent cladistic definition only includes amniotes that are more closely related to birds or lizards than to mammals. > And what about amphibians? What about them? > My proposal is, a monophyletic group need not include all descendents. Then you will have to redefine "monophyletic". > That specimens descended from members of a particular monophyletic group > can be placed in a separate, daughter group that qualifies as a unique > monophyletic group based on traits and features found to be specific to > members of that group. Thus Aves would be a monophyletic group based on > features unique to birds and not found among Archosaurs. You mean "other archosaurs". Aves is a monophyletic group. It just happens to be nested within Archosauria. So a pigeon is both a bird and an archosaur, just as you are both a primate and a mammal. > The fact is, matters are not so clear cut as we would wish in the field > of biology. Instead of clear cut lines we have fuzzy zones. You get > right down to it, Homo habilis is an Australopithecine with upgraded > hips and knees. True, but irrelevant to anything you were saying. >My proposal is, a monophyletic group need not include all descendents. I think it is the definition of monophyletic in evolutionary systematics already: http://en.wikipedia.org/wiki/Evolutionary_systematics > From my searching on the Web I do know that the paraphyletic group > called Thecodonts have been rearranged into (at least) 4 (more or less) [quoted text clipped - 33 lines] > happened if a 1953 Buick had landed on King George III of Britain in > 1758. (Sorry about that, but I just had to vent) On several occasions after combat Washington found bullet holes in his cloths. They missed. If one had made a direct hit our world would be very different and events as small as quamtum made have changed where some of those bullets landed. I get the idea. >> From my searching on the Web I do know that the paraphyletic group >> called Thecodonts have been rearranged into (at least) 4 (more or less) [quoted text clipped - 38 lines] > different and events as small as quamtum made have changed where some of > those bullets landed. I get the idea. How would the world be very different? That's like believing that the wings of a butterfly in Kansas can ultimately cause a hurricane in the Atlantic. The only two other possibilities are that he would have either been injured or killed. In either case he might have been seen with even more awe, but that hardly could have changed history. If wounded he might have been off the field for awhile, but you imply that no other general could have filled his shoes. If killed the same argument applies, and you seem to say that no other person was capable of leadership. Your premise says that the American Revolution could not have occured without him. If that is true then he was the only person wanting it enough to make it happen and that there was no popular desire for independance absent his rhetoric. That is a contradiction to what is taught in your schools, that most of the people wanted it to happen. According to Wikepedia only 40-45% of the population supported the revolution so it was a tyrany of the minority. Do you believe that if Washington had died that this minority would not have prevailed? Z >>> From my searching on the Web I do know that the paraphyletic group >>> called Thecodonts have been rearranged into (at least) 4 (more or less) [quoted text clipped - 58 lines] > > Z The first time was in the French and Indian war. If you don't think not having a George Washington would have changed all history after that point you don't have a clue about cause and effect. Neither you nor I nor our grandparents would have existed. The eggs and sperm that got together to make future generations would not have been the same because the people that would have been our ancestors would have been doing different things. How this would have played into winning or losing the Revolution is wide open to guessing but that history would have been completely different isn't even a question. |
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