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Thread: turtles ancestry closer with 'missing link'

  1. #1 turtles ancestry closer with 'missing link' 
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    There is a debate out there as to just what lineage gave rise to turtles, that appeared randomly at the start of the triassic- as turtles. No intermediate, they just appeared. The forerunners for this ancestry are usually: pareiasaurs, which only posses dermal bone and share nothing else (not even gross anatomy), Captorhinids which are a possibilty, or my favorite: Procolophonids, who have a very turtley skull right down to the otic notch.

    http://news.bbc.co.uk/2/hi/science/nature/7748280.stm

    Well, the skull looks like a captorhinid from that shitty picture on the site, but that doesn't mean much at this point for a decision since it's my opinion of what it looks like anyways. Look what else this tells us. It tells us their origin was likely aquatic, not surprising, but it also demonstrates that the ventral plasteron was the first to form a 'shell'. Embryology actually supports this. Pretty cool, but two big questions still stand in my mind:

    1. When (and HOW/Why) did the shoulder girdle rotate into the ribcage? They are the only known animal ever to do this, and the embryos of modern turtles actually form the girdle outside the ribs, then it rotates in! This results in the most complete armour, give or take, ever.

    2. Just what the hell gave rise to the turtles? I'm certain they aren't diapsids that have been modified, they are definately anapsids- but which lineage? At any rate, they aren't in the reptiles proper, Eureptilia...or anywhere near it. I vote to kick them out once and for all! (and while we're at it, monotremes can get the fuck out of mammalia proper as well)

    Discuss.


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    WYSIWYG Moderator marnixR's Avatar
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    i've always wondered whether turtles were primarily or secondarily anapsid - is there any good evidence out there to decide ?

    as for classification, i thought it was known for quite some time that "reptile" sits there together with "fish" a taxonomically unsatisfactory term

    the more meaningful split would be anapsid / diapsid / synapsid (which would solve the problem you seem to have with the monotremes), with a possible subdivision within the diapsids between the lepidosauromorphs and the archosauromorphs (don't know where that would leave the euryapsids as modified diapsids though)


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    Reisz & Laurin, 1991 said that the turtles were nested within parareptiles, closely related to procoluphonoids based on their study of Owenetta , a basal procolophonian that shares a suite of synapomorphies with turtles.

    Reisz RR & Laurin M. 1991. Owenetta and the origins of turtles. Nature 349: 324-326.

    deBraga & Rieppel proposed that turtles are nested within diapsids as sister group of a clade of Mesozoic marine reptiles, the Sauroterygia.

    deBraga M & Rieppel O. 1997. Reptile phylogeny and the interrelationships of turtles. [i]Zool J Linn Soc 120: 281-354.
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    WYSIWYG Moderator marnixR's Avatar
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    any idea on what grounds these allocations were made - or was it more like shoe-horning of an awkward category ?
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    Forum Freshman RathDinen's Avatar
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    If i'm not mistaken, they use cladistic method to classify species based on the evolutionary ancestry..It works by analysing different taxa to find objective similarities and differences between them, and using those similarities and differences to derive a hierarchical structure showing which taxa are most similar to others. The assumption is that similar taxa are similar because they are related, so that the trees produced by cladistic analysis are approximations to the phylogeny of the group being studied.
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  7. #6 Re: turtles ancestry closer with 'missing link' 
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    Quote Originally Posted by mormoopid
    At any rate, they aren't in the reptiles proper, Eureptilia...or anywhere near it. I vote to kick them out once and for all! (and while we're at it, monotremes can get the fuck out of mammalia proper as well)
    1. What is the reptile 'clade' anyway?

    2. Don't you dare touch my beloved monotremes - they possess Linnaeus' defining characteristics of the group - mammaries and the production of milk, hence mammalia, and it's a clear and distinct synapomorphy.
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    Has no one constructed any DNA phylogenies? Morphological traits can be notoriously misleading if you choose the wrong ones, or don't use enough.

    My favorite example is the fact that anyone who attempted to reconstruct the phylogeny of the great ape clade using only skeletons was never able to recreate the known phylogeny from DNA. They could do it if they included soft tissues, though.
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    WYSIWYG Moderator marnixR's Avatar
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    the problem with a DNA tree is that there's only 3 different types of diapsids to compare turtles with (lepidosaurs, crocodiles and sphenodonts) and no anapsids whatsoever - the tree is just too sparsely populated to give much of a clue, which is often the case when most of comparative taxa are extinct
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    Forum Cosmic Wizard paralith's Avatar
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    Quote Originally Posted by marnixR
    the problem with a DNA tree is that there's only 3 different types of diapsids to compare turtles with (lepidosaurs, crocodiles and sphenodonts) and no anapsids whatsoever - the tree is just too sparsely populated to give much of a clue, which is often the case when most of comparative taxa are extinct
    I gotcha, that makes sense.
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    Eurydiapsids basically just lost the jugal bar. They are diapsids.

    Turtles share many traits with procolophonid skulls that I've seen. One major one is the presence of an otic notch, not to mention the shape, functionality of the jaw (though I'm not sure if the jaw muscles insert on the inside or outside of the skull- turtles are inside).

    Sunshine- what if mammary glands aren't universal to eumammalia? It'd suggest there is quite a few characters that should all but assure the expulsion of monotremes; one character isn't enough to keep them in the eumammalia. For example, what if cynodonts (not mammals) had mammary glands that 'leak' milk like monotremes? What would you say then? Go where the evidence leads. In this case, you have one character which may or may not be present in clades that clearly aren't mammals whereas you have several other morphologies and physiologies that indicate that they maybe don't have a true home within mammalia.

    The reptile clade is a sticky one; I personally would kick turtles right the hell out and kick archosauromorpha out as well (with all their sauropsidan friends to boot). I'd suggest keeping reptilia to lepidosaurs and immediate friends. That's just me, I'm more of a splitter. If leave it to common ancestry though, Aves MUST be included- and so should mammals if you really wanna be inclusive like some do, in reptilia proper.
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    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by mormoopid
    Sunshine- what if mammary glands aren't universal to eumammalia? It'd suggest there is quite a few characters that should all but assure the expulsion of monotremes; one character isn't enough to keep them in the eumammalia. For example, what if cynodonts (not mammals) had mammary glands that 'leak' milk like monotremes? What would you say then? Go where the evidence leads. In this case, you have one character which may or may not be present in clades that clearly aren't mammals whereas you have several other morphologies and physiologies that indicate that they maybe don't have a true home within mammalia.
    I don't know about clades that clearly aren't mammals. The monotremes are clearly within the same clade as mammals - that which excludes all squamates, aves, chelonians, lissamphibia etc of the surviving land vertebrates.

    While the mammaries may be the king characteristic of mammals, monotremes share a number of other characteristics that also make them suitable - the fur, the homoiothermy and so on, as well as anatomical homologies and synapomorphies.

    The question is not whether or not monotremes are in a different clade, but really how extensive you want the clade you specifically call 'Mammals' to extend. Me, I'm happy to have it include the monotremes because we're otherwise left with having to define two separate clades, plus the clade that includes them both, and it seems unnecessary to me. But you say you're a splitter and that's fair enough....

    Unless it is that you have evidence of the monotremes being more closely related to some other clade of existing animals?
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    I'm telling you that monotremes are a clade of more ancient relatives of mammals, outside the eumammalia proper. What if I told you that monotremes actually have a much lower metabolism and therefore 'homeothermy' as you would put it, than eumammalia? Fur is a lousy synapomorphy. Feathers, as an analogy, came far before birds and existed in very clearly nonavian dinosaurs; so logic would follow that fur doesn't mean eumammalia, since I would promise you that the sister group of mammalia also had fur.

    There is no issue whatsoever with kicking them out of mammalia, phylogenetically or cladistically; infact, it solves many issues altogether- the foot venom, the egg laying, the lower metabolism, the unique bone anatomy (especially the forward girdle in the Echidna). They would likely have a common ancestor with mammalia that isn't too far off. You are thinking of crown groups only, it would seem, and that simply doesn't cut it when defining the 'tree of life'.
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    WYSIWYG Moderator marnixR's Avatar
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    all the more reason to insist that mammalia is an artificial classification based on a node cladistic analysis which is not very useful - far better to be more inclusive and consider the synapsid clade as the natural group
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    Forum Professor sunshinewarrior's Avatar
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    Mormoopid

    As Marnix points out, either way you cut it, the categories and their relationships would stay the same. You seem to be getting everything focused on the point of what a "Mammal" is and treating the label as though it has some correct 'essence' of mammal that it needs to meet.

    As far as I am concerned, in despite of your objections (and you say you are a comparative anatomist so I'm sure many, possibly all, of them are valid), it seems an irrelevane to remove the montremes from the mammals: of living species, they quite clearly fit in the same group. Of the fossil record and the history that it shows, they also quite clearly did not split away from the rest of the mammalian line that much before the rest of the mammals (modern ones) began their own series of splits into marsupials etc.

    So I see no point getting worked up about what is in 'essence' a mammal. Monotremes are close enough, easily distinguished and with loads of synapomorphies (there is nothing that will change that - they will never, I venture, ever be seen as more closely related to any other group of vertebrates), for it to be a non-issue for me - just as definition-jousting regarding what exactly is a 'kingdom', what a 'phylum' and so on also seem to me to be irrelevant. There is no 'essence of phylum' in nature that I know of. Nor really any 'essence of mammal'. We make do with what seems most coherent at the time...

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    They aren't mammals though, that's the point. They are a different division within the synapsida dude. They are closer to mammals than any other crown group (extant group), but it doesn't mean that's what they are.



    Look at the OUTGROUP there. Molecularily, morphologically AND systematically they aren't mammals and certainly aren't crown group mammals; you can even look for yourself. They are the sister group to mammals in every single phylogenetic tree I could find.
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    WYSIWYG Moderator marnixR's Avatar
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    still, from a stem cladistic point of view, if one group is found to be the nearest sister group of another (and nearer to that group than any other), there is reason to ally it with that group
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    I'm with Sunshine on this one. It does not matter too much to me in what order or clade or whatever you care to categorize them, except for the purpose of having an intuitive way to know which associative characteristics they posses. Mammals, in layman's terms, means that it has all the classic mammalian characteristics, like a solid jaw, lactation, live birth, etc. If you think about it, marsupials also have a few weird characteristics that one would not assign to mammals at first glance. They also have a cloaca for example. Genetically the comparison becomes clearer, with monotremes splitting earlier from marsupial and placental mammals, but they are still in my mind mammals in a broad sense. IMHO
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    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by mormoopid
    They aren't mammals though, that's the point. They are a different division within the synapsida dude. They are closer to mammals than any other crown group (extant group), but it doesn't mean that's what they are.



    Look at the OUTGROUP there. Molecularily, morphologically AND systematically they aren't mammals and certainly aren't crown group mammals; you can even look for yourself. They are the sister group to mammals in every single phylogenetic tree I could find.
    Of course they're the 'outgroup', nobody denies that.

    Get rid of them, however, and marsupials become the outgroup, and the next candidate for eviction from the mammalian tepee.

    You still haven't given us an argument in principle for why you want to draw the line at one place and exclude the closest outgroup to other mammals extant. Might as well get rid of prosimans because they're the outgroup of primates and so on.
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    WYSIWYG Moderator marnixR's Avatar
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    and let's face it : monotremata are no longer the outgroup when you compare mammals with birds
    imo a classification of mammalia based on current eutheria is in danger of ejecting nearly all of the known mesozoic mammals as the concept is currently understood - if they are taken into account, monotremata no longer stands out, in fact, compared with the diversity of mesozoic mammals, viviparity and eutheria are likely to be exception not the rule
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    the funny part is that I'm usually the biggest lumper on the planet

    Reptilia and mammalia would be considered close if you use Echinoderms are the sister group; so I find your point about them being lumped together based on what the outgroup is kind of inappropriate. We are talking about mammalia and essentially the synapsid branch within tetrapods.

    If you consider the palaeontological rammification of including monotremata within mammalia proper, you might be surprised. They are much older than any other lineage within mammalia, for one, with Ornithorhynuchus like (hell, almost bang on the were so close) dating back to the Cretaceous. Well, the specific genus doesn't but monotremata does. It's a situation where I'm going to ask you consider everything between, at least, to be mammals or just plain remove the monotremata. What, in your estimation, sounds to be the most accurate phylogenetically? Considering that many things inbetween really aren't considered true mammals, I'd suggest that removing them would provide the most accuracy.

    One awesome but strange thing I'm learning from this discussion about how I do things is that I split at higher taxonomic levels quite ruthlessly but I'm known for being a stalward lumper all the way to family level. I guess you could say that I don't approve of lumping big groups that are quite different, but that creates a whole new issue of eventually lumping them together at even higher taxonomic levels anyways. This dilemma faces many paleontologists, that's why using words like Class, Order etc have been dropped in many phylogenetic descriptions.
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    WYSIWYG Moderator marnixR's Avatar
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    the big problem that exists when you want to confine the term mammalia to marsupials and placentals is the rest becomes a hodgepodge of "stem non-mammals", which is as bad as the old "Thecodontia", and doesn't help taxonomy one iota
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    Nobody cares about taxonomy except taxonomists.

    nuff said.

    It's all artificial. Just use it any way that is productive for you. That is taxonomy is for.
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    Yeah thecodontia is brutal, but there actually isn't just a hodgepodge of nonmammals. There is actually alot between base synapids leading up to mammalia, pretty good fossil record there.
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    Just decided to point out how inherantly silly it is to include animals phylogenetically based on what you use as the outgroup by way of examples.

    If you use mammals as the outgroup, pterosaurs would become dinosaurs (with all their fused antorbital and narial fenestral goodness)

    Chondrichthyes would be fused with osteichthyes if you use insects as the outgroup.

    If you use marsupalia as the outgroup, chiroptera and primates are the same

    If you use serpentes as an outgroup, you might as well fuse up chameoleonidae and iguanidae...

    You can see where this is going.


    I know all it takes is stepping up a few levels of taxonomy, say from order to class, for them to unite eventually. However, the split is there for a reason. If you start pushing things together simply because they are 'close', you will wind up with a mishmash garbage of things, like that paraphyletic fuster cluck called reptilia.

    So you see, the outgroup doesn't necessarily determine what should be included or excluded- what determines that is essentially synapomorphy.
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    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by mormoopid
    If you consider the palaeontological rammification of including monotremata within mammalia proper, you might be surprised. They are much older than any other lineage within mammalia, for one, with Ornithorhynuchus like (hell, almost bang on the were so close) dating back to the Cretaceous. Well, the specific genus doesn't but monotremata does. It's a situation where I'm going to ask you consider everything between, at least, to be mammals or just plain remove the monotremata. What, in your estimation, sounds to be the most accurate phylogenetically?
    From what I gather, the placentals started their divergence (at least the modern lot, as long ago as 105mya. The placentals split from the marsupials at least 125mya, and the placentals and marsupials split from the monotremes about 150mya. Pro rata, one does not seem greater than the other in terms of evolutionary time.

    I appreciate that palaentologists have their own ideas and issues but, given that the next nearest split, after the monotremes, takes in all the rest of the living amniotes, and dates back to 350mya or thereabouts, I don't think it unreasonable to group the monotremes firmly with the rest of the mammals.
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    time from divergence has no bearing on relationships at all.

    The main problem I'm having with what you're talking about is that you are comparing 3 groups exclusively and are neglecting to consider that while the monotremes are related to mammals, they are probably more closely related to extinct nonmammal groups unless you are willing to say that every single synapsid that is related to monotremes dating back 150 MYA and probably a great deal further is without a doubt a mammal. Are you willing to be that inclusive with your terms?

    Can you convince me there won't be some type of polyphyly there?

    One huge issue with monotremes, as an aside, is their teeth. Mammals are distinguised from each other anatomically by their teeth a great deal of the time. There are entire sequences of rodents dating back millions of years on just teeth alone; they do the same with shrews as well. Monotremes don't actually have teeth. Echidnas certainly don't, whereas the platypus has some kerratinous pads that it uses. So not only is this a pretty big synapomorphy of this group, suggesting to me that its divergence could be way the hell out there, but you have no actual way of tracking who they are specifically related to. This is compounded by an absolute lack of a 'missing link', until you get back into the cynodontia who probably did lay eggs but also had fur.

    Can you tell me that once you get that far back things are still mammals? Because that's what you're essentially doing by lumping monotremata within mammalia proper.
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    WYSIWYG Moderator marnixR's Avatar
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    Quote Originally Posted by mormoopid
    Monotremes don't actually have teeth. Echidnas certainly don't, whereas the platypus has some kerratinous pads that it uses. So not only is this a pretty big synapomorphy of this group, suggesting to me that its divergence could be way the hell out there, but you have no actual way of tracking who they are specifically related to.
    correct me if i'm wrong here, but didn't Obdurodon have teeth, as well as all other earlier monotremes ? your "synapomorphy" only applies to the last 20 million years

    besides, from a taxonomic point of view, "lack of" has never been a very satisfactory criterion for a synapomorphy - do you want to lump all amniotes apart from birds, bats and pterosaurs together on the "synapomorphy" of lacking wings ?
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    I singled out specifically extant monotremes for a lack of teeth to illustrate that there would be some difficulties in determining who is closest to them within the mammalia. I should have said crown group or extant, sorry.

    It is a pretty big synapomorphy of mammalia simply because of the high imporance of teeth for taxonomic markers in the mammalia. As that part you quoted does say, it means they could be way the hell out there and there's not much way of knowing. This is assuming we're only talking about crown group monotremata since it was a bit of a focus in this discussion.
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    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by mormoopid
    As that part you quoted does say, it means they could be way the hell out there and there's not much way of knowing.
    Can I ask how they could be 'way out there'? And what that means taxonomically?

    I presume you're not claiming that, as part of the monotreme clade, they have evolved a toothles synapomorphy, so that suddenly that makes them more ditant from the mammals - there relatedness to mammals will be the same whether or not they've evolved further synapomorphies and these days we do not need palaentological evidence to confirm that relationship: given that they are extant, the DNA provides superb evidence of relatedness.
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    The only thing the dentition (or lack thereof) shows is that it is more difficult to place them adjacent to a 'next of kin' within the mammalia.

    I'm just going to review what a mammal is here:

    From the omnipotent wikipedia:
    Mammals (formally Mammalia) are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, which they use in feeding their young. They are also characterized by the possession of sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.
    mammary glands almost certainly existed in part in at least the nonmammalian sister group and I dare you to provide logic or evidence to the contrary. Assuming this is even a good trait, which I contest it isn't, you cannot logically include any extinct mammal as having it 'for sure'. So I'll ask: At what point is something with a mammary not a mammal but close to mammals? Shame on you if you say "Well if it has a mammary, it's a mammal duh".

    Sweat glands is marginal at best, there are mammals today that don't even have these. Furthermore, I'd say certainly they existed in other groups. As if nonmammalian outgroups didn't have access to the precursors and couldn't find use in a sweat gland.

    Three middle ear bones? Okay, well lets ask solidly here: When does the Articular, Quadate and Angular become Malleus, Stapes and Incus? Well, I'd say when it no longer functions in movement of the jaw; If I recall, monotremes do have this. feature, so I'll have to grant this point to you.


    Mammals also have a double occipital condyle: they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. Paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, since it would be confusing if they found a fossil that had one feature, but not the other.
    It wouldn't be confusing- it would be a transitional animal. Idiot wikipedia. Anyways, if I recall, monotremata has 2 occipital condyles.


    okay, well now that we are reviewing solid markers that don't more include flexible things (IMO) like hair and milk production, it appears that monotremata are mamamls:

    http://en.wikipedia.org/wiki/Monotreme

    I'd still contend that prototheria really doesn't 'belong' and perhaps should get more credit as an intermediate than being a true mammalian, or at least close enough to to the intermediate that it should give us a pretty good analog for it. I dislike, quite alot, including such basal members within such a well defined and 'advanced' crown group; but, in the interest of having a few of the lowest common denomenators (previously unmentioned in this thread), monotremes have nowhere else to go but mammalia.

    In closing: Monotremata is to mammals what the crums are to a bag of chips- still in the bag, still made of chips, but pretty bad at being a chip.
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    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by mormoopid
    In closing: Monotremata is to mammals what the crums are to a bag of chips- still in the bag, still made of chips, but pretty bad at being a chip.
    Fair enough. Sounds a bit denigratory to me (see Dawkins' The Ancestors' Tale and his notes on the platypunculus), but if we're all happy to live with it, I'm sure the montremes are, too!

    Have a great Crimbo all...
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