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Are birds reptiles?

  1. Aug 5, 2012 #1
    Seems like a stupid question, but from what I could gather aves (birds) are therapods, a kind of dinosaur, and dinosaurs are part of the class reptilia. So are birds reptiles? Are dinosaurs reptiles and are birds dinosaurs? I've tried figuring this out by myself, but sometimes aves is a subgroup of dinosauria, sometimes it is a class in its own right. Moreover, I've read that crocs are closer to birds than to lizards (o_0) Can the taxonomically savvy forumers help?
  2. jcsd
  3. Aug 5, 2012 #2
    I've been wondering the same thing, quite recently. The answer, according to what I found, is that birds are quite clearly NOT reptiles, simply because of the way one defines a reptile. Dictionary.com begins its entry with the words "any cold-blooded vertebrate", which already rules out warm-blooded birds. Wikipedia is even more explicit, identifying them as "amniotes [...] that are neither birds nor mammals."

    I'd recommend the rather exhaustive "History of Classification" section from the same article to gain a better understanding of where the confusion stems from. What it ultimately boils down to, in modern terms, is this:

  4. Aug 5, 2012 #3
    Modern Birds descended from Theropods but they aren't classified as theropods anymore, although they do fall under the theropod clade. Clades are basically individual branches of a phylogenetic tree, including all its sub branches. Our system of classification works only in a short time frame (evolutionarily speaking), as if we have taken a snapshot of all the species that are there right now and then group them based on their morphological characteristics and phylogenetic relationships. So we wouldn't call birds reptiles, we consider them two separate classes. This creature is considered ancestor of all terrestrial tetrapods. That doesn't mean we classify ourselves as fish does it?
  5. Aug 5, 2012 #4
    I've read somewhere that not all dinos were cold-blooded though, does that mean that they are not reptiles?

    I did not know our classification system only worked in a short time span. I guess that helps makes things clearer. But was the class reptilia valid in the time period of the dinos? Were dinos actually reptiles even though some of them were quite different from what we'd call reptiles today?
  6. Aug 5, 2012 #5
    "Reptile" is a human concept. Actually, it's several overlapping concepts. One refers to any member of the scientific, but antiquated, class Reptilia. Another is part of everyday language and might be paraphrased as "lizards, crocodiles, and that sort of thing".

    The principal purpose of both of those was to classify extant species, and whether either one includes some or all dinosaurs depends on how one goes about extending them to extinct species. Which, due to the haphazard way in which they are defined, is quite arbitrary.

    It's like asking whether poodles are wolves, or like that deprecated but very intuitive category called "pachyderms".
  7. Aug 5, 2012 #6
    No. Functional resemblance does not count for much in taxonomy. Structural resemblance counts a bit. However, the absence of one structural trait does not automatically a bit more.
    The naked mole rat is cold blooded and almost hairless. However, it is still considered a mammal. It feeds its babies milk with teats that resemble from modified sweat glands. It has stapes in the middle ear. It has seven vertebrae in its neck. It has interlocking vertebrae like other mammals. The cold bloodness is hypothesized to be a feature that evolved fairly recently.
    Many fish give birth live. Many fish have a type of warm bloodness, different from mammals and birds. They have to flex their muscles to keep warm. So the structure behind the warm bloodness is different from that of mammals. Fish do not have a placenta or uterus that is structurally like mammals. Even though it is warm blooded in terms of temperature, the anatomy behind the warm bloodness is completely different.
    The great white shark is both "warm blooded" and "gives birth live". It is not considered a mammal.
    A structural feature that serves different functions counts for a lot more. Feathers are structually similar in all birds. Yet, basically the same feather in structure can be used for flying (pigeon), swimming (penguin), braking (ostrich) or insulation (all birds). The fact that the feathers are so similar, but are used completely different, suggests that the differences were started by small mutations.

    From a cladastic point of view, the class reptilia is not valid even today. Reptiles are now considered a polyphyletic classification. They did not branch of at the same time.
    Reptiles don't have enough in common to be considered a biologically relevant grouping. The crocodile is not a lizard any more than a dolphin is a fish.
    Squamata (lizards, snakes), Testudinata (turtles, tortoises), Rhynchocephelia (Sphenodon), and crocodilia (alligators, crocodiles, caimans, gavails) are different orders of extant reptiles. However, the evidence is that these four orders branched off at completely different times.
    In terms of a cladogram, crocodilians are closer related to birds then to squamata, testudinata or rhynchocephelia. Some scientists prefer to group crocodilians, birds and dinosaurs into one group called archosaurs.
    There are a few groups of mammal-like reptiles that lived before the dinosaurs. Pelycosaurs, therapsids and gorgonoids were first classified as reptiles, just like the dinosaurs were first classified as reptiles. However, research over the last few years shows that these reptiles are closer related to mammals than they were to dinosaurs.
    There is a lot of controvery about grouping testudinata and squamata together. Some evidence indicates testudinata may be closer related to archosaurs than to squamata. However, the testudinata split off very early from both archosaurs and squamata.
    It appears that in terms of natural history, reptiles are not a well defined group. The four extant orders have a skin deep resemblance to each other. However, they did not split off from each other later than they split off from the mammal line.
    If you include extinct animals in taxonomy, it gets even more difficult to think of reptiles as a monophyletic clade. Before the Permian Triassic extinction 250 MYA, there was a continuum of creatures that could be classified as archosaurs. There were fewer distinctive features between birds, crocodilians and dinosaurs in the Jurassic. However, these animals differed a great deal from squamata, pterodactyls and mososuars.
    Reptiles are a historically useful class derived from Linnean taxonomy. Historical categories are useful for looking up things in the library or even on the internet. If everybody kept reclassifying organisms to keep up with research that is continually being done, no one could keep track of that research. Libraries would go broke reshuffling their books every few months. However, reptiles is not a useful category when correlating facts about nature.

    The cladogram in the following link may be useful.
    Dissected cladogram of repriles, birds, and mammals.
    Last edited: Aug 5, 2012
  8. Aug 6, 2012 #7
    Wow, thanks for the reply.
  9. Aug 6, 2012 #8


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  10. Aug 6, 2012 #9
    So is the old class of reptilia obsolete? It's clear birds share their clade with dinosaurs, but not with lizards or turtles. On the other hand, mammals share their clade with all reptiles, but also with birds. Is there still a proper way to use the term "reptile"?

    EDIT: There's an error on the first line of my post. Birds and crocodilians form a clade, but lizards, snakes, turtles and mammals only form a clade with birds at the level of amniotes.
    Last edited: Aug 6, 2012
  11. Aug 6, 2012 #10


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    Amphibians aren't amniotes. The ancestors of mammals form the group of synapsides. Personally I would not have problems as classifying humans into reptiles. After all it's more than 100 years since people minded Darwin grouping men and apes together.
  12. Aug 6, 2012 #11
    Yes. I deleted that question before seeing your post. My question is whether there is still an acceptable term for the cold blooded amniotes we refer to as reptiles.
  13. Aug 6, 2012 #12


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    Reptiles seem to be all the amniotes excluding the synapsids. Apparently (?) the synapsids have branched off before the other reptiles separated, so reptilia is still a monophyletic group (only if the birds are included).
  14. Aug 7, 2012 #13
    The Reptiles flyed with the same caracteristics than the birds of today. The "écailles" or "plumes" are similars. The constitutions of the bone too. The worse difference is the cold blood versus hot blood. But it's the possible evolution more near of the possible between the ways. In Mamiferes, the temperature is differents too and under pressure of climate it's the best I can imagine.
  15. Aug 8, 2012 #14
    The "amniotes ecluding the synapsids" are the sauropsids. Reptilia, on the other hand, traditionally includes the bulk of the species from both the synapsid and sauropsid branches. The difference between the two is merely that while all of the synapsid reptiles are extinct, some of the sauropsid reptiles survive.

    That's my understanding, at any rate.

    This diagram shows the two non-reptilian crown groups as having been quite literally scooped out of the amniotes, leaving the reptiles as the remainder, which I found conceptually quite helpful:

  16. Aug 8, 2012 #15
    Thanks for the diagram onomatomanic. It confirms what I asked in post 11. Reptiles are the cold blooded amniotes. I guess "reptile" is still a "legitimate" term defined in this way.
    Last edited: Aug 8, 2012
  17. Aug 8, 2012 #16
    Hmmm. That's pretty much the dictionary definition I gave originally, except that that one used "vertebrate" instead of "amniote" and then proceeded to exclude fishes and amphibians more explicitly, which is likely due to the former being a more broadly understood term than the latter.

    In the scientific context, however, a "reptile" is simply any member of the class Reptilia, I should think, and that class likely does include warm-blooded species. For one thing, I don't think that the class Aves is commonly defined in a way which would include all dinosaurs which are believed to have been warm-blooded. For another, more insidiously, there's this:

    It's a can of worms, whichever way you slice it (wheee, mixed metaphor...)
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