Why are Birds Dinosaurs?


Month after month, one of the most popular posts on the Paleocave blog is the How to Read a Cladogram post I did some time ago. I always intended to follow it up with more cladistic fun. So, hold onto your butts, we’re going to let the dinosaurs loose.

Birds are dinosaurs. We’ve all heard this. But does that phrase make any sense? Not really. Dinosaurs, for the most part, are things that were really big, were mostly scaly, had fantastic teeth, and are extinct. Birds, on the other hand, don’t have teeth, are generally small, and are covered in feathers (I know that you know that lots of old school dinosaurs had feathers too, but whatever). So, why do we say that birds are dinosaurs? The answer involves evolution and the meaning of taxonomic names in biology.

Let’s start with what the name of a group should mean. Why would we call one group of animals ‘birds’ and another group of animals ‘dinosaurs’? Presumably all the animals called ‘birds’ should have more in common with each other than they do with the animals in the group called ‘dinosaurs’. Now we could categorize things based on any attribute we want. We could sort all orange animals into a group and call that group ‘tacos’, but I don’t know how useful that would be. Biologists have decided that what taxonomic names should represent is shared evolution, or relatedness. With that in mind, what the group Dinosauria should contain is all animals that are evolutionarily more closely related to each other than they are to non-dinosaurs. Let’s examine that using a cladogram (more on how to read those here).


Relationships among archosaurs

Relationships among archosaurs

Ok, moving from left to right, we’ve got crocodilians (represented by an alligator), pterosaurs (represented by a Pteranodon), an ornithiscian dinosaur (represented by Triceratops) and two sauriscian dinosaurs (represented by T. rex  and a pelican). Group names are located at intersections of the cladogram and the name applies to all the animals from that point upwards. The whole lot of animals on this cladogram are in a branch of the reptiles called the Archosauria (that’s why the name is down at the bottom-most node of the cladogram). Further up the cladogram we have the name Dinosauria. Next I’ll circle up all the animals that belong in the group Dinosauria.



No surprises here (unless you forgot that pterosaurs aren’t dinosaurs). Birds are in the circle because they are in the group Dinosauria. But do they really belong? Well yes, actually they do. If you know how to read the diagram correctly it says that the pelican and the T. rex are the two most closely related animals depicted. Triceratops is the next most closely related animal to those two. Did you catch that? T. rex and the pelican are more closely related to each other than T. rex is to Triceratops. They are both in the group Theropoda (that Triceratops isn’t part of). The T. rex and the pelican have been on the same evolutionary path for longer than any other two things on this diagram. Here I’ll show you the same information diagramed slightly differently.

This makes it more obvious that birds are squarely in Dinosauria.

This makes it more obvious that birds are squarely in Dinosauria.

See, birds are dinosaurs.

See, birds are dinosaurs.

Bird are dinosaurs not just because they evolved from dinosaurs, but because they are more closely related to some of the extinct dinosaurs than those dinosaurs are to each other! So next time that someone tells you that dinosaurs are extinct, you can tell them that, actually, there are probably more species of dinosaur alive today than there were in the Mesozoic!



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16 Responses to Why are Birds Dinosaurs?

  1. Ed CroneNo Gravatar 18 December, 2013 at 2:54 pm #

    This is great! I have a 3 year old who is so very fascinated by dinosaurs and I am learning all new things from when I was in school. Really great stuff!


  2. Ben HillierNo Gravatar 15 April, 2015 at 1:21 am #

    A nice clear explanation. Thanks!

    But there’s something I don’t get. If you trace that Cladogram even further back, you’ll end up with a group called fish, from which all of the Archosaurs descended. Does that mean birds and dinosaurs are all fish? In fact, come to think of it, that means we are also fish? What am I missing?

    • RyanNo Gravatar 15 April, 2015 at 9:08 am #

      You’re not missing anything! From a certain perspective it would be appropriate to call all land vertebrates fish because tetrapods evolved from a sarcopterygian ancestor. At a certain point it’s a judgement call on the usefulness of the definition. Explaining that birds are dinosaurs still keeps dinosaurs a relatively small taxonomic group then expanding fish, already a huge group, to include every land vertebrate even if that is technically the case. This podcast provides a nice discussion of the problem with defining what exactly a ‘fish’ is: http://www.fieldmuseum.org/science/blog/what-fish-episode-1-you-are-all-fishes

      • AntonyNo Gravatar 29 February, 2016 at 2:18 am #

        For me the reason birds are dinosaurs, but that (although technically true) we do not think of all land vertebrates as ‘fish’ is that there are some defining characteristics that define us (and dinosaurs) from what we think of as ‘fish’ (eg, respiratory systems are different). I have not seen any such defining characteristic between birds and dinosaurs, nothing that exists solely with one group and not the other. They share far too much…

        But yes, we are a very strange fish evolved to live on land and breathe air O_o

  3. JohannaNo Gravatar 26 May, 2015 at 6:16 am #

    What exactly does “closer” in “closer related” mean (final paragraph)? I thought the length of the branches in cladograms is abritrary?
    Very nice blog btw.

    • Karlo Marcos ReyesNo Gravatar 8 March, 2016 at 10:56 pm #

      WELL , one actualization , reptiles , amphibians and fish terms were cut off of modern biology for taxonomical errors , like call reptile the dinos and pelicosaurs but not their decedents
      how we will call correctly a lizard?
      a sqamata or the name of the specie <3

  4. AntonyNo Gravatar 29 February, 2016 at 2:07 am #

    “Dinosaurs, for the most part, are things that were really big,”

    Is this even true? Were they mostly big? Sounds unlikely to me, would be a very top-heavy ecosystem. These days there are more small animals than large, since large animals require more small ones or vegetation to survive. Is there any evidence this was different in the past, or are you simply playing into the confirmation bias that exists since a small number of large dinosaurs are far more famous than the many small ones?

  5. YOUR MUMNo Gravatar 14 February, 2017 at 2:34 am #


  6. JoeNo Gravatar 6 July, 2017 at 3:17 pm #

    The general consensus these days is indeed that birds are a surviving line of dinosaurs, but the consensus is not so broad as to render the opposing view as a fringe position.

    There are some very old fossils (i.e. predating arxheopteryx) that have features more similar to modern birds than feathered dinosaurs from later periods. Based on these finds and competitive anatomical features of modern birds and non-avian dinosaurs a minority of reputable scholars argue that birds did not evolve from dinosaurs but rather split off the line leading to dinosaurs and represent a closely related but nevertheless distinct line of archosaur.

    If this line of reasoning is true then the relationship of birds to dinosaurs is akin to the relationship of dinosaurs to pterodactyls or of true mammals to stem mammals (i.e. ‘mammal like reptiles’)..

    The preponderance of the evidence seems to favor the view that birds are dinosaurs. However I don’t think this is quite as well established as many writings on the topic suggest.

    There is a Wikipedia article describing dinosaur size for example which describes the range of known dinosaur sizes by citing some sauropod as the largest known dinosaur before citing the smallest known dinosaur (only a few inches long) by its Latin taxonomic name. Intrigued by this tiny dinosaur I had never heard of before, I looked it up and saw that they were referring to the hummingbird.

    Now, I recognize that the preponderance of evidence suggests that birds are dinosaurs, but that still struck me as rather odd. In part because whoever wrote it was ostensibly attempting to obfuscate the fact that they were referring to a hummingbird by using its obscure Latin taxonomical name, and in part because In part because I don’t think the idea that birds are dinosaurs is yet sufficiently firmly established that one can cite the hummingbird as the smallest known dinosaur without making it clear that the smallest known ‘dinosaur’ in question is in fact the common modern day hummingbird. This struck me as a tad overzealous and as implying that the notion that birds are dinosaurs is more firmly established than it actually is.

    I realize Wikipedia, as an open source encyclopedia which anyone can contribute to, is vulnerable to less than encyclopedic information, but I have seen similar references outside of Wikipedia.

    It makes me a little concerned that some people, including reputable scientists, find the idea that dinosaurs didn’t entirely go extinct but continue to live amongst us in the form of birds, to be so ‘cool’ that the question isn’t always being considered with the appropriate degree of scientific scrutiny.

    I also tend to think that our classification of dinosaurs has always been strongly influenced by the way we first cable to know about them. I think the fact that we classify this large group of related extinct animals all as “dinosaurs” is an accident of history. If archeopteryx had been the first ‘dinosaur’ to be discovered perhaps we would have been more cautious in our taxonomical classification of subsequent finds. Instead the earliest unearthed specimens tended to be the larger, superficially reptilian looking types and we came to think of most of these creatures as belonging to the same ‘class’.

    But what if the major genera of dinosaurs had never gone extinct? We live in a world where we have historically assigned the animals we are to one of a few major classes (i.e. Fish, Amphibians, Reptiles, and Mammals, etc). I have a sneaking suspicion that if the vast array of creatures we call dinosaurs had never gone extinct, we might well have regarded the major orders of dinosauria as distinct classes (as different from one another as birds and reptiles etc). I doubt we would think of creatures as different and distantly related as Tyrannosaurs and Sairopods and Ceratopsians as belonging to one group; I think this is an awkward artifact of the way we came to know of these extinct animals.

    A hasty generalization on a grand scale that may take decades or centuries to correct.

    • Antony TysonNo Gravatar 7 July, 2017 at 1:39 pm #

      Interesting, but the fact remains that all birds, extant or extinct, are more closely related both phylogenetically and chronologically to a T Rex than a T Rex is related to a Stegosaurus. So if we are to classify T Rex and Stegosaurus ‘dinosaurs’ birds should be too, regardless of whether they split off before dinosaurs such as T Rex or are descendants of dinosaurs like T Rex.

      But the end point about how broad the tent is that includes ‘dinosaurs’, and that maybe we would not consider them the ‘same’ in that respect, makes sense to me. Maybe ‘dinosaur’ is a Victorian idea that’s past it’s best and needs to go the same way as the luminerferous ether. However, I haven’t heard any issues about to phylogeny of dinosaurs so maybe not… Maybe I’ll look that up sometime…


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