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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13 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 #



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