Why are Birds Dinosaurs?

nationalgeographic.com

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

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.

Dinosaurs!

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.

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!