Most of the time when we think of birds, we think of the things that make them birds, and not the things that make them dinosaurs. But that is because we often have the relationship between dinosaurs and birds reversed in our little primate minds; Much of what is bird-like is not exclusive to birds, but rather, to a larger group of dinosaurs. Birds have taken these particular traits in novel directions, but these traits existed independently of all the birdiness we usually attribute to our feathered, flying, bipedal friends, long ago, before the Great Extinction.
You’ll hear people tell you that birds are dinosaurs, and that is supposed to blow you away and make you go all gaga about birds and evolution. People also say that hyraxes are elephants, but they are so distant in their relationship that it is little different than saying that deer are a type of pronghorn antelope. It is much stranger and illustrative of the ways of evolution to point out that whales are a form of “hoofed animal” (unlikely but true). Also, there are reasons why statements like this are scientifically incorrect even if there is ‘some truth’ to them. The idea that birds are dinosaurs could be thought of as a trivial fact of evolution … birds are really no more dinosaurs than whales are bovids, functionally, even though they actually are, phylogenetially … but that would be missing a greater and very interesting point. It is much more interesting to consider the fact that some (actually, many) dinosaurs were bird-like, and of course, this similarity had to do with their common ancestry.
For example, there seems to have been dinosaurs that used feathers as their body covering. In nature, you’ve got your leathery skin (like elephants or crocodiles), you’ve got your fur (like beavers and red deer), you’ve got your scales (like fish and pangolins) and you’ve got your feathers (like many theropod dinosaurs …. oh, and birds). We know that features on birds facilitate, in fact allow, bird-like flight. There are other uses of feathers but flight is key in birds, and it is easy to imagine that feathers evolved to facilitate flight. If so, then other uses (as insulation, armor, signalizing to con specifics, feathering the nest, etc.) are convenient secondary uses. And, that may be true regarding recent evolution of feathers. Indeed, flightless birds have feathers and they use them only for these other uses, and there are probably features of modern feathers that evolved long after the other dinosaurs disappeared in that great cosmic collision 65 million years ago. However, it would appear that during one fairly long period of time in the past there were a number of species (of dinosaur) that used feathers in a number of ways such as those just mentioned (and possibly other ways), and one subgroup of these dinosaurs would later be birds. Some of those other groups, which have left no progeny, may even have used feathers to facilitate gliding or some sort of flight, but we can not assume that flight was the reason that feathers first evolved. Almost certainly, it is not.
Also, there may have been some dinosaurs that you would never look at and say “what kind of bird is that?” sporting bird-like lungs. Birds have interesting lungs that allow a more or less continuous flow of air across surfaces that exchange gasses between the atmosphere and the blood. Some dinosaurs used this technique as well. There was probably a time when running faster and for a longer period of time was strongly selected for and highly efficient lungs therefore evolved in these dinosaurs. Or may be there was some other reason that efficiency in breathing was extra important. It is very easy to see the super efficient lung of birds as a great adaptation to flight, and it is, by most definitions of “adaptation,” but the features that make bird lungs unique in the world today were probably already in place and being used by a bunch of different dinosaur species, a subset of which were in the “bird clade” and the others absolutely, definitively, not. I would guess that there are aspects of the bird respiratory system that have been fine-tuned, perhaps significantly, as flight became increasingly important, but the basic idea of pushing a more or less continuous stream of air across gas-exchanging tissues may have been well in place before anything you’d call a ‘bird’ emerged, and used by non-bird dinosaurs.
Bipedalism is somewhat rare these days in the animal world. Among mammals, bipedalism is found in several rodents, a number of marsupials, and one primate. Facultative (occasional, used when needed) bipedalism occurs in one group of carnivores. But overall, it’s not common if you exclude bird and bats, the two main flying vertebrate groups. But bipedalism is a feature shared by many dinosaurs that did not fly, and counts as another example of a trait we link to birds found broadly in many dinosaurs that were bird relatives before birds ever flew.
It is probably helpful to have a sense of how birds and dinosaurs (or should I say, “other dinosaurs”) relate to each other on a phylogenetic tree. In words, modern birds are called Neornithes. Neornithes together with some now extinct close relatives to the modern birds is referred to as Avialae. A larger group that includes Archaeopteryx, is called Aves. Aves together with some dinosaurs form the group Theropoda (the Theropods). The living forms that are most closely related to Theropods are the Crocodilia (crocs and gators), and together, Theropods and Crocodilia (sometimes spelled Crocodylia) are members of the grop Archosauria, which includes all the extinct forms we call dinosaurs as well as pterosaurs (probably). Which means that Archosauria gave rise to flight at least twice. That’s actually a somewhat simplified version of what happened, and may obscure the key point, so let me state it a different way.
A four-legged somewhat croc-like ancestor gave rise to:
- More croc-shaped creatures
- More four-legged creatures including many famous dinosaurs;
- Flying dinosaurs (Pterosaurs)
- Two-legged creatures, which included many famous two-legged dinosaurs like T-Rex
- A bunch of dinosaurs that had various features that birds are now famous for, which included a bunch of things that went extinct and were not birds but were birdlike in important ways
Birds have feathers, interesting respiratory systems, and are bipedal, and these (and some other bony features) were shared with this larger group that included but was not limited to birds. Meanwhile, birds of today have a number of features that evolved within the bird-only subgroup, such as a beak without teeth. Certain other features are of unknown relations. Did the unique bird brain exist in any of these non-birds, or for that matter, in any other dinosaurs? What about colorful feathers used for various purposes? The nature, distribution, and evolution of bird song is unclear.
One of the most interesting differences between birds and dinosaurs has to do with their eggs. A subset of dinosaurs including birds had changes in their skeleton that allowed for larger egs and/or more eggs to be managed by the female, for instance. Perhaps birds were able to have all their young hatch with greater synchrony and to incubate more quickly or to greater maturity than other dinosaurs. This is a topic we’ll come back to.
In the meantime, rest assured that not only are birds really dinosaurs, but that this is much less strange than it sounds when you realize that very many dinosaurs, typically not the ones you see on TV and in movies, were actually very bird-like in some rather unexpected ways.
For more information see the following sources:
Birdish adaptations in dinosaurs: Aerosteon riocoloradensis
Living Dinosaurs: The Evolutionary History of Modern Birds