You sometimes hear people saying that birds are related to dinosaurs, but that’s really not true – birds aren’t related to dinosaurs… they are dinosaurs! 65 million years ago, a huge extinction wiped out all dinosaur groups except for one – that group of dinosaurs went on to become all the birds we see today. But let’s start from the beginning.


Branching out to birds. Image via Palaeos.

Dinosaurs were a diverse group of animals which first emerged during the Triassic, 231.4 million years ago. They were the dominant life forms on land for 135 million years, until the great extinction wiped most of them out. The cause of that extinction is still a matter of debate, but the most likely option seems to be an asteroid impact (though volcano eruption is also quite possible).

Dinosaurs are split into two major groups – Saurischia and Ornithischia.

  • Saurischia include all the carnivorous dinosaurs (theropods) and the giant, herbivorous dinosaurs (sauropods). The theropods are the more “popular” carnivorous dinosaurs, such as T-Rex or the velociraptor while the sauropods are the largest animals to ever walk on land;
  • Ornithischia are an order of beaked, herbivorous dinosaurs. The more notable members are the Stegosaurus and Iguanodon.

Interestingly enough, birds evolved from the Saurischian dinosaurs. Modern paleontology indicates that birds may have started to emerge during the Jurassic, some 150 million years ago.

… and birds

Birds are a member of Maniraptora, a group of theropods. It may seem strange that they actually emerged from dinosaurs, but today, most paleontologists agree that several dinosaurs were covered in feathers. Their bones were also very light, they laid eggs… they have many things in common even with modern birds.

Of course, the transition took place over millions and millions of years, with several key transitions. The most well known transitional fossil is Archaeopterix.

Archaeopterix fossil. Image via Science Views.

Archaeopteryx lived in the Late Jurassic period around 150 million years ago, when Europe was an archipelago of islands in a shallow warm tropical sea. The species is one of the clearest examples of transitional species, showing characters from both dinosaurs and birds. Most of the fossils found include impressions of feathers. Because these feathers are of an advanced form (flight feathers), these fossils are evidence that the evolution of feathers began before the Late Jurassic.

However, it took quite a while before birds started diversifying. For tens of millions of years, birds still had clawed wings and teeth. Truly modern birds appeared emerged 100 million years ago, way before the massive extinction which wiped out the dinosaurs. While they were never really on top of the food chain, they are the only branch of dinosaurs to survive past the Cretaceous (if you haven’t already, get used to the idea: birds are dinosaurs).

Modern birds are characterized by a beak with no teeth and a high metabolic rate and rate of growth. Most can fly, with some exceptions. They also have several other adaptations specifically aimed at flying.

So, birds are reptiles?

Modern birds are very diverse. Image via Ashley Barron.

Well… yes, in a way, birds are reptiles – but it’s a bit more complicated than that. Biologists use two types of classification systems, the Linnaean and the phylogenetic. Depending on the classification system you use, birds are or aren’t reptiles. In the Linnaean classification, a reptile is an animal that is cold blooded and has scales – so in this sense, a bird is not a reptile. But in modern biology, people tend to follow the phylogenetic classification – that is, animals are grouped by their ancestry, and in this way, birds are reptiles. It’s a bid hard to follow, but in essence, things aren’t very complicated – both these systems have different uses.

The question which emerges now is: if birds are indeed reptiles, what are their closest relatives? Strangely enough, birds are most closely related to crocodiles. But while crocodiles have remained mostly unchanged for tens of millions of years, birds have changed and adapted – which is why we see this stunning bird diversity today.