People often say that birds are related to dinosaurs, but that’s really not true – birds aren’t related to dinosaurs… they are dinosaurs!
About 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.
Dinosaurs were a diverse group of reptiles that first emerged during the Triassic period, 231.4 million years ago. They were the dominant life forms on land for 135 million years, until a 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 generally split into two major groups – Saurischia and Ornithischia.
The Saurischia include all the carnivorous dinosaurs (theropods) and the giant, herbivorous dinosaurs (sauropods). The theropods are the more “popular” carnivorous dinosaurs, such as the T-Rex or the velociraptor, while the sauropods are the largest animals to ever walk on land;
The Ornithischia are an order of beaked, herbivorous dinosaurs. The more notable dinosaurs of the group 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, which makes a bit more acceptable. Their bones were also very light, they laid eggs, and despite their overall look, 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, the so-called missing link between reptiles and birds.
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, which only rarely become conserved in fossils. 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?
Well… yes, in a way, birds are reptiles in a sense, but it’s 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 definitely 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 kind of are reptiles. It’s counterintuitive, but the question is only particularly useful for classification purposes, and 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.
Andrei's background is in geophysics, and he's been fascinated by it ever since he was a child. Feeling that there is a gap between scientists and the general audience, he started ZME Science -- and the results are what you see today.