Birds are a reminder of a distant, strange time in Earth’s history when giant, terrible lizards roamed the land.
It seems unbelievable that tiny hummingbirds and the enormous Tyrannosaurus rex have anything in common, yet they share a branch on the family tree of life.
Are birds dinosaurs?
Birds are the last surviving dinosaurs, according to the phylogeny of vertebrates, but technically they are considered reptiles, or avian dinosaurs. Evidence strongly suggests that birds and dinosaurs shared a common therapod ancestor, during the Jurassic Period, about 150 million years ago.
The Missing Link Between Reptiles and Birds
For most of the 19th and 20th century biologists did not really understand how birds evolved.
In 1861 an astonishing discovery was made in Germany: a fossil that seemed to be the missing link between reptiles and birds – the Archaeopteryx.
This was the first time that scientists considered that birds could have evolved from reptiles. It took over 130 years for scientists to realize that birds are actually dinosaurs.
What is a Dinosaur?
It seems like a silly question because generally, people have quite a good idea of what dinosaurs are. When asked, most children can rattle off a list of their favorite dinosaur celebrities.
But from an evolutionary perspective, what is the definition of a dinosaur?
Consider the vertebrate phylogeny – the family tree that traces the lineages of different vertebrates all the way back to a shared, common ancestor.
Dinosauria is a clade, or group, which includes everything, from the modern pigeon to the mighty Tyrannosaurus rex.
Dinosaurs are an incredibly diverse group of organisms.
There are over 10,000 extant species of avian dinosaurs, or birds, and over 1,000 non-avian species have been described by paleontologists around the world.
From the fossil record, we know that they varied hugely in size, some were bipedal, while others walked on four feet, and they filled almost every terrestrial niche.
The Origin and Evolution of Birds
During the Triassic Period, 231.4 million years ago, a diverse group of reptiles emerged – the dinosaurs.
They dominated life on earth until around 65 million years ago, when a massive extinction event caused all groups of dinosaurs to go extinct, except for one – the birds.
Not only are birds related to dinosaurs, in an evolutionary sense birds are dinosaurs.
There are two major groups of dinosaurs – Ornithischia and Saurischia.
The Stegosaurus, Triceratops and Iguanodon belong to this group. These were beaked, herbivorous dinosaurs.
The name “Ornithischia” translates to “bird-hipped”, referring to the physical trait that these dinosaurs share with modern birds – a pelvis with a backward-pointing pubis.
Surprisingly, birds are more closely related to the other group of dinosaurs.
The Saurischia group, to which birds are most closely related, include the theropods – carnivorous dinosaurs – and the sauropods – large herbivorous dinosaurs.
The theropods ran on two legs hunting their prey, while the sauropods remained on four legs and grew to be the largest animals to ever walk on land.
Birds, or Aves, were derived from a group of theropods called Maniraptora. Dromaeosaurs, like Velociraptor, are also Maniraptorans and therefore closely related to birds.
They share characteristics such as light, hollow bones and reproduction by egg laying.
While extant birds share many characteristics with extinct dinosaurs, they are also entirely different.
Environmental changes and over 150 million years of evolution have led to modern birds looking, behaving, and functioning completely differently to their dinosaurian ancestors.
The discovery of dinosaur fossils that had feathers, wings and other features that are characteristic of birds was key to the finding that birds are in fact dinosaurs.
Just a year after Charles Darwin published his work On the Origin of Species by Means of Natural Selection in 1860, an intriguing fossil was discovered in Germany that seemed to prove just the point that Darwin was trying to make.
The exquisite specimen, named Archaeopteryx, had sharp claws at the ends of its legs, a long, bony tail, and other reptilian features, but more surprisingly, it had wings and feathers like a bird.
This transitional form between a bird and a reptile was enough to convince some of Darwin’s theory of evolution, but it did not persuade scientists that birds could have evolved from dinosaurs.
Until the 1960’s, dinosaurs were perceived as giant, sluggish lizards and it was assumed that birds had evolved from some kind of reptile.
It was not until the discovery of Deinonychusin in 1964 that the paradigm started to shift. It kicked off a period that paleontologists call the “dinosaur renaissance.”
Deinonychus was a relatively small, predatory theropod that lived around 115 million years ago. Most striking about this dinosaur was the fact that it had feathers.
The way paleontologists envisioned dinosaurs and their behavior was completely transformed.
The feathered Deinonychus appeared to be an intelligent pack-hunter that ran at great speeds – something a giant, cold-blooded lizard would not be able to do.
Dinosaurs started to be seen as more dynamic, exciting animals.
In the early 1990’s another feathered therapod dinosaur fossil was discovered in China – Sinosauropteryx.
This finding shed light on how feathers evolved, as Sinosauropteryx had long, filamentous structures, or protofeathers.
The discoveries of these fossils in Asia have bridged the gap in our understanding of how birds evolved from reptiles and have conclusively shown that not only are birds related to dinosaurs, but they are also dinosaurs.
Are Birds Dinosaurs or Reptiles?
Although birds are considered living dinosaurs, they are also in a sense reptile. Whether birds are or are not reptiles all depends on the biological classification system you use.
According to the Linnaean classification system, the term reptile refers to animals that are cold-blooded with scales, like snakes and lizards.
Birds are warm-blooded and have feathers, so in this system they certainly are not reptiles.
The phylogenetic classification system, which most modern biologists use, groups animals according to their ancestry, tracing their evolutionary lineages all the way back to a common ancestor.
Birds and dinosaurs do share a common therapod ancestor, so in this system they are reptiles.
Today, almost all paleontologists agree that birds are classified as coelurosaurian theropod dinosaurs.