Are Birds Dinosaurs? What Fossils and Science Show

What does it mean to say birds are dinosaurs?

In modern biology, animals are grouped by shared ancestry. A group such as “dinosaurs” includes a common ancestor and all of its descendants. Because birds descend from theropod dinosaurs, they belong inside the dinosaur family tree, just as bats belong among the mammals.

That is why scientists now often say “non-avian dinosaurs” for the extinct dinosaurs such as Triceratops and Stegosaurus, and “avian dinosaurs” for birds. With roughly 11,000 living species, birds are by far the most successful group of dinosaurs alive today.

Birds did not evolve from Tyrannosaurus rex, though. They arose from a branch of small, lightly built theropods called maniraptorans. Their closest extinct relatives are the dromaeosaurs, such as Velociraptor, and the troodontids.

Archaeopteryx: the first clue

The first strong evidence came from Germany. In 1861, quarry workers in the Solnhofen limestone of Bavaria found a fossil of Archaeopteryx, an animal about the size of a magpie that lived about 150 million years ago.

Archaeopteryx had feathered wings like a bird, but it also had teeth, clawed fingers and a long bony tail like a small dinosaur. Only two years after Darwin published On the Origin of Species, it looked like a perfect link between reptiles and birds.

In the 1860s and 1870s, the English biologist Thomas Henry Huxley pointed out how closely the skeleton of small theropods such as Compsognathus matched that of birds. The idea then fell out of favor for decades, until the American paleontologist John Ostrom described the agile, bird-like predator Deinonychus in 1969 and revived it.

Feathered dinosaurs from China

The debate was settled in the 1990s. In 1996, scientists described Sinosauropteryx from Liaoning Province in northeastern China, the first non-bird dinosaur found with a coat of simple, hair-like feathers.

Since then, thousands of exceptionally preserved fossils have been found in China and elsewhere. They show a full range of feathers, from fuzzy filaments to complex flight feathers:

• Microraptor, a small dromaeosaur, had long flight feathers on both its arms and legs, giving it four wing-like limbs.
• Anchiornis, about 160 million years old, was covered in feathers, and pigment cells preserved in its fossils show it was patterned in black, white and reddish brown.
• Yutyrannus, a relative of T. rex about 9 m (30 ft) long, had long, filament-like feathers.
• Velociraptor itself had quill knobs on its forearm bones, the bumps where large wing feathers attach in living birds.

Feathers therefore evolved in dinosaurs long before flight. They probably first helped with warmth and display, and only later were used for gliding and flying.

The evidence in bones, eggs and nests

Feathers are not the only link. Birds and theropods share dozens of skeletal features:

Behavior fits too. Fossils of the oviraptorosaur Citipati from the Gobi Desert of Mongolia were found crouched on nests of eggs with their arms spread over them, brooding just like a bird. The small troodontid Mei long was preserved asleep with its head tucked under its arm, a typical bird sleeping posture.

Some theropod eggs also show bird-like features, including pigmented shells and a similar shell structure. One difference is teeth: theropods had them, and so did the earliest birds, but living birds have lost them, as explained in do birds have teeth?.

Why some birds look prehistoric

Every bird is a dinosaur, but a few look the part more than others. The southern cassowary of New Guinea and Australia has scaly legs, three-toed feet with a long inner claw, and a tall crest on its head, rather like the crests of some feathered oviraptorosaurs.

The shoebill of African swamps stands motionless with a massive bill and a fixed stare, and online it is often called a “dinosaur bird”. The ostrich runs on long, powerful legs in a way that recalls fast-running theropods.

These birds are not more closely related to dinosaurs than a sparrow is. All living birds are equally dinosaurs. Their “prehistoric” look comes from features such as large size, scaly feet and big claws that remind people of fossil reconstructions.

Why did birds survive?

About 66 million years ago, an asteroid struck what is now the Yucatán Peninsula in Mexico. The impact and its effects, including darkness, cold and the collapse of food chains, wiped out about three-quarters of all species, including every non-bird dinosaur.

Some birds survived. Scientists think the survivors were small, ground-dwelling birds that could eat seeds and did not depend on forests, which were destroyed over large areas. After the extinction, birds diversified rapidly into the groups we know today.

Myths and facts

Myth: Birds are only related to dinosaurs, not dinosaurs themselves. In modern classification, birds sit inside the dinosaur family tree, so they are dinosaurs in the same way that humans are mammals.

Myth: Pterosaurs and marine reptiles were dinosaurs. Flying pterosaurs and swimming plesiosaurs were reptiles that lived at the same time, but they were not dinosaurs, and birds did not evolve from them.

Fact: Crocodiles are the closest living relatives of birds outside the dinosaurs. Birds and crocodilians are the two surviving branches of the archosaurs, the reptile group that also included the pterosaurs.