Are birds really dinosaurs?

Scientists say dinosaurs are not all extinct, because the birds around us are dinosaurs too. Here is the evidence behind that conclusion.

Birds belong to the dinosaur family tree

Biologists group living things by evolutionary history: a group has to include a common ancestor and all of its descendants. Since the evidence shows that birds descend from theropods, the meat-eating dinosaurs, birds count as dinosaurs, in the same way that humans count as mammals.^[1]

That is why scientists often say "non-avian dinosaurs" for the dinosaurs that died out 66 million years ago. In terms of family history, Tyrannosaurus is a closer relative of a chicken than of Triceratops or Stegosaurus.^[1]

The orange line is the lineage that leads to birds; green dots mark where bird-like features appear. (Simplified, showing only a few dinosaurs; how the base of the tree splits is still debated.)

From Archaeopteryx to Deinonychus

In 1861 a fossil of Archaeopteryx was found in Germany. It had wings and feathers like a bird, but also teeth, clawed fingers and a long bony tail. A few years later Thomas Huxley pointed out how similar the skeletons of birds and small meat-eating dinosaurs are.^[1][3]

The idea then went quiet for decades, until in 1969 John Ostrom described Deinonychus, a fast, agile predator, and showed that it shared many features with birds. His work brought scientists back to the question in earnest.^[1]

Evidence from bones and behavior

The V-shaped fused collarbone, or wishbone (furcula), that we see in chickens is found in many theropods, including Tyrannosaurus and Velociraptor.^[5]

Theropods also had hollow bones with air spaces linked to air sacs, like the breathing system of birds, and many had a half-moon-shaped wrist bone (semilunate carpal) that let them fold their hands against the body, much as birds fold their wings.^[1]

Some fossils even record behavior. Citipati from Mongolia died sitting on its nest with its arms spread over the eggs, the same pose birds use to brood today.^[6]

Dinosaurs had feathers

In 1996 scientists described Sinosauropteryx from Liaoning, China, the first non-avian dinosaur found with traces of a coat of filament-like feathers. Many more feathered dinosaurs followed, such as Microraptor, with long flight-style feathers on both arms and legs, and Yutyrannus, a feathered relative of Tyrannosaurus about 9 meters long.^[2][4]

Even Velociraptor, whose feathers have not been found directly, has small bumps on its forearm bone (quill knobs) where feathers attach in modern birds. An accurate Velociraptor should have feathers, not the bare scaly skin seen in the movies.^[2]

There are more than ten thousand species of birds alive today. Dinosaurs did not vanish completely: they still fly and walk all around us.^[9]

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