Read more: Living dinosaurs: How birds took over the world
Only now can we say beyond reasonable doubt that birds don’t just have anatomy like dinosaurs – they are dinosaurs
Almost as soon as archaeopteryx was discovered, scientists noticed its striking anatomical similarity with dinosaurs. Yet it has taken until now to establish beyond reasonable doubt that this is because birds are dinosaurs.
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Darwin’s friend Thomas Huxley first championed the idea of a bird-dinosaur relationship back in the 1860s. However, the idea fell out of favour in the early 20th century when Gerhard Heilmann, a Danish artist and scientist, published a hugely influential book, The Origin of Birds, arguing that birds evolved directly from a primitive archosaur, a reptilian group which also gave rise to dinosaurs, pterosaurs and crocodiles.
Heilmann’s key point was that theropods – the suborder of dinosaurs containing the species that archaeopteryx most resembled (see chart) – did not have the collarbones that are fused in birds to form the wishbone, or furcula. This led him to conclude that birds could not have evolved from theropods as it would have meant losing their collarbones and then re-evolving them. This view dominated for most of the 20th century.
Opinion began to swing back towards the dinosaurs in the 1970s, when John Ostrom of Yale University noted that a small predatory theropod named deinonychus had particularly striking anatomical similarities to archaeopteryx. Later work confirmed that birds share dozens of anatomical features with theropods, and are especially similar to two families, the dromaeosaurs (including deinonychus) and troodontids.
Around the same time it started to become clear that some theropods did indeed have wishbones (to be fair to Heilmann, they rarely fossilise). Over the past 30 years the wishbone argument has been demolished as more and more theropod remains have been discovered. “The furcula was present in the vast majority of theropod groups and originated with theropods,” says Alan Turner of Stony Brook University in New York, who recently co-authored a review of current furcula knowledge (). “The only living animals that have a furcula are birds.”
The discovery of feathered dinosaurs in China in the mid-1990s tilted the argument even further in favour of the dinosaur-bird link. There are now around 25 known species of feathered theropod, some with feathers so well developed they would not look out of place on a modern bird. There are also hints that dinosaurs other than theropods had feathers ().
“The feathers of some theropod dinosaurs would not look out of place on a modern bird”
Feathered dinosaurs didn’t quite clinch the argument, however, as they only dated back 130 million years at most. Archaeopteryx, with its modern-looking feathers and wings, is 15 million years older. How could birds be descended from bird-like dinosaurs when none were known from older rocks?
This niggling problem was resolved last year with the discovery of Anchiornis, a feathered troodontid that is at least 6 million years older than archaeopteryx. Although it was not a direct ancestor of birds, it demonstrates that bird-like dinosaurs pre-dated the oldest known bird ().
Other lingering doubts have also been laid to rest in the past year or so, most notably a technical argument over the details of forelimb anatomy. The overwhelming majority of scientists are now convinced that birds are theropod dinosaurs which share a common ancestor with dromaeosaurs and troodontids, although a few remain unconvinced.
