As little as 15 years ago, the boundary between birds and dinosaurs was a fairly sharp one. On one side was Archaeopteryx, a 150 million-year-old magpie-sized creature from Bavaria, southern Germany, long considered the world’s first bird.
On the other was Deinonychus, an emu-sized meat-eating dinosaur or theropod, and a member of the group known as Dromaeosauridae, made famous as the “raptor” in Stephen Spielberg’s Jurassic Park movies.
Numerous similarities between the skeletons of Deinonychus and Archaeopteryx – first recognised by palaeontologist John Ostrom in the late 1960s – provided important evidence that birds had evolved from small two-legged theropod dinosaurs, an idea first championed by Thomas Huxley in 1859.
The feathered Archaeopteryx, unlike its modern-day counterparts, had a long bony tail, a toothed beak and three unfused and clawed fingers.
Deinonychus, on the other hand, was much larger than Archaeopteryx, was not known to have been feathered, and clearly couldn’t fly.
So although these two animals provided a glimpse across what had previously been an insurmountable evolutionary gap, the bridge between dinosaurs and birds was still very flimsy.
Indeed, many palaeontolgists argued that the differences between Archaeopteryx and Deinonychus were too great to link birds to dinosaurs, and any similarities were likely to be due to convergent evolution.
Then, starting in 1996, a flurry of new discoveries changed the game forever.
Feathered dinosaurs, many of them dromaeosaurids like Deinonychus, others only distantly related, began to emerge from the 125 million-year-old rocks of Liaoning Province, north-eastern China.
Some were covered in hair-like downy tuffs, others sported feathered tails, arms and even legs. All of a sudden, the boundary between birds and theropod dinosaurs became very fuzzy.
Many features long regarded as the hallmark of birds – feathers, a wish-bone, long forelimbs and the capacity to fly – had clearly evolved within the theropods.
How to define a bird in a way that distinguished the group from dinosaurs became very tricky. But the consensus among palaeontologists was that Archaeopteryx should still be considered the first “true” bird.
Its evolutionary position still defined the point at which dinosaurs transitioned into birds … until today.
Now we have Xiaotingia, a small, chicken-sized, feathered theropod, also from Liaoning, but from rocks about 155 million years old.
Although its fossils are not as spectacular as those of Archaeopteryx, the skeleton of Xiaotingia (see image above) shows that several features, including long, robust forearms, that were thought to be distinctive of avialans (a group containing the ancestors of modern birds), may actually be characteristic of the more inclusive group of bird-like dinosaurs and birds – Paraves.
Most significantly, however, detailed analysis of the evolutionary position of Xiaotingia within Paraves indicates that both it and Archaeopteryx are not avialans.
In other words, neither are “true” birds. Instead, both animals fall within their own group, Archaeopteryidae, within Deinonychosauria, alongside dromaeosaurids (such as Deinonychus) and troodontids.
These animals have much shorter, higher skulls, blunt teeth and were possibly herbivorous. While this arrangement is still only tentative, it indicates that the evolutionary transition from birds to dinosaurs will need to be carefully re-evaluated … again.
Should we be surprised by all this? Probably not.
As more and more bird-like dinosaurs and dinosaur-like birds are discovered, it is becoming increasingly apparent that the transition from dinosaurs to birds was a gradual transformation over many million of years, and one that involved many different types of animals and a large portion of the dinosaurian family tree.
Out of this feathery tangle, the ancestors of today’s birds eventually emerged.
Archaeopteryx will always be an iconic fossil in the history of evolutionary biology, but for now its title as the world’s first bird seems to have been lost.
Who knows what the next chapter of this exciting story will bring. I for one will be looking forward to it.