The first birds

The story

A feather in the stone

In 1861, in a limestone quarry near Solnhofen, a small village in Bavaria, Germany, workers split a slab and found inside the fossil of an animal never seen before. It had a long bony tail, legs with claws, a mouth full of teeth, and all around the body and arms, pressed into the stone, feathers. It was something halfway between a reptile and a bird. The animal was named Archaeopteryx, from the Greek for “ancient wing” or “ancient feather”.

The timing was perfect. Only two years earlier, in 1859, Charles Darwin had published On the Origin of Species, arguing that living things descend from one another through long transformations. To his critics Darwin could not yet show the intermediate forms his theory predicted. Archaeopteryx looked just like one of those. The Bavarian doctor who had received the fossil, Karl Häberlein, sold it to the British Museum in London, where it was studied by the anatomist Richard Owen. Its most famous defender was another Englishman, Thomas Huxley, a great supporter of Darwin.

The limestone of wonders

The Solnhofen limestone is made of very fine mud, laid down 150 million years ago in calm lagoons short of oxygen, where bodies were neither eaten nor disturbed. That is why it has preserved fossils in an extraordinary way: jellyfish, insects, fish, pterosaurs, even the shading of feathers. This limestone has also been used for centuries in printing: lithography, a printing technique invented at the end of the 18th century, was done on slabs of this very material.

Today about fourteen skeletons of Archaeopteryx are known, all from the Solnhofen area. The finest is the Berlin specimen, found in 1874–1875 and bought by the Berlin museum in 1881, with feathers clearly visible. The animal was about the size of a crow or a magpie, around half a metre, and lived in an archipelago of tropical islands, with a vegetation of shrubs.

Bird or dinosaur?

For a long time Archaeopteryx was called “the first bird”. But the more fossils were discovered, the more it looked like a small feathered dinosaur. In the 1960s the American John Ostrom studied Deinonychus, a predatory dinosaur, and saw that its limbs were remarkably like those of Archaeopteryx. From there the idea was reborn that birds are dinosaurs. Then, from 1996, in China, in Liaoning province, fossils of dinosaurs with feathers or with fuzz-like filaments were found: Sinosauropteryx, four-winged Microraptor, and even Yutyrannus, a relative of Tyrannosaurus about 8 to 9 metres long, covered in feathers.

Today most palaeontologists think that birds are a branch of the theropod dinosaurs, those meat-eaters that walked on two legs. Archaeopteryx is an example of what was going on in that change. For this reason the title of “first bird” is debated: it depends on how a bird is defined, and on where a line is drawn in a gradual transformation. Some studies, such as one in 2011 on a Chinese fossil, Anchiornis, have even suggested that Archaeopteryx was a relative of Velociraptor and not the founder of the birds. The debate is still open.

Could it fly?

The wing feathers of Archaeopteryx are asymmetrical, with one side narrower than the other, like those of birds that fly today: it is a sign that the animal could make use of the air. It did not have, however, the big keel on the chest that in modern birds supports the flight muscles. A 2018 study of a fossil, done with X-rays, suggested that it could fly with short flaps of its wings, perhaps to escape predators or to get up into trees. Other researchers think it was more of a glider. Here too science has not settled the question.

One thing the fossils show is that feathers did not arise for flying: they were already present in dinosaurs that did not fly, probably for keeping warm or for showing off to their mates. Only later were they used for flight. It is an example of how evolution reuses features that already exist.