The evolution of Archaeopteryx is stranger than anyone imagined
BY MICHAEL MARSHALL at NewScientist.com
ARCHAEOPTERYX has been an icon of palaeontology for over 150 years. Its discovery, a few years after the publication of On the Origin of Species, couldn’t have been better timed. Charles Darwin’s theory predicted that the fossil record should be full of transitional forms, as one species gradually evolved into another. Yet these missing links were, well, missing. Then, the strange birdlike dinosaur was unearthed – and instantly became a poster child for evolution.
After all this time, you might think there is little left to discover about the “first bird”. In fact, much of its story has yet to be told. Only this February it emerged that the original fossil – a feather – was not what it seemed. And in recent years, we have found other contenders for the title of first bird. Nevertheless, new insights into the origins and lifestyle of Archaeopteryx reveal it to be a real trailblazer, making an epic journey over sea before settling on remote islands – a trip that shaped its evolution in a way that certainly would have intrigued Darwin.
The first Archaeopteryx skeleton was found in Germany in 1861, close to – and shortly after – the feather. It was about the size of a crow, and headless. Only with the discovery of a second skeleton, a decade later, did it become clear that instead of a birdlike beak, Archaeopteryx had a snout filled with teeth. Eleven specimens have been found in total, although one vanished mysteriously in 1991 after the death of its owner. Those that remain reveal an animal that lived about 150 million years ago in what is now western Europe.
Other birdlike dinosaurs lived in China, according to fossils unearthed since the 1990s. Like Archaeopteryx and modern birds, they are theropods, a group of dinosaurs with hollow bones and three-toed limbs that include tyrannosaurs and velociraptors. The oldest of the Chinese fossils date from 160 million years ago, and reveal an animal that had feathers but probably couldn’t fly. The first creatures that closely resemble modern birds – theropods capable of flight and with beaks – date from around 125 million years ago.
So Archaeopteryx was just one of several early birdlike dinosaurs. But details of its story are still emerging, and they shed new light on its evolution. To understand how, we must travel back 175 million years to when the supercontinent Pangaea began to break apart. An ocean called the Tethys pushed in from the east, creating Laurasia in the north – a combination of what are now North America, Europe and Asia – and Gondwana in the south. Much of Europe was flooded, forming an inland sea with a couple of Madagascar-sized islands in the east and smaller ones further west. These remote western islands, known as the Solnhofen archipelago, are where Archaeopteryx lived.
The islands were tropical, lying 500 kilometres from the equator, and surrounded by coral reefs. Artists’ impressions often show Archaeopteryx flapping from tree to tree, but that’s probably wrong, says Oliver Rauhut of the Bavarian State Collections of Palaeontology and Geology in Germany: “There’s a complete lack of any evidence for large trees.” Instead, small shrubs appear to have dominated. Also, the very idea that Archaeopteryx could fly has been controversial. It may have been birdlike with feathers, but it lacked key markers of flight, such as a solid breastbone to anchor its wings.
Determined to resolve the debate, in 2011, Ryan Carney at the University of South Florida carried out a microscopic examination of an Archaeopteryx feather. This revealed pigmented structures called melanosomes indicating that the plumage was matt black, which would have aided flight. “In modern birds, black melanin pigment substantially increases the strength and durability of feathers,” says Carney. Unfortunately, his finding didn’t settle the matter: the feather he scrutinised was the original Archaeopteryx fossil, which according to a new analysis probably belonged to another dinosaur. Evidence Carney has gathered since, however, makes a strong case. Unlike other studies, which tend to focus on a single anatomical feature, he looked at the bigger picture, such as how the muscles and joints moved the entire forelimb. Based on this analysis, in 2016, Carney concluded that Archaeopteryxcould fly.
This fits with what Rauhut and his colleagues have reported as well. Examining the eleventh specimen, they found that its body was covered in pennaceous feathers, the kind that in modern birds have evolved for flight. Their shape indicated that this process had already begun in Archaeopteryx. But it probably wasn’t a very adept flier. Last year, another group compared the internal structure of Archaeopteryx bones with those of modern birds and concluded that it was able to fly, but only for short bursts, like a pheasant. “It’s not a great style of flight,” says Rauhut.
Not glamorous, but it was enough to make an epic journey. In a 2017 paper, Rauhaut argued that birdlike dinosaurs evolved on the Asian part of Laurasia then migrated west towards what is now Europe. Archaeopteryx alone appears to have crossed the inland sea to reach the Solnhofen archipelago.
Flight may have helped Archaeopteryx evade predators but it is unlikely to have been skilled enough to catch its own prey on the wing. We know it hunted by day, however, because it lacks the large eye sockets found in nocturnal animals. But what did it eat? The shape of its teeth suggests insects. Rauhut notes that each Archaeopteryx specimen has distinctive teeth, hinting that they were adapted for different diets depending on what was available on the island where it lived. For instance, the eighth has large, cone-shaped back teeth, suitable for cracking hard foods. This hints it ate insects with tough shells, perhaps beetles, he says.
It isn’t just their teeth that differ. The bones in each Archaeopteryx skeleton also vary in size and proportions, raising questions about whether they really are all from the same species, as some research has concluded. Other work has grouped them into at least two species: Archaeopteryx lithographica and Archaeopteryx siemensii. And a paper on the eighth specimen published last year suggested it belonged to a new species.
All of this controversy over classification has an intriguing implication. It suggests a parallel between Archaeopteryx and the Galapagos finches studied by Darwin. These birds also live on remote islands and are adapted to the different opportunities offered by their particular island habitat. Thought to number 17 species, they are a classic example of adaptive radiation, where a multitude of species evolves rapidly from a single one. Archaeopteryx may have done the same thing on the Solnhofen archipelago. Perhaps they are the Darwin’s finches of the Jurassic.