A new analysis of a pigeon-sized Archaeopteryx fossil in the collection of the Field Museum in Chicago is revealing an array of previously unknown features of the earliest-known bird, providing insight into its feathers, hands, feet and head.
The specimen, unearthed in southern Germany, is one of the most complete and best preserved of the 14 known fossils of Archaeopteryx identified since 1861. The discovery of the first Archaeopteryx fossil, with its blend of reptile-like and bird-like features, caused a sensation, lending support to British naturalist Charles Darwin's ideas about evolution and showing that birds had descended from dinosaurs.
The new study, examining the Chicago fossil using UV light to make out soft tissues and CT scans to discern minute details still embedded in the rock, shows that 164 years later there is more to learn about this celebrated creature that took flight 150 million years ago during the Jurassic Period.
The researchers identified anatomical traits indicating that while Archaeopteryx was capable of flight, it probably spent a lot of time on the ground and may have been able to climb trees.
The scientists identified for the first time in an Archaeopteryx fossil the presence of specialized feathers called tertials on both wings. These innermost flight feathers of the wing are attached to the elongated humerus bone in the upper arm. Birds evolved from small feathered dinosaurs, which lacked tertials. The discovery of them in Archaeopteryx, according to the researchers, suggests that tertials, present in many birds today, evolved specifically for flight.
Feathered dinosaurs lacking tertials would have had a gap between the feathered surface of their upper arms and the body.
"To generate lift, the aerodynamic surface must be continuous with the body. So in order for flight using feathered wings to evolve, dinosaurs had to fill this gap - as we see in Archaeopteryx," said Field Museum paleontologist Jingmai O'Connor, lead author of the study published on Wednesday in the journal Nature.
"Although we have studied Archaeopteryx for over 160 years, so much basic information is still controversial. Is it a bird? Could it fly? The presence of tertials supports the interpretation that the answer to both these questions is 'yes,'" O'Connor added.
The delicate specimen, preserved in three dimensions rather than squashed flat like many fossils, was painstakingly prepared to protect soft tissue remains, which glowed under ultraviolet light.
Birds are the only members of the dinosaur lineage to have survived a mass extinction 66 million years ago, caused by an asteroid striking Earth. Archaeopteryx boasted reptilian traits like teeth, a long and bony tail, and claws on its hands, alongside bird-like traits like wings formed by large, asymmetrical feathers.
The soft tissue of its toe pads appears to have been adapted for spending a lot of its life on the ground, consistent with the limited flight capabilities that Archaeopteryx is believed to have possessed.
"That's not to say it couldn't perch. It could do so still pretty well. But the point being that near the beginning of powered flight, Archaeopteryx was still spending most of its time on the ground," said study co-author Alex Clark, a doctoral student in evolutionary biology at the University of Chicago and the Field Museum.
The soft tissue on the hand suggests that the first and third fingers were mobile and could be used for climbing.
An examination of Archaeopteryx's palate - roof of the mouth - confirmed that its skull was immobile, unlike many living birds. But there was skeletal evidence of the first stages in the evolution of a trait that lets the beak move independently from the braincase, as seen in modern birds.
The fossil possesses the only complete Archaeopteryx vertebral column, including two tiny vertebrae at the tip of the tail showing it had 24 vertebrae, one more than previously thought.
The museum last year announced the acquisition of the fossil, which it said had been in the hands of a series of private collectors since being unearthed sometime before 1990.
"This specimen is arguably the best Archaeopteryx ever found and we're learning a ton of new things from it," O'Connor said.
"I consider Archaeopteryx to be the most important fossil species of all time. It is, after all, the icon of evolution, and evolution is the unifying concept of the biological sciences. Not only is Archaeopteryx the oldest-known fossil bird, with birds today being the most successful lineage of land vertebrates, it is the species that demonstrates that birds are living dinosaurs," O'Connor said.
