Two feathered skeletons unearthed in Early Cretaceous sediments of China belong to the oldest relatives of today’s birds ever discovered. Archaeornithura meemannae enjoyed an aerodynamic lifestyle and also waded around in lakes. The findings, published in Nature Communications this week, push back the evolutionary record of modern birds by six million years.
Ornithuromorpha is the name of the evolutionary branch that gave rise to every bird species alive nowadays. And back in the Mesozoic Era, between 252 and 66 million years ago, ornithuromorphs represented half of all the birds around. Another branch called Enantiornithes, on the other hand, left behind no living descendants: These Mesozoic birds had teeth and clawed wings, but they died out at the Cretaceous-Paleogene boundary along with pterosaurs, plesiosaurs, and all non-avian dinosaurs.
A team led by Min Wang and Zhonghe Zhou from the Chinese Academy of Sciences examined the two partial skeletons, complete with feathers, unearthed from the Sichakou basin of Hebei Province in northeastern China. Based on the geological layers where these fossils were extracted from, the newly discovered species lived about 130.7 million years ago in the Early Cretaceous. That means it predates the previous earliest record holder for this branch: a 125-million-year-old specimen from the Lower Cretaceous Yixian Formation.
These fossils have more in common with a modern bird than any found from such an early stage in their evolution, Wang says. In addition to the exceptionally preserved plumage, they had other anatomical features that indicate maneuverability during flight, such as the small projection on the front edge of their wings. Their fan-shaped tail feathers, highly fused bones at the ends of wings, and U-shaped wishbone resemble our birds today, Science explains.
Furthermore, the hummingbird-sized species may have also waded -- they had no feathers on their upper leg. The siltstone they were embedded in suggests that the area may have once been a lake. Coming out of the safety of trees to take advantage of the open, semi-aquatic spaces meant they had more high protein food choices, Zhou says, and having more predation pressure likely favored the evolution of adept fliers. These fossils, according to the team, reveal the origins of features that likely helped modern birds survive the mass extinction event tens of millions of years later.
"The new bird is quite derived and has many advanced features of modern birds, and thus is far away from the transitional history of dinosaurs-birds," Wang tells The Washington Post. And while the species represents the earliest ornithuromorph bird on record, "the most primitive bird of Ornithuromorpha is most likely from older deposits than what we discovered now."
The Greek “archae” and “ornithura” mean “ancient ornithuromorph,” and the species name honors paleontologist Meemann Chang.
Images: Zongda Zhang (top), Wang et al., Nature Communications (bottom)