Rare fossils reveal evolution of bird-like dinosaur

During an expedition to Xinjiang, China, an international team of researchers discovered an entirely new species of dinosaur, along with a second new intermediate species. Xiyunykus pengi and Bannykus wulatensis are alvarezsaurs, which is an enigmatic group of dinosaurs that share many characteristics of birds. They have slender bodies with a bird-like skull that contains many small teeth – a contrast to the usual large, sharp cutting teeth of their meat-eating relatives.

“When we described the first well-known alvarezsaur, Mononykus, in 1993, we were amazed at the contrast between its mole-like arms and its roadrunner-like body, but there were few fossils connecting it back to other theropod groups,” says James Clark, a biology professor at George Washington University.

Early members of the alvarezsaurs group had relatively long arms with strong-clawed hands and larger, meat-eating teeth. But over time, the group evolved into dinosaurs that had mole-like arms with a single claw. This new discovery of previously undescribed specimens gives researchers a look into the intermediate period when this shift in specialized features occurred.

“It can be hard to pin down the relationships of highly specialized animals. But fossil species with transitional features, like Xiyunykus and Bannykus, are tremendously helpful because they link bizarre anatomical features to more typical ones,” explains Jonah Choiniere, an associate professor at Wits University and member of the research team.

These fossils were found during an expedition co-led by Dr. Clark and Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences. The partnership between GW and the academy has now led to nine species of dinosaurs being discovered. “Our international field teams have been tremendously productive over the years,” says Dr. Xu. “This research showcases just some of our incredible discoveries.”

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By Connor Ertz, Earth.com Staff Writer

Image Credit: James Clark