Rare Intermediate Fossils Give Insight into Evolution of Bird-Like Dinosaur

A discovery made by an international team of researchers fills in a missing link in the evolutionary transition of the alvarezsaurs.

August 22, 2018

alvarezsaur species

An artist’s reconstruction of alvarezsaur species: Haplocheirus (left), Xiyunykus, Bannykus and Shuvuuia. Note the lengthening of the jaws, reduction of the teeth and changes in the hand and arm. (Reconstruction by Viktor Radermacher)

An international team of researchers discovered a new species of dinosaur, Xiyunykus pengi, during an expedition to Xinjiang, China. The discovery is the ninth species of dinosaur identified as part of a partnership between George Washington University and the Chinese Academy of Sciences.

The findings were published Aug. 23 in Current Biology along with the description of a second new intermediate species, Bannykus wulatensis. Xiyunykus and Bannykus are both alvarezsaurs, a mysterious group of dinosaurs that share many characteristics with birds. Their bodies are slender, with a bird-like skull and many small teeth instead of 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,” said James Clark, the Ronald Weintraub Professor of Biology at the Columbian College of Arts and Sciences.


An international team of researchers discovered a new species of dinosaur, Xiyunykus pengi, during an expedition to Xinjiang, China. Shown are the bones before they were removed from the rock. (Photo: James Clark)

Alvarezsaurs did not always look this way. Early members of the group had relatively long arms with strong-clawed hands and typical meat-eating teeth. Over time, the alvarezsaurs evolved into dinosaurs with mole-like arms and a single claw. The discovery of new specimens allowed researchers to uncover an important shift in how the specialized features of the alvarezsaurs evolved.

"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," said Jonah Choiniere, Ph.D. ’10, an associate professor at Wits University and member of the research team.

The fossils were discovered 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 fieldwork and research were supported by a U.S. National Science Foundation grant.

“Our international field teams have been tremendously productive over the years,” Dr. Xu said. “This research showcases just some of our incredible discoveries.”