Rare intermediate fossils give researchers insight into evolution of bird-like dinosaur

August 23, 2018, George Washington University
Xiyunykus bones in the lab before their removal from the rock. Credit: James Clark

An international team of researchers discovered a new species of dinosaur, Xiyunykus pengi, during an expedition to Xinjiang, China. The discovery is the latest stemming from a partnership between the George Washington University and the Chinese Academy of Sciences. The findings were published today in Current Biology along with the description of a second new intermediate species, Bannykus wulatensis.

Xiyunykus and Bannykus are both alvarezsaurs, an enigmatic group of 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," James Clark, the Ronald Weintraub Professor of Biology at the GW Columbian College of Arts and Sciences, said.

However, 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 the new specimens allowed the 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," Jonah Choiniere, an associate professor at Wits University and member of the research team, said.

Bannykus resoration. Credit: SHI Aijuan

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. Xiyunykus pengi is the ninth of dinosaur identified by the partnership between GW and the academy.

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

Xiyunykus skeleton linedrawing. Credit: SHI Aijuan
Alvarezsaurs restoration. Credit: Vikto Radermacher

Explore further: Dinosaur discovery helps solve piece of evolutionary puzzle (w/ Video)

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3.5 / 5 (4) Aug 23, 2018
Probably went back from eating other larger animals to the very plentiful insect species of the era. Heavy meat eating teeth are not needed, but smaller teeth for smaller prey, and an arm with a claw that may have eased digging in termite or ant nests. Perhaps even for removing flakes of bark to get at the grubs and beetles underneath.

It is possible that the prey they had previously dined on, with the larger teeth, either evolved better defenses or died out, in either case the critter would have to find different food source similar to what it had (meat vs plant matter) and going Back to insects, which had likely been an early food for animals to begin with, would have been the natural answer as bugs and insects seem to be everywhere.

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