Paleontologists in China have found fossil fragments from two new dinosaur species — named Xiyunykus pengi and Bannykus wulatensis — that walked the Earth approximately 120 million years ago (Cretaceous period).

Xiyunykus pengi and Bannykus wulatensis are both alvarezsaurs, an enigmatic 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.

“Alvarezsaurs are weird animals. With their strong, clawed hands and weak jaws, they appear to be the dinosaurian analogue to today’s aardvarks and anteaters,” said Professor Jonah Choiniere, from the University of the Witwatersrand.

Xiyunykus pengi was discovered in 2005 in Xinjiang, north-western China.

Bannykus wulatensis was discovered a few years later, in 2009, in Inner Mongolia, north-central China.

“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 Professor James Clark, a paleontologist at the George Washington University.

“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.”

“This transition plays out in an incremental fashion over more than 50 million years. It could one day potentially serve as a classic example of macroevolution akin to the ‘horse series’ of North America,” said Dr. Xing Xu, a researcher at the Institute of Vertebrate Palaeontology and Palaeoanthropology, Chinese Academy of Sciences.

The discovery of Xiyunykus pengi and Bannykus wulatensis allowed the team 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,” Professor Choiniere said.

“But fossil species with transitional features, like Xiyunykus pengi and Bannykus wulatensis, are tremendously helpful because they link bizarre anatomical features to more typical ones.”

“The new fossils have long arms, and so show that alvarezsaurs evolved short arms only later in their evolutionary history, in species with small body sizes,” said Oxford University’s Professor Roger Benson.

“This is quite different to what happens in the classic example of tyrannosaurs, which have short arms and giant size.”

The findings appear in the journal Current Biology.

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Xing Xu et al. Two Early Cretaceous Fossils Document Transitional Stages in Alvarezsaurian Dinosaur Evolution. Current Biology, published online August 23, 2018; doi: 10.1016/j.cub.2018.07.057