Xiyunykus and Bannykus: Two Newly Discovered Chinese Dinosaurs Are Evolutionary 'Missing Link'

This illustration shows where two newly discovered dinosaur species fit in the lineage of alvarezsaurs (from left): Haplocheirus, Xiyunykus, Bannykus and Shuvuuia. The species show lengthening of the jaws, reduction of the teeth and changes in the hand and arm over time. Viktor Radermacher

Paleontologists have reported the discovery of two new dinosaurs in northwest China, which could prove to be the evolutionary "missing links" in an unusual lineage of predators that roamed the Earth between 160 and 90 million years ago.

According to a study published in the journal Current Biology, the two species—which have been named Xiyunykus and Bannykus—belong to the diverse subgroup Theropoda, which includes all of the flesh-eating dinosaurs. Theropods all walked on their hind legs, although they ranged in size from the 2-foot long Microraptor to the 40-foot Tyrannosaurus rex.

One lineage of small theropods eventually evolved into modern birds (meaning birds are themselves part of the subgroup), while another, known as the alvarezsauroids, developed into strange-looking insect-eaters with short arms and hands characterized by an enlarged finger which may have been used to dig into nests.

To date, researchers have been unable to explain how the anatomy and behavior of alvarezsauroids changed over time because there is a large evolutionary gap of 70 million years between the later insect-eaters and what is thought to be the earliest known member, Haplocheirus, which lived around 160 million years ago (although some experts have questioned whether it truly belongs to the group).

Xiyunykus and Bannykus, however, are the first alvarezsauroids to be found that lived during the evolutionary gap, according to the researchers. Their fossils are thought to be 130-120 million years old (Mya) and 120-110 Mya respectively.

"Before the discovery of Bannykus and Xiyunykus, the only known early alvarezsaurian was Haplocheirus," Corwin Sullivan, a paleontologist from the University of Alberta in Canada and author of the study, told Newsweek. "All other known alvarezsaurians were Late Cretaceous in age, younger than about 100 million years. Haplocheirus looks in many ways like a fairly typical small theropod dinosaur—a long-armed predator with grasping hands."

"The Late Cretaceous alvarezsaurians, by contrast, have small simplified teeth, short but robust arms with enlarged thumbs, and hind legs that seem specialized for running," he said. "They may have foraged over long distances, searching for insect nests that they could break into with their large thumb claws as a prelude to devouring the insects with their small teeth."

According to Sullivan, Xiyunykus and Bannykus are important because they fall squarely into the evolutionary gap and thus can be considered "missing links," "intermediates" or "transitional forms" between Haplocheirus and the Late Cretaceous alvarezsaurians.

They newly discovered species confirm that Haplocheirus is indeed a primitive alvarezsaurian and provide strong evidence that northern Asia is where the group originated and much of its susbequent evolution took place. Significantly, the fossils provide important insights into various anatomical and inferred behavioral changes.

"Bannykus resembles the Late Cretaceous alvarezsaurians in some features of the skull and in having somewhat shortened arms with enlarged thumbs," Sullivan said. "It may have already been engaging in nest-digging behavior, or perhaps in digging for another purpose. However, the pelvis and hind legs still look fairly primitive in both Xiyunykus and Bannykus, indicating that in the course of alvarezsaurian evolution the skull and forelimbs began to undergo significant modification earlier than the hindlimbs."

Based on the available evidence, the researchers estimate that Xiyunykus weighed around 15 kilograms (33 pounds) and measured slightly more than 27 inches at the hip. Bannykus was larger, weighing around 24 kilograms (53 lbs). Both had slender builds and would likely have been fast runners.

Sullivan notes that while the new findings are intriguing, to fully understand what happened during this group's evolution many more fossils will be needed—especially given that the set of bones representing Xiyunykus and Bannykus are incomplete.

Xiynykus was discovered in 2005 in the Junggar Basin, northwestern China, while Bannykus was discovered a few years later in 2009 in western Inner Mongolia, north-central China, according to Xing Xu, co-author of the study from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences.

This article has been updated to include additional comments from the researchers.