Rare Dinosaur Fossils Uncovered In China Are The ‘Missing Link’ In The Mystery Of The Alvarezsaurs

Zina Deretsky/National Science FoundationWikimedia Commons/Cropped and Resized

The fascinating alvarezsaurs are one of the most enigmatic groups of theropod dinosaurs to emerge in the Mesozoic Era. These bird-like creatures started off as meat-eating dinosaurs in the Jurassic Period and took a peculiar turn in their evolution, eventually ending up as insect eaters in the Cretaceous Period.

Small of stature, alvarezsaurs had long legs but extremely short and robust forelimbs, which seem to have played a key role in their remarkable transformation, reveals a new study published today in the journal Current Biology.

In fact, these creatures evolved having somewhat longer arms adorned with three claws, as seen in the earlier alvarezsaurs of the Jurassic, to dinosaurs with smaller arms and only one functional claw — the alvarezsaurs version that lived on through the Cretaceous.

The conclusion comes from two newly discovered alvarezsaurs fossils, belonging to two new species that shed new light into the evolution of this bizarre group of theropods — a dinosaur family that also includes Tyrannosaurus rex, the Inquisitr previously reported.

Discovered a decade ago in China during two separate expeditions co-led by researchers from the Chinese Academy of Sciences (CAS), George Washington University in Washington, D.C., and the University of the Witwatersrand in Johannesburg, South Africa, these rare fossils depict an intermediary stage in the group’s evolution, reports the U.S. university.

The first fossil, unearthed in 2005 in the Junggar Basin of Xinjiang, northwest China, has been dubbed Xiyunykus pengi. Unlike the big toothed alvarezsaurs of the Jurassic, Xiyunykus had a slender body with a bird-like skull and small teeth, perfect for chomping down on insects.

The same kind of features can be seen in the second fossil as well. Named Bannykus wulatensis, the species was uncovered in 2009 in a region of north-central China known as Inner Mongolia.

As study co-author Jonah Choiniere puts it, “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 Choiniere, who is a professor at Wits University’s Evolutionary Studies Institute.

The two new alvarezsaurs species show adaptations that would have helped these dinosaurs feed on insects living in colonies, notes Wits University. Their large claw is believed to have been used at slashing open anthills and rotting logs to expose the tasty insects within.

According to the Chinese media outlet Xinhua, the newfound species represent the “missing link” in alvarezsaurs evolution and help unravel the mystery of how these creatures adapted to a new diet and lost their fingers in the process.

In fact, Xiyunykus and Bannykus stand at the threshold between the very first alvarezsaurs and the very last of their kind and fill in a gap spanning 90 million years, from the Late Jurassic to the Late Cretaceous, that occurred in the evolution of these theropods.

“The two new dinosaurs are from the midpoint of the 90-million-year gap between the earliest and latest known alvarezsaurs,” said study lead author Xing Xu, a researcher at the CAS’ Institute of Vertebrate Paleontology and Paleoanthropology.

“They help complete the evolutionary chain, from the earliest alvarezsaurs with relatively long arms and three claws, to the Bannykus and Xiyunykus with long arms and one powerful finger and two slimmer and shorter ones, and finally to the most recent alvarezsaurs with short arms and only one functional finger and two other very tiny ones,” Xu explained.