An international team of researchers has found two specimens of Alvarezsaurus in China that have never been seen before. These new fossilized theropods solve a mystery in the evolution of these dinosaurs.
Only Xiyunykus and Bannykus were needed to complete the Alvarezsaurus family portrait, according to the recent study published in the journal Current Biology.
These are the names that researchers gave to these two dinosaurs, which fill a 70-million-year evolutionary gap between their ancestors from the Jurassic era ( Alvarezsaurus Haplocheirus ) and their Cretaceous descendants.
“Until now, there was a lack of connection between Haplocheiruses and late Cretaceous Alvarezsaurus,” says Corwin Sullivan. A paleontologist at the University of Alberta, Mr. Sullivan spent 10 years in China and contributed to this discovery.
Morphologically, both fossils have long legs similar to those of Haplocheiruses. But they have short, sturdy arms ending in one sharp claw instead of three, a reduced skull and smaller teeth, like the newer Alvarezsaurus.
They present all the characteristics of a missing link between two stages of evolution of this dinosaur. “These animals tell us that the skull and fore-limbs of the Alvarezsaurus first changed,” says Sullivan.
Like other theropods, Xiyunykus and Bannykus were carnivorous and bipedal. They should not weigh more than 25 kg. However, everything suggests that these Alvarezsaurus did not feed on mammals, but on insects.
“It’s quite strange in terms of evolution … they do not look like typical theropods” like the Tyrannosaurus, Sullivan observes.
The paleontologist can return to study the two skeletons aged 120 million years in China, where they will be preserved.