The discovery of two new species of alvarezsaurians offers clues to the evolution of bird-like dinosaurs

Xiyunykus pengi fossil (Image courtesy James Clark)
Xiyunykus pengi fossil (Image courtesy James Clark)

An article published in the journal “Current Biology” describes the discovery of two species of dinosaurs belonging to the Alvarezsauria group, which shares many characteristics with modern birds, in north-western China. A team of researchers studied the fossils of the species that were named Xiyunykus pengi and Bannykus wulatensis recognizing some characteristics intermediate between the oldest species of that group and others that lived several million years later, showing for example how the morphology of their arms evolved slowly.

In the last decades in China and Mongolia many dinosaurs have been discovered with characteristics similar to those of modern birds and this led to a series of changes in the classification of these groups of animals that is not yet finished. Today among the theropod dinosaurs there’s the large group of miniraptors (Miniraptora) which includes birds and various groups of dinosaurs including the alvarezsaurians (Alvarezsauria), a group created following the discoveries begin with that of Mononykus olecranus, a small dinosaur described at beginning of the 1990s.

In subsequent decades several dinosaurs were discovered with characteristics more or less similar to those of modern birds and this made it necessary to create several new branches at the taxonomic level with families, superfamilies and clades still to be defined with precision to accommodate new species that lived at different times.

Reconstructing the relationships is one of the most complex problems in the field of paleontology and for example there’s the genus Haplocheirus, represented only by the species Haplocheirus sollers, which lived at the end of the Jurassic period, which seems different from all other species of alvarezsaurians, so much that some paleontologists have doubts about its inclusion int this group.

One of the problems in the reconstruction of the history of alvarezsaurians and therefore also of their relationships is that there are gaps in fossil records. Among the consequences, for example, it’s difficult to understand the phases of the evolution of these dinosaurs’ arms between the end of the Jurassic period and the Late Cretaceous, about 70 million years later. The discovery of two new alvarezsaurians that lived in the Early Cretaceous offers some answers.

The partial skeleton of Xiyunykus pengi found indicates that this animal had a mass estimated around 15 kg and probably had not yet reached its adult stage, having an age at the time of death estimated around 9 years.

The partial skeleton of Bannykus wulatensis found was larger, with a mass estimated around 24 kg, and its species was more massive because its age at the time of death was estimated at around 8 years and it probably had not yet reached the adult stage as well.

The fossils were discovered during an expedition led by James Clark and Xing Xu, who subsequently participated in their study. The bottom image (Courtesy George Washington University. All rights reserved) shows an artistic reconstruction of the alvarezsaurians Haplocheirus, Xiyunykus, Bannykus, and Shuvuuia, which illustrates the changes in their jaws and the reduction of their teeth and in their arms and in particular in their claws.

Slowly, the new discoveries are allowing paleontologists to reconstruct the evolution of alvarezsaurians and in general of miniraptors with increasing precision. This is also allowing to reconstruct the transition from dinosaurs to birds by understanding which anatomical characteristics already existed in dinosaurs and if some of them evolved parallel in different groups.

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