'New dinosaurs fill evolutionary gap of 70 million years'

Two newly discovered dinosaurs may be missing links in an unusual lineage of predators that lived between 160 million and 90 million years ago, a study suggests.

The two species, Xiyunykus and Bannykus, were theropods -- a group of bipedal, largely carnivorous dinosaurs, said researchers from the University of Alberta in Canada.

Some theropods eventually gave rise to birds, while another branch, the alvarezsauroids, evolved into strange-looking insectivores with short arms and hands with an enlarged finger for digging into nests.

However, until now, little was understood about how this change happened because of the 70-million-year evolutionary gap separating the insect-eating alvarezsauroids from the earliest known member of the group, Haplocheirus.

"The significance of Xiyunykus and Bannykus is that they fall within that gap and shed light on patterns of evolution within Alvarezsauroidea," said Corwin Sullivan, a paleontologist at the University of Alberta.

"These specimens greatly improve the scientific community's understanding of the early stages of alvarezsauroid evolution and give us a better idea of what early alvarezsauroids were like," Sullivan said.

The new specimens reveal clues about how the creatures' diet shifted from meat to insects, Sullivan noted.

The forelimbs show some adaptations for digging, which would later become more exaggerated, and some features of their skulls also resemble those of insectivorous alvarezsauroids.

"The hindlimbs are less modified, suggesting the arms and head of alvarezsauroids underwent significant change before the legs did," said Sullivan.

"Xiyunykus and Bannykus are currently represented by one incomplete specimen apiece. Those specimens provide a good deal of intriguing information, but we'll need many more fossils before we can be confident that we have a clear understanding of how alvarezsauroids, to put it bluntly, got so weird," Sullivan said.