With the discovery of two dinosaur fossils in China, scientists think they have found the 70-million-year "missing link" between markedly different dinosaurs from the late Jurassic and the upper Cretaceous periods.

The two species, called Bannykus and Xiyunykus, were members of the alvarezsaurid group of dinosaurs, insectivores that had short arms and small hands with one enlarged finger. But they came from the lineage of theropods, which were three-fingered and thought to all be carnivores.

Over millions of years, some of them evolved from fierce hunters and meat eaters to a creature that subsisted largely on insects, but until now little was understood about how this change occurred because a vast evolutionary gap separated the alvarezsauroids of the late Cretaceous period from the earliest known member of the group in the late Jurassic period.

Artist’s reconstruction of important alvarezsaur species, left to right: Haplocheirus, Xiyunykus, Bannykus and Shuvuuia. Note the lengthening of the jaws, reduction of the teeth and changes in the hand and arm. (Viktor Radermacher)

"When we see a transition like that in the fossil record, we always want to know how it happened," says Corwin Sullivan, a professor of paleontology at the University of Alberta, and co-author of a paper on the discovery, published last week in the journal Current Biology.

Sullivan, curator of the Philip J. Currie Dinosaur Museum near Grande Prairie, Alta., spent a decade with the Institute for Vertebrate Paleontology in Beijing, which is researching the dinosaurs.

Bannykus and Xiyunykus existed around 120 million years ago, during the early Cretaceous period. They fill in a vital role in the fossil record between an earlier ancestor known as haplocheirus which lived 160 million years ago, and later versions of alvarezsauridae, the insect eater, which was known to live around 90 million years ago.

The left hand of Bannykus, showing the large first claw on the thumb and the smaller second and third finger. (Prof. Jonah Choiniere)

"So these animals are, in a sense, missing links," explains Sullivan. The largest of the two, Bannykus, would have weighed about 24 kilograms, and was found in Inner Mongolia. The smaller, Xiyunykus, weighed about 15 kilograms and was discovered in northern China.

Evolutionary shifts tell a story

There are two evolutionary shifts which are of particular interest to the researchers.

The teeth on both of the newly discovered fossils are dramatically different from their ancestors, suggesting a change in diet, says Sullivan.

This graphic, included in the study, shows where Xiyunykus and Bannykus fit on the evolutionary scale, and how their fingers developed into a claw. (Xu et al/Current Biology)

"The teeth are quite a bit smaller — and in particular in the alvarezsaurids of the late Cretaceous, which are usually interpreted as specialized for insect eating, the teeth get very small, they lose their serrations on a very fine scale."

The other fundamental change in the species is the development of the pronounced large claw at the end of its tiny forearm, rather than the three-fingered hand and longer arms its ancestors had.

Sullivan says the development of the claws suggests the dinosaurs adapted to digging, likely for insects.

This evolutionary shift suggests a scarcity of prey for smaller dinosaurs as larger species were dominating the landscape. There would have been far less competition for insects, Sullivan speculates.

"It's probably a question of exploiting a food resource that was available. They would have been competing with other theropods and other kinds of predators."

The discovery also reveals clues about the evolutionary origin of alvarezsaurian dinosaurs. Sullivan believes they came from the area that is now northern China, and from there travelled to other parts of the world including North and South America.

The fossils of Xiyunykus and Bannykus will remain at the Institute for Vertebrate Paleontology in Beijing for further study.