Dinosaur discovery helps fill 70-million-year evolutionary gap

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

The two species, calledBannykusand Xiyunykus, were members of thealvarezsauridgroup of dinosaurs, insectivores that hadshort arms and small hands withone enlarged finger. But they came from the lineage of theropods, whichwere three-fingered and thought to all be carnivores.

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

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

BannykusandXiyunykusexisted 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.

"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 kilogramsand was discovered in northern China.

Evolutionary shifts tella story

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

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

"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-fingeredhand 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 othertheropodsand 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.