Fossils found in China may give clues to the evolution of Tyrannosaurus rex.

中国发现的化石可能会给暴龙的进化带来一些线索。

Tyrannosaurus rex may have had much smaller ancestors

Uncovered near the city of Jiayuguan, the fossil finds come from a novel（新奇的） tyrannosaur（恐龙） dubbed（复制） Xiongguanlong baimoensis.

The fossils date from the middle of the Cretaceous（白垩纪） period, and may be a "missing link", tying the familiar big T rex to its much smaller ancestors.

The fossils show early signs of the features that became pronounced（显著的，断然的） with later tyrannosaurs.

Paleontological（古生物学的） knowledge about the family of dinosaurs known as tyrannosaurs is based around two distinct（独特的，不同的） groups of fossils from different parts of the Cretaceous period, which ran from approximately 145 to 65 million years ago.

One group dates from an early part of the period, the Barremian, and the other is from tens of millions of years later.

Hadrosaurs - duck-billed dinosaurs - spread rapidly in the late Cretaceous

Physical form

Before now it has been hard for palaeontologists to trace the lineage（家系，血统） from one group to the other.

"We've got a 40-50 million year gap in which we have very little fossil record," said Peter Makovicky, associate curator（管理者） at the Field Museum in Chicago, who helped to lead the US/Chinese team that uncovered the fossil.

But, he said, X baimoensis was a "nice link" between those two groups.

"We're filling in that part of the fossil record," he said.

Writing in the Royal Society's journal Proceedings B, Dr Makovicky and colleagues suggest that X baimoensis is a "phylogenetic（动植物种类史的）, morphological（形态学的）, and temporal（暂时的，现世的） link" between the two distinct groups of tyrannosaurs.

The fossil has some hallmarks（品质证明） of large tyrannosaurs such as a boxy skull, reinforced temple bones to support large jaw muscles, modified front nipping teeth and a stronger spine（脊柱） to support a large head.

But it also shows features absent from older tyrannosaurs, such as a long thin snout（鼻子）.

An adult would have stood about 1.5m tall at the hip and weighed about 270kg. By contrast, an adult T rex was about 4m tall at the hip and weighed more than 5 tonnes.

Wider net

The same edition of Proceedings B features papers about two other sets of dinosaur fossils.

One discovery was made in China by many of the palaeontologists who found the tyrannosaur. The samples found in the Yujingzi Basin came from a dinosaur that resembled the modern ostrich（鸵鸟）.

While many of these ornithomimosaurs（似鸟龙类） have been found before, analysis of the bones of the new species, dubbed Beishanlong grandis, suggest it was one of the biggest.

The specimen found by the palaeontologists was thought to be 6m tall and weigh about 626kg.

Alongside in Proceedings B was work on the remains of a duck-billed dinosaur found in Uzbekistan called Levnesovia transoxiana.

Analysis of the fossils, by Hans-Dieter Sues of the Smithsonian in Washington and Alexander Averianov of the Russian Academy of Sciences, may shed light on the waves of expansion hadrosaurs（鸭嘴龙） undertook during the late Cretaceous.