Two-clawed and parrot-sized: new T.rex cousin unveiled
A tiny distant cousin of Tyrannosaurus rex has been discovered in China with only a single claw on each forelimb.
Linhenykus monodactylus weighed no more than a large parrot and was found in sediments between 84 and 75 million years old.
The dinosaur belongs to a sub-branch of the theropods, the dinosaur group which includes T.rex and Velociraptor, and which gave rise to modern birds.
Details are published in Proceedings of the National Academy of Sciences.
The new species was named after the Chinese city of Linhe, Inner Mongolia, near where its fossilised remains was uncovered in what is called the Upper Cretaceous Wulansuhai Formation.
The international team found a partial skeleton, including bones of the vertebrae, forelimb, hind limbs, and a partial pelvis.
It is part of the Alvarezsauroidea family of theropods, a group of small, long-legged dinosaurs, known for their strange and tiny arms.
Michael Pittman of University College London, who was part of the team, says the animal would hardly have been intimidating.
"You'd see a very small animal, probably below your hip height, with a very small skull. It's not very threatening because its teeth are very small compared to other carnivorous dinosaurs and there's some evidence it may have been an insectivore," he told the BBC.
What is striking is the animal's unusual claws.
"Non-avian theropods start with five fingers but evolved to have only three fingers in later forms," he says. "Tyrannosaurs were unusual in having just two fingers but the one-fingered Linhenykus shows how extensive and complex theropod hand modifications really were."
The suggestion is that this mono-digit theropod may represent the end of one evolutionary pathway, in which unused digits disappear as part of a process of natural selection.
"Vestigial structures, like legs in whales and snakes, may appear and disappear seemingly randomly in the course of evolution," says Jonah Choiniere of the American Museum of Natural History, who also worked on the find.
"Linhenykus highlights complexity in evolution of these vestigial fingers."
Dr Paul Barrett of London's Natural History Museum said the discovery was a "nice specimen" to add to a sub-group already known for its weirdness.
"Alvarezsauroids are already known to be an unusual group of theropods with very bizarre hands used primarily for digging, and this new find confirms there was some variation in how weird these hands were."