Researchers in China have unearthed a miniature single-clawed dinosaur, the only such creature known to have just one finger.
The newly named species of theropod, Linhenykus monodactylus, would have been about a metre tall and as heavy as a parrot, says the study in the Proceedings of the National Academy of Sciences.
Most theropods, which were carnivores that gave rise to modern birds, had three fingers per hand, but this one just had a single large claw that it likely used for digging into insect nests, in an odd but evolutionarily useful adaptation.
"Non-avian theropods start with five fingers but evolved to have only three fingers in later forms," says study co-author Michael Pittman of the Department of Earth Sciences at University College London.
"Tyrannosaurs were unusual in having just two fingers, but the one-fingered Linhenykus shows how extensive and complex theropod hand modifications really were."
'Natural selection' in action
Scientists are not sure why Linhenykus evolved to no longer have his other two digits, but the study hypothesises that "their disappearance may simply reflect the fact that they were no longer being actively maintained by natural selection."
And this sort of thing happened all the time in the history of the wild world, says co-author Jonah Choiniere from the Division of Paleontology at the American Museum of Natural History.
"Vestigial structures, like legs in whales and snakes, may appear and disappear seemingly randomly in the course of evolution," says Choiniere.
The remains of the dinosaur were dug up near the border between Mongolia and China in rocks of the Upper Cretaceous Wulansuhai Formation, which dates to between 84 and 75 million years ago.
Researchers found a partial skeleton at the site, including vertebral bones, a forelimb, part of a pelvis and almost complete hind limbs.