Linhenykus monodactylus, the newly discovered parrot-sized dinosaur with one curiously large claw on each of its hands.
Credit: Julius T. Csotonyi
SYDNEY: A new species of dinosaur, with the unique distinction of bearing just a single large claw on each of its hands, has been identified from a fossil exhumed in China.
As the first known one-fingered dinosaur, the discovery of the parrot-sized Linhenykus monodactylus reveals a complex pattern of evolution within the theropods - a largely carnivorous group of dinosaurs that includes the notorious Tyrannosaurus rex and Velociraptor.
“Linhenykus is important for understanding the evolution of the alvarezsaurs [a theropod family originally thought to represent the earliest flightless birds]”, said palaeontologist Jonah Choiniere of the American Museum of Natural History, a member of the international team whose discovery is reported in the latest issue of the Proceedings of the National Academy of Sciences.
Vestigial outer fingers
“It shows us there was an incredible diversity of alvarezsaur species in the Late Cretaceous of what is now China and Mongolia... whatever these alvarezsaurs were doing, they were successful at it.”
Most theropods had three-fingered hands, but in the Alvarezsauroidea branch there appears to have been an evolutionary trend towards a reduction in the size of the two outer fingers, until eventually they became useless.
While closely related alavrezsaurs retain these vestigial fingers, “Linhenykus goes a step further - it lacks side fingers altogether”, Choiniere explained. “It seems that in alvarezsaurs, like in whales, snakes and some other vertebrates, vestigial structures may disappear and reappear seemingly randomly.”
“Bones scattered across the ground”
The international team of palaeontologists discovered the fossil near the border of China and Mongolia, preserved in rocks of the Upper Cretaceous Wulansuhai Formation, dating from 84 to 75 million years ago.
“In a typical fashion, I was kicking rocks and complaining to [co-author Michael Pittman] that we would never find anything out on the flat areas of the desert when I looked down and spotted bones scattered across the ground”, Choiniere said.
From the skeletal fragments uncovered they were able to paint an accurate picture of what Linehenykus must have looked like - a relatively small alavarezsaur with just one curiously large claw on each of its hands.
Specialised digger?
Having lived alongside numerous closely-related, similar-sized dinosaurs, the unique skeletal structure of Linehenykus suggests a degree of specialisation, probably associated with the way it foraged for food.
Indeed, with a ‘biomechanical tool-kit’ akin to mammalian diggers such as the giant armadillo Priodontes, it is thought Linehenykus may have been adept at digging insect nests from the ground.
According to Erich Fitzgerald, a palaeontologist from Museum Victoria, “what’s really interesting about the pattern of reduction of the digits is that we previously thought it was limited to groups such as the Tyrannosaurs, but clearly it may have been more widespread through therapod dinosaurs.”
“It re-affirms how little we know about the anatomical diversity of dinosaurs. They were not limited to being either giants or large predators but from an anatomical perspective were probably as ecologically diverse as large land mammals are today.”
