The strange creature had a graceful swan-like neck but also scythe-like claws, a reptilian tail, and a beak lined with teeth.
Halszkaraptor escuilliei, which lived 75 million years ago, was about the size of a modern swan and is thought to have been semi-aquatic.
A bizarre feathered dinosaur resembling a nightmarish mutant swan has been identified by scientists.
It walked on two legs on land, but probably used its flippered forearms to manoeuvre in water.
The dinosaur is the first of the large family of meat-eaters, called theropods, known to have adopted the lifestyle of a present day water bird.
Other theropods include the mighty Tyrannosaurus rex and agile Velociraptor, star of the movie Jurassic Park.
Dr Andrea Cau, from the Geological Museum Capellini in Bologna, Italy, said: “The first time I examined the specimen, I even questioned whether it was a genuine fossil.
“When we look beyond fossil dinosaurs, we find most of Halszkaraptor’s unusual features among aquatic reptiles and swimming birds. The peculiar morphology of Halszkaraptor fits best with that of an amphibious predator that was adapted to a combined terrestrial and aquatic ecology: a peculiar lifestyle that was previously unreported in these dinosaurs.”
He added that Halszkaraptor had sickle-shaped “killer claws” on its feet similar to those of Velociraptor.
Scientists at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, used an advanced form of X-ray imaging to create a 3D reconstruction of the creature.
The scan revealed hidden details within the rock in which the fossil was embedded, including numerous teeth.
ESRF researcher Vincent Beyrand said: “We also identified a neurovascular mesh inside its snout that resembles those of modern crocodiles to a remarkable degree. These aspects suggest that Halszka was an aquatic predator.”
A description of Halszkaraptor appears in the latest issue of Nature journal.
The unusually well preserved fossil skeleton was discovered in southern Mongolia before spending years in the hands of private collectors around the world.
It was one of many fossils illegally exported from Mongolia. The scientists, who acquired the specimen in 2015, plan to return it to the country.