Paleontologists have discovered what they say is a completely unexpected semi-aquatic theropod dinosaur that lived 75 million years ago in what is now Mongolia.

The new dinosaur, named Halszkaraptor escuilliei, is a strange new anomaly in the dinosaur world.

It walked on two legs on land, with postural adaptations similar to short-tailed birds (like ducks), but used its flipper-like forelimbs to maneuver in water (like penguins and other aquatic birds), relying on its long neck for foraging and ambush hunting.

“The first time I examined the specimen, I even questioned whether it was a genuine fossil,” said Dr. Andrea Cau, a researcher at the Geological and Palaeontological Museum ‘Giovanni Capellini’ in Bologna, Italy.

“The unexpected mix of traits makes it difficult to place Halszkaraptor escuilliei within traditional classifications.”

In order to ascertain the integrity of the specimen, it was visualized and reconstructed in 3D using synchrotron multi-resolution X-ray microtomography.

“This technique is currently the most powerful and sensitive method to image internal details without damaging invaluable fossils,” said Dr. Paul Tafforeau, from the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.

“Our first goal was to demonstrate that this bizarre and unexpected fossil is indeed a genuine animal: multi-resolution scanning confirmed that the skeleton is not a composite assembled from parts of different dinosaurs,” added Dr. Dennis Voeten, also from the ESRF.

“We implemented new methods for the acquisition and optimization of tomographic scan data, which not only confirmed the integrity of the specimen, but also revealed additional paleontological information,” said ESRF researcher Dr. Vincent Fernandez.

The synchrotron was even able to reveal, in astonishing detail, those parts of the skeleton that have remained deep within the rock ever since the dinosaur got buried.

“Our analysis demonstrated that numerous teeth, which are not visible externally, are still preserved inside the mouth,” said ESRF scientist Dr. Vincent Beyrand.

“We also identified a neurovascular mesh inside its snout that resembles those of modern crocodiles to a remarkable degree. These aspects suggest that Halszkaraptor escuilliei was an aquatic predator.”

Although the new dinosaur is unique in many ways, certain parts of the skeleton, including the sickle-shaped ‘killer claws’ on its feet, are shared with well-known dinosaurs such as Velociraptor.

The research is published in the journal Nature.

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Andrea Cau et al. Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs. Nature, published online December 6, 2017; doi: 10.1038/nature24679