If it walks like a duck and swims like a duck, it might be a dinosaur. Scientists have discovered a flippered theropod dinosaur that appears to have spent much of its life in water.

The fossil of Halszkaraptor escuilliei, described in the journal Nature, reveals a strange dinosaur that defies paleontologists’ expectations: one that mixes the traits of theropod dinosaurs with those of aquatic or semi-aquatic birds and reptiles today.

“The first time I saw the fossil I was shocked,” said lead author Andrea Cau, a paleontologist at the Geological and Palaeontological Museum Giovanni Capellini in Italy. “It was so unexpected and bizarre.”

H. escuilliei lived some 75 million years ago, during the Cretaceous Period. It was a theropod, a largely carnivorous group of dinosaurs whose members included Tyrannosaurus rex and the ancestors of all living birds.

H. escuilliei, called Halszka for short, was part of the dromaeosauridae, a group of feathered theropods that included velociraptor and that were not birds or bird ancestors, but closely related to them. While no feathers survived on this specimen, Halszka probably sported plumage and it had a somewhat bird-like bill that was still not a true beak (in part because it housed several teeth).

Halszka had a long, swan-like neck, was the size of a goose, and it probably spent much of its time in lakes and rivers eating small fish, crustaceans and small animals such as lizards, Cau said. In this dino-eat-dino world, its predators may have included fellow theropods like velociraptor.

(Royal Belgian Institute of Natural Sciences)

This fossil, which is still partly embedded in rock, was originally poached from Mongolia, passing through several private collections before a French fossil dealer acquired it in 2015 and donated it to the Royal Belgian Institute of Natural Sciences. It quickly struck researchers as an oddball.

“We’re used to thinking of dromaeosaurs in the context of the classic raptors — velociraptor and Deinonychus and Utahraptor, because we now know they’re totally feathered and so forth — as sort of knife-footed murder-birds,” said Thomas Holtz Jr., a vertebrate paleontologist at the University of Maryland in College Park who was not involved with the study.

But this new fossil, he said, is one “weird … looking dromaeosaur.”

This dinosaur did have a curved sickle-like claw on the second toe of the foot that is typical of dinosaurs like velociraptor, but it was not especially long and probably wasn’t used that much for hunting, Holtz said. Meanwhile, its “arms” were small and appeared to have been modified for use as flippers, which could have helped it paddle through the water.

Unlike penguins and other aquatic birds today, Halszka would not have been a diver, Holtz said. Instead, it probably would have used its long neck to dart out and grab prey close to the water’s surface.

The overall result was a sort of “pseudo-goose … something that could wade out into the water and dab around for some small-bodied prey,” Holtz said.

The animal’s hind legs, meanwhile, appear to have been modified for standing in a more upright position — modifications that can be found in birds today such as ostriches and ducks, Cau said.

Because this fossil had been stolen from its original resting site in Mongolia, the researchers had to make sure that their fossil was authentic.

The scientists subjected Halszka to synchrotron multi-resolution X-ray microtomography, producing a high-resolution digital scan of the whole fossil. This allowed them to see that the structure of the rock around different parts of the specimen remained the same, confirming that the fossil had not been cobbled together from different parts.

This technique also allowed them to see the bones that were still embedded in the rock, the teeth within the bill and even a neurovascular mesh in its snout that’s similar to what’s found in aquatic reptiles like crocodiles today.

“Halszkaraptor shows aquatic and swimming adaptations not seen in other dinosaurs,” Cau said.

Halszka wasn’t the only dinosaur with this weird mix of traits, as it turns out: Two other Mongolian fossils — one found in 1970, the other in 1992 — may represent two other species that together with Halszka define a new group of amphibious or semi-aquatic dinosaurs.

The next step, Cau said, is to keep analyzing the six terabytes of scan data the scientists pulled form this single fossil. Once their study of this particular fossil is done, it will be returned to Mongolia.

amina.khan@latimes.com

Follow @aminawrite on Twitter for more science news and "like" Los Angeles Times Science & Health on Facebook.

MORE IN SCIENCE

Fires, droughts and hurricanes: What’s the link between climate change and natural disasters?

Eating for your health is also better for the environment, study shows

These 215 fossil pterosaur eggs are revealing new clues about these flying reptiles