This Duck-Like Dinosaur Could Swim. That Isn’t the Strangest Thing About it.

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If it looked like a duck, did it swim like a duck?

Paleontologists studying an unusual fossil have identified a new dinosaur, related to the velociraptor, that had a neck like a swan, a snout like a goose and forelimbs like flippers. The creature’s hodgepodge of features — so strange that extra work was needed to verify the fossil’s authenticity — suggest that it might have lived on both land and in water, they reported in a paper published Wednesday in Nature.

If so, that would make the species, known as Halszkaraptor escuilliei, only the second swimming dinosaur ever found, after the ferocious spinosaurus (though even that is debated).

The clues that suggest H. escuilliei was semiaquatic include its long neck and hooked, crocodile-like teeth, which it may have used to plunge face first at fish from the water surface. The dinosaur also had a snout filled with sensory nerves that are typically seen in crocodiles and used to detect movement and temperature changes in water. Its forelimbs were shaped less like wings and more like flippers, similar to those used by marine reptiles like the plesiosaur to swim.

“It was designed for swimming,” said Pascal Godefroit, a paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels and an author on the paper, “but which kind of swimming we don’t know.”

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Perhaps it used its flipper-like forelimbs to maneuver like a penguin. Or its long legs to wade through water like a crane or heron. Dr. Godefroit said the dinosaur lacked webbed feet, so it is unlikely that it swam like a swan, gracefully gliding on the water’s surface while feverishly paddling below. Instead, its legs and clawed feet were better suited for running on land. But if the dinosaur used its feet to propel itself in the water, it would not have been as effective a swimmer as a swan or duck, Dr. Godefroit said. It also was probably not a diver like some water fowl.

Though the prehistoric oceans were teeming with fierce marine predators like the ichthyosaurs and mesosaurs, those leviathans were not dinosaurs, but rather marine reptiles like sea turtles. The newly discovered H. escuilliei was a turkey-sized raptor. This finding helps show that the raptor group included not only terrestrial killing machines like deinonychus and potentially flying members like microraptor, but also swimmers.

The newly identified dinosaur’s past is as shrouded in mystery as its swimming abilities. The research team knows only that it is about 75 million years old and came from Mongolia, but not when it was found. That’s because after it was unearthed it was sold on the black market. For years, perhaps decades, it was held in private collections in Japan and Britain before ending up in the hands of researchers.

This checkered past, combined with its bizarre characteristics — duckbill, swan neck as long as its body, sickle-shaped toe claw — left some paleontologists dubious about its authenticity. The team, which was led by Andrea Cau, a paleontologist from the Geological and Palaeontological Museum Giovanni Capellini in Italy, had to make sure it wasn’t just a heap of bones and plaster glued together. So they brought it to a synchrotron to blast with X-rays and peek inside.

“It looked like an alien or something, it’s like a mixture of things that could have been put together,” said Vincent Fernandez a paleontologist at the European Synchrotron Radiation Facility and an author on the paper. After the team scanned the specimen, they had Dr. Fernandez, who was just joining the team, inspect the image on a computer screen. At first he did not know whether it was real or not.

“They said ‘convince me it’s a fake,’” Dr. Fernandez said. “I thought it would be very obvious. But I looked at it for hours and hours and I couldn’t find anything.”

It was real. The synchrotron also revealed a series of sharp teeth hidden in its mouth.

Thomas Richard Holtz, a paleontologist from the University of Maryland who was not involved in the study, said that at first he thought the fossil was a fake. The results from the synchrotron convinced him that wasn’t the case.

“We’re used to thinking of raptors in the form of velociraptors, looking like knife-footed murder birds,” he said. “Now we know it also produced the non-bird dinosaur equivalent to a goose.”

He said that the dinosaur’s characteristics support the idea that it swam, and suggest that it most likely waded in the water until it got too deep and then it used its forelimbs to push itself forward. But he added that further investigation was needed to reveal this strange dinoduck’s swim stroke of choice.