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.
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