A long-mysterious dinosaur now has a nearly complete skeleton, revealing a form much more unusual than scientists had predicted.

Nearly 50 years ago, paleontologists uncovered some fearsome dino arms. At 2.4 meters long, they held the record for the longest forelimbs of any two-legged animal. But these remains, which belonged to a species of dinosaur that would be named Deinocheirus, didn’t yield much information – other than the arms, paleontologists only found a few ribs and pieces of vertebrae.

These remains were unique enough to distinguish the skeleton from other, previously known species. But the pieces weren’t complete enough to paint an accurate picture of the creature they’d once belonged to.

Now, researchers report in a new Nature paper, the hunt for the Deinocheirus is complete. After finding two more partial skeletons in 2006 and 2009 (and then tracking down missing pieces from each dig that had already been poached and sold into private collections when researchers arrived), Deinocheirus is complete enough to find its place in the tree of life – and for scientists to model its appearance and gait in the video looped above.

Deinocheirus mirificus, (whose name means “unusual horrible hand”) is indeed a member of the ornithomimosaurs, a group of dinosaurs that vaguely resembled modern ostriches, as was theorized when its arms were discovered. But it was by far the biggest, and it had a whole host of features that haven’t been seen in its cousins.

The dinosaur, which lived 70 million years ago around Mongolia, had a large, tooth-less snout that flared out like a duck’s bill. Its curved spine probably formed a sail-like fin, and its feet were unusually broad. These flat-bottomed toes may have helped the dinosaur forage for food in aquatic areas by keeping it from sinking into mud. Because its bill is similar to an herbivore’s but its stomach contents seem to contain fossilized fish, the researchers believe that Deinocheirus mirificus was omnivorous.

The researchers write in their study that Deinocheirus’s surprising figure should serve as a reminder that incomplete skeletons can be very misleading.

“The discovery of the original specimen almost half a century ago suggested that this was an unusual dinosaur, but did not prepare us for how distinctive Deinocheirus is—a true cautionary tale in predicting body forms from partial skeletons, even for animals in which the rela- tionships are known,” the authors write.