The creatures – named Oksoko avarsan – grew to around two metres long with only two functional digits on each forearm.
They had a large, toothless beak similar to the type seen in parrots today and would feed on other animals as well as plants.
Researchers said the remarkably well-preserved fossils provided the first evidence of digit loss in the family of dinosaurs known as oviraptors.
Just like the T Rex, Oksoko avarsan had only two fingers – but all the other known members of its family had three.
The discovery that they could evolve forelimb adaptations suggests the group could alter their diets and lifestyles, and enabled them to diversify and multiply, the team said.
The fossil remains of four young dinosaurs were preserved resting together, pointed to the probability that Oksoko avarsan, like many other prehistoric species, were social as juveniles.
Dr Gregory Funston, of the University of Edinburgh’s School of GeoSciences, who led the study, said: “Oksoko avarsan is interesting because the skeletons are very complete and the way they were preserved resting together shows that juveniles roamed together in groups.
“But more importantly, its two-fingered hand prompted us to look at the way the hand and forelimb changed throughout the evolution of oviraptors, which hadn’t been studied before.
“This revealed some unexpected trends that are a key piece in the puzzle of why oviraptors were so diverse before the extinction that killed the dinosaurs.
“Oviraptorosaurs were highly adaptable, which may have enabled them to diversify the end of the Cretaceous.”
Researchers studied the reduction in size, and eventual loss, of a third finger across the oviraptors’ evolutionary history.
The creatures’ arms and hands changed drastically in tandem with migrations to new geographic areas, specifically to what is now North America and the Gobi Desert.
New dinosaur linked to birds
The study, published in the journal Royal Society Open Science, was funded by The Royal Society and the Natural Sciences and Engineering Council of Canada.
It also involved researchers from the University of Alberta and Philip J Currie Dinosaur Museum in Canada, Hokkaido University in Japan, and the Mongolian Academy of Sciences.
Additional reporting by agencies.