Mononykus was a small, for a dinosaur, no more than about six and a half feet long (2 m), with long, skinny legs.
The biggest mystery surrounding Mononykus is the single claws on its front limbs (similar to T. rex). These had powerful muscles attached to them that would have made them very strong, but even so, scientists have not been able to find a practical use for these claws. They were too short to be of use in deference or for feeding, so perhaps they were used as part of a mating display. Given the strong muscles behind them, even a theory has its problems. It used to be thought that the claws would have been used for been explained.
Mononykus was very nimble and could run at high speeds, something that would have been useful in the open desert plains where it lived. It had a small skull,and its teeth were small and pointed, suggesting that it mainly ate insects and small animals, such as lizards and mammals. Large eyes allowed Mononykus to hunt by night, when it was cooler and there would have been fewer predators about.
The first Mononykus fossils were found near the famous Flaming Cliffs in Mongolia during the 1920s,but it was not until 1922 that more complete skeletal material was discovered and the animal could be named.
Mononykus has proved to be something of a problem to science. Its skeleton has a remarkably bird-like sternum. In fact, Mononykus has so many bird-like qualities that there is an ongoing argument as to whether it is actually an unusual type of bird or an unusual type of theropod dinosaur. The issue has not yet been resolved, but whether it was a bird or a dinosaur, Mononykus is usually reconstructed with a covering of feathers. In life these would have provided it with insulation during colder months. They could also have provided it with camouflage, or perhaps have been used as part of a colorful mating display.
In Walking With... series
Nigel Marven was able to catch one at night to show its primitive feathers and small forelimbs. Later on, another Mononykus ate a scorpion that Nigel had caught earlier and put into a jar. Then a bunch of them are seen running past Nigel running away from a Tarbosaurus. Nigel encounters them for a last time at a prehistoric lake, where the Mononykus were drinking alongside several Saurolophus.