A small theropod with bird-like features.
It was first unearthed in Mongolia in 1923. However, it was not until 70 more years that it was recognized as a unique and unusual dinosaur (bird?).

Mononykus has a light skeleton, an elongated tail and slender legs. Most striking of all is its unusually short single-clawed forelimbs.

The claw is disproportionately big and robust for that of a sole finger. Phalange (finger bone), ulna (lower arm bone) and humerus (upper arm bone) are all similar in length. The keeled sternum of Mononykus (indicated by an arrow in the figure) seems to have supported heavy muscles. These features (short arms with large muscle attachment area) are also seen in some digging animals like moles, however, the long slenderhindlegs and long flexible neck indicate that this animal was not fossorial.

Mononykus has several key features which relate to birds. For example, harboring the small, but keeled sternum is one of the typical features of birds. Reduction of the lower end of the fibula (not articulating with the ankle bone (=tarsus) ) is another feature common between them. This is why Mononykus is sometimes described as a primitive bird, however, its forelimbs are in no way similar to the avian wings.You can see all of these features in Nakasato.

It is not obvious whether its keeled sternum is a remnant of the avian counterpart or just the result of convergence. It might have originally evolved to support some function related to the highly derived forelimbs.

Today, many scientists consider birds as extant dinosaurs. Mononykus may serve as a link between dinosaurs and birds.

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