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Mononykus

© Beverly Eschberger

In the 1920's, Roy Chapman Andrews led the first expeditions into the Gobi Desert by the American Museum of Natural History. Among the bones that he brought back were a few vertebrae, the hind limbs, and parts of the pelvis of what was only referred to as an "unidentified bird-like dinosaur."

In the 1990's, a new group of paleontologists from the museum returned to Mongolia's Flaming Cliffs. They found the articulated bones of a turkey-sized dinosaur with long bird-like hind limbs and tiny front limbs. Examining the bones, they realized that they had discovered a new species of theropod dinosaur, that they named Mononychus olecranus. "Mononychus" meaning "one claw", and "olecranus" referring to the olecranon fossa, a hollow or groove at the end of the humerus closest to where it joins with the radius and ulna. The genus name was later changed to Mononykus when it was learned that the other spelling was the name of a genus of beetle. (See my article "Brontosaurus versus Apatosaurus" for more information about how biologists name animals.)

The paleontologists realized that Mononykus was not simply a "bird-like dinosaur", but was actually a primitive bird with short, single-clawed forelimbs instead of wings. Its discoverers have postulated that Mononykus may have occupied a niche similar to our modern aardvarks or anteaters, using its clawed forelimbs to dig up ant and termite mounds. You can compare the Mononykus forelimb to that of the human.

Why is Mononykus considered to be a bird, rather than a dinosaur? It has some characteristics that are seen in the non-avian dinosaurs, such as teeth, a long tail, and foot bones that are separated from one another, rather than fused as in modern birds. However, it shares certain characteristics with birds, such as a breast bone with a keel, a fibula (a small bone attached to the tibia) that is reduced to a small spike, and fused wrist bones. More information about the relationship between birds and dinosaurs.

Mononykus also presented a puzzle. The fossil remains were determined to be about 70 million years old (from the late Cretaceous Period), and had therefore appeared later than other fossil birds that were older but more advanced. Archaeopteryx, about 145 million years old (early Cretaceous Period), was probably able to fly, although we do not know how well. Sinornis and Iberomesornis are extinct birds that appeared about 40 million years before Mononykus, but both were able to fly. Also, many of the fully-flighted ancestors of our modern birds lived at the same time as Mononykus, as did Hesperornis a diving bird.

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