Mononykus: Beast of the Week

 This week we'll be going over an interesting little dinosaur with unique arms.  Check out Mononykus olecranus! 

Mononykus was a small, bird-like, likely feathered dinosaur that lived in what is now Mongolia, during the late Cretaceous period, about 70 million years ago.  It only measured a little over 1 meter (between 3 and 4 feet) long from nose to tail and was likely a meat-eater, specializing in insects, when alive. The name, Mononykus, translates to "one claw" because it only had one large functional claw on each hand.  

Life reconstruction of Monoykus in watercolor by Christopher DiPiazza.

As the name implies, Mononykus possessed only one claw on each hand, which is perhaps its most distinctive trait.  The claws are proportionally large and curved, and are backed up by extremely stubby but powerful arms.  In fact, despite their length, Mononykus' ulnas (outer forearm bone) extend out past the upper arm bones, implying they had large muscle attachments there in life.  We see similar features in living animals like aardvarks, which use those kinds of muscles and large claws to dig into termite mounds.  Even though aardvarks and dinosaurs are not closely related at all, this may provide insight as to how Mononykus was using its unusual anatomy.  

Arm bones of a modern Aardvark(top) and Mononykus (bottom).  Note the extended ulna.

Other parts of Mononykus' anatomy imply it may have been digging for insects other than the arms and hand claws.  Its vertebra would have been interlocked in a way that stiffened the spine in life, making it more sturdy, another trait that would benefit digging.  The inside of the skull suggests Mononykus had superb hearing, comparable to that of modern owls, who are specially adapted for hunting prey they cannot see.  Again, this is a useful trait to have for an animal that is hunting for insects underground or inside of dead logs.  Other, more completely known, relatives of Mononykus, imply it likely had a narrow, toothless, beaklike mouth, and large, forward-facing eyes.  

Mononykus may have used its superb hearing to detect insects underground or inside logs, then would have needed to press its body against that surface in order to dig them out with its short arms.

As stated before, Mononykus had extremely short arms, which seems to be the only trait inconsistent with animals that dig for their food.  The arms are so short, in fact, that the only way for Mononykus to have even reached a surface to dig is if it's torso was pressed up against it.  I can imagine Mononykus listening to the outside of a dead log, and then rapidly positioning itself to a diffing position with its body pressed against the log, vigorously tearing at the bark with its claws, and then hopping back up to a standing position to listen again or root out the now exposed insects with its beak-like snout.  

Mononykus skeletal mount on display at the American Museum of Natural History in New York.

Mononykus also had extremely long and slender legs.  This implies it was a fast runner when alive, which would have been a useful trait for avoiding predators, but it also has another function that may also be related to finding food.  Long legs means Mononykus could have more easily walked long distances without getting tired.  Since we know Mononykus lived in a desert, it would have needed to regularly travel across large expanses of land in order to find areas where insects actually were, like oasises, or sparsely occurring logs in an otherwise barren environment.  

When alive, Mononykus would have shared its environment with other dinosaurs, like Prenocephale, Gallimimus, Deinocheirus, Therizinosaurus, and the large tyrannosaur, Tarbosaurus, to name a few.  It likely avoided confrontation with the vast majority of these other animals by possibly being nocturnal.  

References

Choiniere, Jonah N.; Neenan, James M.; Schmitz, Lars; Ford, David P.; Chapelle, Kimberley E. J.; Balanoff, Amy M.; Sipla, Justin S.; Georgi, Justin A.; Walsh, Stig A.; Norell, Mark A.; Xu, Xing; Clark, James M.; Benson, Roger B. J. (2021-05-07). "Evolution of vision and hearing modalities in theropod dinosaurs". Science. 372 (6542): 610–613.

Perle, A.; Norell, M. A.; Chiappe, L. M.; Clark, J. M. (1993). "Flightless bird from the Cretaceous of Mongolia". Nature. 362: 623−626.

Perle, A.; Chiappe, L. M.; Rinchen, B.; Clark, J. M.; Norell, M. A. (1994). "Skeletal morphology of Mononykus olecranus (Theropoda, Avialae) from the late Cretaceous of Mongolia". American Museum Novitates (3105): 1−29.