When it was first described in 1993, Mononykus was one of the strangest dinosaurs known. It had the slender, light build of some of the "ostrich mimic" dinosaurs, yet it possessed two stubby, one-clawed hands and a few other subtle characteristics that placed it in a new group called the alvarezsaurs. Since that time, multiple species of alvarezsaur have been found, and the latest discovery has just been announced in the journal PNAS.
Named Linhenykus monodactylus, the new dinosaur is known from a partial skeleton found in the 84- to 75-million-year-old fossil deposits of Inner Mongolia. It was not a very large dinosaur—as Dave Hone commented at Archosaur Musings, "the living animal would probably have been able to sand comfortably in the palm of your hand"—but what makes it stand out are its heavily built forearms.
Like many of its close relatives, Linhenykus had only one functional finger—a single, stout digit tipped with a heavy-duty claw. Where Linhenykus differs, however, is that it lacked any additional fingers. Other alvarezsaurs discovered so far had tiny, vestigial fingers that were still retained alongside the primary finger. Even in Mononykus, where only the functional finger has been found, there were small indentations in the bone of the hand which suggest that it also had two additional, tiny fingers. Not so in Linhenykus. There is a small, second bone of the palm of the hand next to the large finger, and since this small bit of bone could not have supported a finger we can say that Linhenykus is the first one-fingered dinosaur known.
Curiously, however, the loss of the additional fingers in Linhenykus was not the culmination of a long-term evolutionary trend among the alvarezsaurs. When compared to other members of this group, Linhenykus fell out closer to the base of the family tree than species which retained the vestigial fingers. This means that the anatomy of Linhenykus represents a pattern of mosaic evolution: It retained a suite of archaic characteristics seen among early members of the group, but it also had peculiar specializations not seen among later species such as Mononykus. The loss of the vestigial fingers in Linhenykus was a specialization not yet seen among any other alvarezsaurs.
Further discoveries and future analyses will flesh out the evolutionary pattern seen among these dinosaurs, but one of the recurring questions is why alvarezsaurs had such unique forelimbs. How did they evolve, and what were they used for? These are two distinct questions—even if we can determine the function of a particular trait, that does not necessarily explain how that trait evolved in the first place.
At the moment, the favored hypothesis is that Mononykus, Linhenykus and their relatives used their claws for digging into ant and termite nests. As pointed out by Phil Senter in a 2005 Paleobiology study, the forelimbs of Mononykus were modified so that the palms of their hands faced downward and they were capable of scratch digging with their functional fingers. No one had yet found a preserved termite or ant nest that was raided by an alvarezsaur, but, given the similarity of their claws to those of modern anteaters and pangolins, the idea that these dinosaurs feasted on insect colonies remains the most popular explanation for their unique anatomy.
Xu, X., Sullivan, C., Pittman, M., Choiniere, J., Hone, D., Upchurch, P., Tan, Q., Xiao, D., Tan, L., & Han, F. (2011). A monodactyl nonavian dinosaur and the complex evolution of the alvarezsauroid hand Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1011052108
Senter, P. (2005). Function in the stunted forelimbs of Mononykus olecranus (Theropoda), a dinosaurian anteater Paleobiology, 31 (3), 373-381 DOI: 10.1666/0094-8373(2005)0312.0.CO;2