A year ago today at Archosaur Musings, Dave Hone wrote about the alvarezsaurs in anticipation of the description of Haplocheirus being published. He wrote, "It’s going to be a big year for alvarezsaurs in China if all the papers come through in time." They didn't quite manage to all come through in time. A couple months after Haplocheirus, Xixianykus was described. Then, all went quiet on the alvarezsaur front as the ceratopsians dominated the rest of 2010.
Now, with the announced description of Linhenykus, fanciers of the still-obscure group of theropods can break out the cigars and celebrate. As described in PNAS by a team of scientists led by Xu Xing, Late Cretaceous Linhenykus is unique among the alvarezsaurs for possessing a single finger - no reduced nubbins, no hints at additional fingers simply missing from the remains. The reduction in forelimbs and loss of digits, along with keeled sternums perfect for anchoring robust muscles, have led to the popularity of a hypothesized "termite digger" lifestyle among them.
Linhenykus monodactylus by Julius Csotonyi, from the University College of London press release.
Evolution is a tricky thing, and Linhenykus is a good reminder of the falsehood of easy, linear models. If you take a look at Linhenykus and see its single finger, you might come to the conclusion that it represents the peak of alvarezsaur evolution, the storyline of which is the reduction and loss of digits. But that's not the case. When compared to all other members of the family, Linhenykus lands in the middle, an offshoot whose loss of fingers occured separately from later forms.
One thing that's interesting about the alvarezsaurs is that they're one family that almost always is depicted as feathered. Having only been around for the last two decades, they were at times thought to be true birds. Even so, they fit well within the maniraptoran family of theropods; a cladogram put forth by Lindsay Zanno in 2009 has them nestled between forms that are known to be feathered, the therizinosaurs and the oviraptorosaurs.
Will alvarezsaurs ever achieve the mass-popularity of their maniraptoran kin? I personally love them, but I'm not sure that the words "diminutive, possibly insectivorous theropod" really get most folks juiced up. But the questions they pose are juicy ones indeed, and anyone who enjoys setting their mind to such puzzles will probably be occupied with these little guys for a long time.