Yep it’s another new paper and another new alvarezsaur. Way back when with Xixianykus I mentioned that there were more to come. One of these was Haplocheirus and now Linhenykus is on the scene. As with Linheraptor this was another find by Johan Choiniere when out with Mike Pittman, and as I recall was found just the day before that cracking dromaeosaur.
This specimen had eroded out of a nodule, rather unusually for Bayan Mandahu, but the bones were in superb condition and aside from the head and tail, was pretty much complete. It’s a tiny thing, the living animal would probably have been able to sand comfortably in the palm of your hand, but even so it has one rather unusual feature – it has only one finger.
Now the mononykines (more properly parvicursorines) the sub-group of alvarezsaurs to which this belongs are, if you know your languages, defined by their single large claw on the hand (mono = one, nykus = claw). But this is in conjunction with either one or two smaller fingers on the hand, not too dissimilar to Cyclopes, the pygmy anteater. Linhenykus however has just one functional finger, the first non-avian dinosaur known with this feature. There is a second metacarpal (one of the palm bones) preserved, but this has no phalanges attached and indeed tapers to a point distally showing that the hand really didn’t have any other fingers. This is a one fingered theropod. Actually the presence of that second metacarpal is rather handy, without it it would be easy to conclude that the rest of the hand was just missing, but being present and in a condition where it would not support any phalanges shows that we do have the whole hand and this was genuinely one fingered.
Interestingly, Linhenykus is not an especially derived alvarezsaur despite the obviously highly modified hand even compared to other alvarezsaurs (see the hands and phylogenies image). Its main claw is not as big as later relatives so it is derived in terms of digit reduction but basal in terms of claw development and robusticity. This suggests at least the possibility that the other digits were effectively vestigial in derived alvarezsaurs. If they were non-functional then they would be under neutral selection, they might be lost, or hang around in various lineages and their reduction or loss could be down to nothing more than chance. This is of course a working hypothesis and it will be very interesting to see if other alvarezsaurs turn up showing similar variability in the state of their non-specialised digits.
Xu, X., Sullivan, C., Pittman, M., Choiniere, J., Hone, D.W.E., Upchurch, P., Tan, Q., Xiao, D., & Lin, T. 2011. The first known monodactyl non-avian dinosaur and the complex evolution of alvarezsauroid hand. PNAS. in press.