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RE: Linhenykus, new Inner Mongolian alvarezsauroid

From: Ben Creisler

In case the full citation and abstract have not been 
posted yet:

Xing Xu, Corwin Sullivan, Michael Pittman, Jonah N. 
Choiniere, David Hone, Paul Upchurch, Qingwei Tan, Dong 
Xiao, Lin Tan, and Fenglu Han (2011) 

A monodactyl nonavian dinosaur and the complex evolution 
of the alvarezsauroid hand.
Proceedings of the National Academy of Sciences (PNAS) 
(advance online publication)
doi: 10.1073/pnas.1011052108 


Digital reduction is a striking evolutionary phenomenon 
that is clearly exemplified in theropod dinosaurs by the 
functionally didactyl manus of tyrannosaurids, the flight-
adapted manus of birds (Aves), and the tridactyl but 
digit II-dominated manus of alvarezsauroids. The 
enlargement of manual digit II in alvarezsauroids and the 
concurrent reduction of the lateral digits have been 
interpreted as adaptations for digging, although no 
detailed biomechanical analysis of hand function has so 
far been carried out for this group. In the derived 
alvarezsauroid clade Parvicursorinae, the lateral digits 
are so small as to be presumably vestigial. Here we 
report a new alvarezsauroid, Linhenykus monodactylus gen. 
et sp. nov., based on a specimen from the Upper 
Cretaceous Wulansuhai Formation of Inner Mongolia, China. 
Cladistic analysis identifies Linhenykus as the most 
basal parvicursorine, and digit II of the manus retains a 
slender morphology and other primitive features. However, 
Linhenykus is also highly apomorphic in exhibiting the 
most extreme reduction of the lateral manual digits seen 
in any alvarezsauroid. Phalanges are retained only on the 
most medial digit (digit II), making Linhenykus the only 
known functionally monodactyl nonavian dinosaur. Other 
parvicursorines are more primitive in retaining a 
tridactyl manus but more derived in that digit II is 
highly robust and shows other apomorphic features in both 
of its phalanges. The unexpected combination of features 
seen in the hand of Linhenykus points to a complex mosaic 
pattern of manual evolution in alvarezsauroids, with loss 
of the presumably vestigial outer digits being decoupled 
from change in the form of digit II. 

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