Around the time Haplocheirus hit the journals I commented that this would likely be a good year for Chinese alvarezsaurs. Obviously I rather had some insider knowledge and here’s part of that reason – Xixianykus zhangi. This is a new and really rather small alvarezsaur from Xixa county in Henan, China – a place far more famous for its fossil eggs and isolated baryonychine tooth.
Aside from being the latest taxon in a rather small clade, and having a fair amount of the skeleton intact, Xixinykus has some more interesting things going for it. For a start it does seem to be especially cursorial (that is, adapted for running) which can be seen by a number of anatomical specialisations especially (unsurprisingly) in the legs but also in the body. Despite likely only being likely around half a metre long, the legs on this thing are about 20 cm long. Being fast requires both a long stride length and / or as high stride frequency and Xixianykus as the former in spades at least. This is also combined with proportionally increasing distal parts of the limb (a short femur and a long tibia / metatarsus) which is another good indicator of cursoriality and is higher for Xixianykus than almost all other theropods. Whether or not is did run much is another question entirely, but when it did, it was probably quick. It was also efficient – there are structural adaptations in the body that would have reduced swaying, cutting down energy loss and the short femur also draws muscle mass up the legs making them more efficient (you don’t have to move all that heavy leg muscualture so far than if it was, to take an extreme example, on the foot say).
This is also a fairly old taxon, among the derived alvarezsaurs (the parvicrusorines) this is phylogenetically one of the most basal and the oldest. It’s dated to Santonian-Conacian as opposed to the others which are either Campanian or Maastrichtian. Based on the available material this suggests the possibility of an Asian origin for the group that later dispersed to North America.
Finally, I should add a quick, but large, ‘thank you’ to Matt Van Rooijen author of the Optimistic Painting blog for his reconstruction of the animal. Please don’t rip it off, it’s his artwork, on loan (if you like) to the Musings. If you want to use it, ask him, even if you do see it on various media sites.
A basal parvicursorine (Theropoda: Alvarezsauridae) from the Upper Cretaceous of China
XING XU, DE-YOU WANG, CORWIN SULLIVAN, DAVID W. E. HONE, FENG-LU HAN, RONG-HAO YAN & FU-MING DU (P.R. China). Zootaxa， 2413 1-19.