An articulated alvarezsaur pes

Some well-know and well studied taxa can often have little bits consistently missing from their fossils and it can take many years before every part of their anatomy is known. In the case of the derived alvarezsaurs it was only very recently that a complete pes was known with the discovery of Albinykus. This was rather unfortunate for me, as it somewhat renders much of my new paper redundant.

Yes a couple of years ago while out prospecting at Bayan Mandahu I came across this delightful little alvarezsaur pes. What is nice about it though is that it has digit I intact, something unknown in the clade (well, when I found it and started work describing it). The piece was actually found within a good stones throw of the points where both Linheraptor and Linhenykus were found, so that’s clearly a good spot to be working in. In this case we refer the pes to Linhenykus given the similarity in form and that it was found in the same locality, and perhaps even the same horizon since both the holotype and this specimen were preserved in nodules.

Linhenykus pes from Hone et al. 2012. Scale bar is 10 mm

So much so not especially interesting perhaps, having been beaten to the punch. True, but we were able to expand our discussion a little in looking at the shape and structure of the metatarsus. Alvarezsaurs like a number of derived theropod groups have a ‘pinched’ metatarsal III resulting in the arctometatarsalian condition. The evolution, structure and function of this set-up has been discussed at some length in a variety of papers, but we were able to provide some extra little details which seem not to have been looked at previously.

While it recognised that despite the proximal and posterior constriction, some metatarsal IIIs have a mediolateral expansion – in other words, looking at the front of the element, it can look quite fat a way up the shaft. Our examination of this and subsequently other specimens suggests that this is a thin flange of bone that sits on top of mts II and IV. This doesn’t seem to have been mentioned before (and I was pleased to see that the referees, both of whom have looked at this extensively, agreed with this assessment) and is potentially quite odd. The structure of the arctometatarsus seems linked to running efficiency and reducing motion between elements. However, having a flange of bone here on the anterior face would presumably help stop mts II and IV moving forwards relative to III, or III posteriorly relative to the others, or even both. Only this seems a rather unlikely problem for even a highly cursorial animal. This is clearly beyond the scope of a short descriptive paper to deal with, but it is (I think) an interesting observation.

Related to this, the exact shape and structure of mt III in this specimen compared to the holotype of Linhenykus is a little different. One might expect such a derived and specialised condition to be highly conservative which suggests the selection is potentially quite weak on this specific structure. It also hints that there may be more variables and details to the arctometatarsus than previous realised.

As a final little note, this paper is out in Acta Palaeontologica Polonica so is freely accessible, though currently as a horrible uncorrected proof, as indeed is the long and detailed description of Linhenykus that follows last year’s brief work in PNAS.

Hone, D.W.E., Choiniere, J.N., Tan, Q. & Xu, X. An articulated pes from a small parvicursorine alvarezsaroid (Dinosauria: Theropoda) from Inner Mongolia, China. Acta Palaeontologica Polonica, in press.
Xu, X., Upchurch, P., Ma, Q., Pittman, M., Choiniere, J., Sullivan, C., Hone, D.W.E., Tan, Q., Tan, L., Xiao, D., & Han, F. Osteology of the alvarezsauroid Linhenykus monodactylus from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China, and comments on alvarezsauroid biogeography. Acta Palaeontologica Polonica, in press.

4 Responses to “An articulated alvarezsaur pes”

  1. 1 Jaime A. Headden 31/01/2012 at 1:22 pm

    What’s awesome here is that in animals that have this distal expansion of MTIII, for example CMN 8538 (holotype of Macrophalangia canadensis = Chirostenotes pergracilis), the cross-section of the shaft at this point is quadrangular, with only slight concavity of the sides where MTII and MTIV articulate and abut MTIII. There is no distinct overlying flange, a unique and ridiculous oversight for otherwise studied alvarezsaur pes! I was actually surprised when I read this, and jumped back into the lit to find out where I recalled MTIII’s being compared extensively in their cross-section, and all I can find his Tom Holtz’s thesis. Wow.

    Incidentally, “Ornithomimuselegans Parks also appears to have a flange on MTIV that abuts MTIII medially, a feature that appears to be diagnostic for the taxon and/or present in a range of specimens from the late Campanian up to the late Maastrichtian which have been referred to that taxon.

    Such strange feet in arctomet and subarctomet theropods.

  2. 2 Anonymous 31/01/2012 at 4:27 pm

    So…does this lend support to the alvarezsaur+ornithomimosaur hypothesis, or does it just indicate the arctometatarsalian condition is homoplastic and widespread amongst Coelurosauria?

    • 3 David Hone 31/01/2012 at 7:28 pm

      Well this specimen specifically says nothing about it – we already knew that the arctometatarsal condition was massively homoplastic as it’s absent in so many basal forms and the actual details of the anatomy are different between most of the groups (though we do summarise all of this). What it does is talk about some of the details of the anatomy and putative functions.

  1. 1 Arctometatarsal origins « Dave Hone's Archosaur Musings Trackback on 08/02/2012 at 2:01 pm
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