Arctometatarsal origins

A comment on my recent post about alvarezsaur arctometatarsals made me realise there was a bit more scope for talking about this issue of the origins of this structure. Arctometatarsalian pedes are known in tyrannosaurs, troodontids, alvarezsaurs, ornithomimids and oviraptorosaurs. In other words a pretty big selection of derived theropods have at least some taxa with this condition.

This begs the obvious question of whether or not this is homologous? After all, this could easily plot on a cladogram as originating before the tyrannosaurs and being maintained throughout the derived theropods only to be lost in therizinosaurs and droimaeosaurs + birds. However, there are two good reasons to think that this convergence and not homology with the characteristic being acquired multiple times.

First off are the details of the actual pes. Although obviously we would expect different lineages to adapt and modify such a structure independently, there are some very clear differences between quite how the middle metarsal is pinched and in which way by which of the surrounding elements. It’s subjective of course but they do look quite different in form.

Secondly and more importantly, the character is not shared by all of the taxa in those various groups and especially not basal forms. Early tyrannosauroids like Dilong and Guanlong don’t have it, nor does the basal alvarezsauroid Haplocheirus, and it’s not present in at least some early troodontids, ornithomimids or oviraptorosaurs. So it is not a simple plot of acquisition before tyrannosaurs and occasional lost, but instead more parsimonious to infer that it has been gained independently multiple times (each with a slightly different form) than been lost multiple times in all those basal forms (i.e. lost in each of Guanlong and Dilong and any other tyrannosaurs or even lost at the base and then acquired again) and been modified repeatedly along each lineage.

What has drive this convergence is likely the benefits of such a structure. Work on the functional morphology of such a foot suggests that it increases running efficiency and may also provide increased turning ability. In short, this is a feature of active runners, something that certainly matches at least some other anatomical specialisations seen a number of these groups.

2 Responses to “Arctometatarsal origins”

  1. 1 Andrea Cau 08/02/2012 at 4:46 pm

    An interesting question is why arctometatarsus evolved only in coelurosaurs (and several times there) and never in other possible cursorial forms as gracile-limbed ceratosaurs or megaraptorians. Is it possible that arctometatarsal foot evolved only in coelurosaurs because it exapted from a trait present only in coelurosaurs? I remember a paper co-authored by Snively where something is discussed about arctometatarsus prerequisite. Perhaps, the last common ancestor of all arctometatarsalian forms had evolved something necessary (but not sufficient) for the evolution of the arctometatarsus.

  2. 2 Jaime A. Headden 08/02/2012 at 6:44 pm

    Rather than “pinching,” formation of the metatarsus into a “cannon bone” structure, a la horses, occurs in some basal abelisaurians, such as Elaphrosaurus bambergi and Velocisaurus unicus (?=Noasaurus leali). This involves the extreme reduction in diameter of the second and fourth metatarsals, and firm attachment to the third. While it doesn’t involve the equine-style snap ligaments described by Holtz and Snively (and coauthors).

    “Cannonization” of the metatarsus involves dealing with compressive forces, and thus increased running performance, while the “loose” third metatarsal involves the snap ligaments that seem to enable better control during turning and keeping the pes in form. This seems to be suited for, in tyrannosaurs and ornithomimosaurs, better stability during running and turns at larger sizes … the unusual shape of the bone in alvarezsaurs may actually involve something else entirely, as the third metatarsal fits between the second and third in a more unusual, more “extensoral” position. Its positional significance may qualify a different function with similar morphology to that of other animals. But that, of course, merely emphasizes convergent acquisition.

    Holtz, T. R., Jr. 1992. An unusual structure of the metatarsus of Theropoda (Archosauria: Dinosauria: Saurischia) of the Cretaceous. Dissertation for the completion of a PhD in the Department of Geology & Geophysics, Yale University.
    Holtz, T. R., Jr. 1995. The arctometatarsalian pes, an unusual structure of the metatarsus of Cretaceous Theropoda (Dinosauria: Saurischia). Journal of Vertebrate Paleontology 14:480-519.
    Holtz, T. R., Jr. 2001. Arctometatarsalia revisited: the problem of homoplasy in reconstructing theropod phylogeny. pp. 99-122. In J.A. Gauthier (ed.), New Perspectives on the Origin and Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom. Yale University Press.
    Snively, E., Russell, A. P. & Powell, G. L. 2004. Evolutionary morphology of the coelurosaurian arctometatarsus: descriptive, morphometric, and phylogenetic approaches. Zoological Journal of the Linnean Society 142:525-553.
    Snively, E. & Russell, A. P. 2003. A kinematic model of tyrannosaurid (Dinosauria, Theropoda) arctometatarsus function. Journal of Morphology 255:215-227.

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