Friday, May 27, 2011

Accessory trochanters in therizinosaurs

While doing my work on the TWG matrix, I noticed something interesting.  A brief intro to the structures described here is helpful.  Theropods have an anterior trochanter (also called lesser trochanter) on the front of their proximal femur.  In birds and various maniraptoriforms it partially or completely fuses to the greater trochanter to form a trochanteric crest.  First recognized in Microvenator, a lot of taxa also have another trochanter right below the anterior trochanter, called the accessory trochanter.  Supposedly, therizinosauroids have a low, separate anterior trochanter but no accessory trochanter.  I think people have just been confusing their accessory trochanter for an anterior trochanter, while the real anterior trochanter is fused in a trochanteric crest.  Note the figure below...

The top row is from Hutchinson's (2001) femoral paper.  The anterior trochanter is 'lt' while the accessory trochanter is 'at'.  I've outlined the accessory trochanter in purple in all figures.  On the left bottom we have basal therizinosaur Falcarius (from Zanno, 2010a), which Zanno correctly realized has both an accessory trochanter and an anterior trochanter which is closely appressed to the greater.  The outline drawing is of Alxasaurus (from Russell and Dong, 1994), which was supposed to have a low cylindrical anterior trochanter.  But note that it is low in position like an accessory trochanter instead and that the greater trochanter is as wide as those taxa which incorporate the anterior trochanter into a trochanteric crest.  Next on the bottom row is an anterior view of Segnosaurus (from Zanno, 2010b).  Here too the anterior trochanter is supposed to be low (labeled flt), but again I think it matches an accessory trochanter more.  Finally, in the lower right is Chirostenotes (from Currie and Russell, 1988) in anterior view.  It was described as having a very low anterior trochanter, which would be quite unlike any avetheropod, but makes sense as an accessory trochanter.  As ornithomimosaurs and basal oviraptorosaurs have the most distinct accessory trochanters, it makes sense that therizinosaurs and Chirostenotes would too.

References- Currie and Russell, 1988. Osteology and relationships of Chirostenotes pergracilis (Saurischia, Theropoda) from the Judith River (Oldman) Formation of Alberta, Canada. Canadian Journal of Earth Sciences. 25, 972-986.
Hutchinson, 2001. The evolution of femoral osteology and soft tissues on the line to extant birds (Neornithes). Zoological Journal of the Linnean Society. 131, 169-197.
Russell and Dong, 1994. The affinities of a new theropod from the Alxa Desert, Inner Mongolia, People’s Republic of China. Canadian Journal of Earth Sciences. 30, 2107-2127.
Zanno, 2010a. Osteology of Falcarius utahensis: Characterizing the anatomy of basal therizinosaurs. Zoological Journal of the Linnaean Society. 158, 196-230.
Zanno, 2010b. A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora). Journal of Systematic Palaeontology. 8(4), 503-543.


  1. Interesting!
    Graphically, I feel a circle around the area, or a highlighter-type markation (yellow or such) would read better. Your lines are not so readily legible.

  2. Be sure to click on the figure to enlarge. :) The circle would have been a good option though.