Re: Extreme foot development

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 04:46 PM 7/21/99 +0100, you wrote:
>Hi,
>
>While watching Attenborough's "Life of Birds" I was struck (not
>literally!) by the structure of the ostrich's foot, which has lost
>several toes as an adaptation for running (it carries it's weight on two
>toes, one being much larger than the other).

The troodontid _Tochisaurus_ (ostrich dinosaur, as the name turns out...)
has metatarsal IV and digit IV larger than digit III.  And, of course, all
troodontids couple extreme cursorial proportions with a two-toed foot.

>In the evolution of mammals this has happened too, and in the case of
>the horse has gone one step (gah!) further.
>
>Is there any known case of a dinosaur going to this extreme (they had
>enough time after all) ? I know I
>have seen drawings of hadrosaurs with a "hoof-like" foot, and indeed
>have seen footprints of 3-toed dinosaurs which were supposed to be
>iguanadon-like). Did any of the bipedal "fast runners" evolve in this
>"single-toed" direction?

_Velocisaurus_, a small and enigmatic ?ceratosaur from the Late K of
Argentina has greatly reduced metatatarsals II and IV, while mt III is long,
slender, and more powerfully developed.  Structurally the foot resembles
some of the more derived three-toed horses (the ones where digits II and IV
were present but very reduced).  It might have potentially led to one-toed
runners.

(Based on a comment Jim Farlow once named, I've dubbed this morphology the
"antarctometatarsalian" condition... :-)

                        Thomas R. Holtz, Jr.
                        Vertebrate Paleontologist
Department of Geology                   Director, Earth, Life & Time Program
University of Maryland                  College Park Scholars
College Park, MD  20742       
Webpage: http://www.geol.umd.edu        Phone:301-405-4084
Email:tholtz@geol.umd.edu               Fax:  301-314-9661