Re: More on Digit Loss in Theropods

["Tim Williams" <tijawi@hotmail.com>]

Jaime Headden wrote:

> In cursorial animals, such as the horse (and the giraffe) and ostrich and
> some cranes, the digits are reduced to between two and one. The more 
> cursorial, the fewer toes.

There is some evidence for this in small theropods.  _Tochisaurus_ and 
_Velocisaurus_ show a configuration of the pedal bones consistent with a 
dominant weight-bearing digit, with one digit taking on an increased share 
of the load at the expense of the others:

http://www.cmnh.org/fun/dinosaur-archive/1999Jul/msg00403.html

> hence, cursorial theropods like ornithomimids had only three absolute toes, 
> even though all
> theropods had only three effective toes with the possible exclusion of 
> segnosaurs

As I'm sure Jaime is aware, some theropods had even *less* than three 
effective toes - at least during terrestrial locomotion.  Troodontids, 
dromaeosaurids and _Rahonavis_ were effectively didactyl when walking or 
running.

> (I am not sure
> the hallux actually participates in locomotion, as only the 
> unguo-phalangeal joint can touch the ground).

In modern birds (and I would say in non-avian theropods too) the hallux 
plays no role in terrestrial locomotion.  Many terrestrial and aquatic birds 
(i.e. birds that do not perch in trees) show an abbreviated, elevated or 
non-existent hallux.


Tim

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