Re: dinosaur vs. bird locomotion

[Stephen_Gatesy@Brown.edu (Steve Gatesy)]

Jaime A. Headden wrote:

>  But having read several Gatesy (incl. 1990) papers,
>and especially enjoying Gatesy and Dial (1996) and
>Gatesy and Middleton (1999) [the last a truly novel
>piece of work] I have come to understand that bird
>locomotion (containing several modes of locomotion) is
>only incomparable due to the development of powered
>flight, in such that the tail and hindlimbs were
>decoupled, and in all living birds (ancestrally), the
>forelimbs became a primary thrust generator, whereas
>flightlessness reduces this to a secondary system.

Thanks for the citations, Jaime, but I think several concepts are getting
confused here.  My work on caudofemoral muscle evolution didn't deal with
flight, only hind limb function during terrestrial locomotion.  I concluded
that basal theropods probably used hip extension (retraction of the
relatively vertical femur) during slow speed walking.  In contrast, extant
birds primarily rely on knee flexion (retraction of the tibia relative to a
more stable, horizontal femur) at low speeds.  I proposed a shift from a
caudofemoral retraction mechanism (primitive) to a hamstring-powered knee
flexion mechanism on the line from ancestral theropod to modern bird.
However, I suggested that this transition began in coelurosaurs PRIOR TO
FLIGHT, and that it might not have been completed until the evolution of
Ornithurae, AFTER THE ORIGIN OF FLIGHT.

I think that issues of decoupling and modularity have more to do with
elaboration of the locomotor repertoire within birds.  We've suggested that
flight allowed for the evolution of unusual hind limbs (think of a
flamingo, for example) that would probably not be viable in primitively
flightless forms.

The question of non-avian dinosaur locomotion with respect to birds or
mammals is entirely dependent on which aspects of limb position, motion, or
control you are comparing.  I think the mammalian comparison that started
the discussion may have involved work on hip motion and femoral position.
In this respect, many non-avian dinosaurs were probably more like mammals
than like living birds when walking.  I've only done theropods, so check
out recent papers by Matt Carrano for data on all other dinosaurs.

Steve


Stephen M. Gatesy
Dept. of Ecology & Evolutionary Biology
Box G-B209
Brown University
Providence, RI 02912
401-863-3770
401-863-7544 (fax)
Web Page: http://www.brown.edu/Departments/EEB/main.html