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Limb proportions (was RE: Many, many thoughts & responses re: the Hutchinson-Garcia paper)

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Fam Jansma
> Dann wrote:
> >
> >Lions and leopards are much slower than many of their prey. In fact,
> >ambush predators in general seem to be fairly robust creatures - not
> >unlike a certain large theropod? If you're not chasing down gazelles for
> >a living, why be an olympic sprinter?
> >
> Dann makes a good point here, but there is still Horner's "C.rex" who is
> said to have had a femur and tibia of equal length, making it possible for
> it to have hunted actively. The femur has to be shorter or at
> least of equal
> length to the tibia to have a hunting predator, which isn't the
> case in the
> laterT.rex.


...okay, got that over with.

Horner has never (count the number of times, never) been able to establish
any functional link between limb proportions and hunting ability; he just
claims this in the popular press, and people take him at his word.

Here are the observations (consult various papers by me, Carranno,
Christiansen, Paul, Currie, and others for the details):

* In theropods (and indeed most dinosaurs) the tibia is longer than the
femur at small body size, but due to negative allometry the tibia grows at a
slower rate than does the femur with larger body size.
* Consequently, if the dinosaur ontogenetically or phylogenetically reaches
a large enough size, its femur will be longer than its tibia.
* Not all dinosaurs scale along the same curve, however.  In particular,
ornithomimids and tyrannosaurs have longer tibiae than expected at a given
femur length than you would find in a more basal theropod of the same size.
* The reason _T. rex_ specimens often have longer femora than tibiae is that
_T. rex_ is a big species, and consequently many of the individuals found
are just over the cross-over point.  However, the "C rex" specimen is
presumably of a body size right AT the cross-over point, and there are
individuals below it (with longer tibiae than metatarsi).

> A well known fact to everyone on the list is that the femur in
> T.rex is longer than the tibia, so it was only able to walk.

As noted above, this is not a fact.

Incidentally, although Horner hypes up the tibia/femur relationships,
mammalogists have traditionally used the metatarsus/femur relationship as a
proxy of "cursoriality" (whatever that precisely means: see recent post by
Hutchinson).  Using this metric, ornithomimosaurs and tyrannosaurs scale
much more "cursorially" than any other comparable sized theropods, and even
more so than ornithischians of the same body size.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796