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Re: Reevolving bones?



>Yeah, yeah. We're talking about using the first toe as well as the second
>through fourth for walking on. Theropods, even the largest and heaviest, like
>_Tyrannosaurus rex_, walked with only toes 2-4 bearing their weight. If
>segnosaurs were theropods, then (among other reversals) they must have
>re-evolved the weight-bearing function for the first toe, something no other
>theropods, from ceratosaurians to birds, ever did.

        Jeez, doesn't anyone read the literature anymore?!?  ;-D  There
_are_ Maastrichtian theropod footprints (_Saurexallopus_) which were made
by animals that were functionally tetradactyl...and they're not
therizinosauroids, either.  They demonstrate that some theropods did lower
and reassert use of their hallux (or, as has been mentioned, shortened the
other digits to a point where the hallux was again in contact with the
ground).  However, since, in the _Saurexallopus_ prints, the hallux is
_not_ retroverted, this argues for some kind of selection pressure to have
the hallux be useful in the foot, rather than simply contact the ground by
happenstance.  See:

Harris, J.D., Johnson, K.R., Hicks, J. and Tauxe, L.  1996.  Four-toed
theropod footprints and a paleomagnetic age from the Whetstone Falls Member
of the Harebell Formation (Upper Cretaceous:  Maastrichtian), northwestern
Wyoming.  Cretaceous Research 17(4):  381-401.

and

Harris, J.D.  1997.  Four-toed theropod footprints and a paleomagnetic age
from the Whetstone Falls Member of the Harebell Formation (Upper
Cretaceous:  Maastrichtian), northwestern Wyoming:  a correction.
Cretaceous Research 18(1):  139.




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Jerry D. Harris                       (214) 768-2750
Dept. of Geological Sciences          FAX:  768-2701
Southern Methodist University
Box 750395                            jdharris@post.smu.edu
Dallas  TX  75275-0395                (Compuserve:  102354,2222)

"Science _does_ have all the answers -- we just don't have all
the science."
                        -- James Morrow