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Re: Cursorial adaptations (was T.rex and elephants)



_T.rex_ and elephants?  What the?!

I hate to beat a _Hyracotherium_, but I can suffer in silence no more. 
Let's look at some of the documented cursorial adaptations we see in extant
vertebrates.  We're going for a rapid rate of long strides achieved with an
economy of effort.  Here are some of the adaptations we see:

The legs are long relative to the body.  The distal limb elements are
elongated relative to the proximal elements.  Therefore, the metatarsus
(and phalanges in the case of some digitigrade forms) are lengthened, and
the femur is shortened.  

In the quadrupedal dogs and cats the spine is supple, further increasing
stride length.

Some of the principle actuating muscles are affixed to the bone closer to
the pivot of motion.  In effect, this utilizes short lever arm "high gear"
mechanics.  This comes at the expense of power.

To reduce the load required to raise the body when running, larger animals
in particular must reduce the weight of their limbs.  This may entail the
reduction or absence of non-cursor muscles (loss of swiveling muscles, for
instance) and reduction of the ulna in forelegs and the fibula in hindlegs.
 In addition, the bulk of the leg muscle mass is concentrated proximally,
closer to the fulcrum of the swinging limb.  This may entail the complete
loss of muscles in the metatarsus and phalanges.  The phalanges may be
tightly bundled together and reduced in number.

The joints utilized in running are modified to form simple braced hinges
for strength and to reduce the risk of injury during high speed running. 
In hoofed animals, the suspensory ligament snaps the fetlock joint back
rapidly as the foot leaves the ground, straightening the leg and pushing
the leg upward.

The fastest animals tend to be lean.  In running dinosaurs, hollow bones
would provide an additional advantage.

I offer the following reference:

Hildebrand, Milton  1960.  How animals run.  pp.26-33 of Vertebrates:
Adaptation.  _Readings From Scientific American_  W.H. Freeman and Company,
San Francisco.

As you can see, an elephant has NONE of these adaptations.  Did anyone (who
has not seen _George of the Jungle_) have any doubts about this?  Elephants
are graviportal.  End of story.

And as for our cherished _Tyrannosaurus rex_.  What of him?  In my view,
_Tyrannosaurus_ clearly does not share all of the cursorial adaptations of
modern mammals.  But that's OK.  Tyrannosaurs had more of the
aforementioned cursorial adaptations than any other animal in its size
class and , hence, would have been fast enough.  Maybe not as fast as
Bakker describes them, but they were definitely in the race.  They were not
so adapted for cursorial pursuit as modern cheetahs, but so what?  Their
prey, which may have included such genera as _Edmontosaurus_ and
_Triceratops_, were NOT gazelles!  And if _T.rex_ was an active hunter, it
may have been able to mortally wound most animals with a single shoveling
bite, so marathon runs need not have been a part of its repertoire.

I now direct you to illustration 6-5 on page 144 of Gregory S. Paul's
_Predatory Dinosaurs of the World_: "What _Tyrannosaurus rex_ would have
looked like if it had been the slow, elephantine animal some think it was."
 If that's your view of _T.rex_, I give up.

Ralph Miller III <gbabcock@best.com>