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Many, many thoughts & responses re: the Hutchinson-Garcia paper



Okay, where to start?

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Richard W Travsky
>
> Ok, so what about other large carnivores?
>
As I stated in my summary, and as the authors state, these problems would
affect all comparably sized dinosaurs.  In fact, the non-tyrannosaurs would
arguably suffer MORE, lacking a) more cursorial limb proportions; b)
arctometatarsi; and c) less hip muscle mass (as Tracy pointed out,
tyrannosauroids (and ornithomimosaurs, and alvarezsaurids) have ilia that
contact on the midline; this is quite probably an adaptation to increase
overall muscle mass in the legs).

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Toby White
>
> A *really* slow theropod is almost harder to understand than a rocket
> raptor.

Let's keep things in perspective: 5 m/s is still fast for a big animal, and
that is BELOW the walk/run transition.  Incidentally (for thought
experiment's sake), if a _T. rex_ of MOR 555's size could move at Fr = 3.8
(the dimensionless "speed" of Day et al.'s Middle Jurassic tracksite), that
would be 9.1 m/s.

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> David Marjanovic
>
> What about various weight estimates? They used 6 t... did they
> also try 4 t,
> for example? What about, say, the elastic effects of tendons and
> cartilage?
>
As for the latter, these are not included in the model as far as I can tell;
I don't want to speak for John, but I would guess he would agree that this
(like any) model can always be amplified with further information.  Of the
criticisms raised so far, this is one of the most significant (i.e.,
incorporation of the role of elastic structures in biomechanical studies in
locomotion).

As for the former, they chose the 6 tonne mass for MOR 555 (and remember,
they are explicitly using a particular individual, not a generalized form)
based on Farlow et al.'s 1995 paper.  However, between the equations in the
paper and the Supplementary Information, ANYONE is free to recalculate based
on their own reconstuctions.  Ain't science fun?

As for comments based on the postural differences between _T. rex_,
_Alligator_ (why do people not comment on that aspect!  I find that even
more problematic!!), and _Gallus_: their model seems to be very robust for a
variety of postures and gaits and limb orientations, and is being developed
in the world's preeminent zoological biomechanical labs.  Does that mean
that they are correct?  No.  On the other hand, if the counter argument is
"Fully adult _T. rex_ MUST have been capable of Fr >2.5, because they
wouldn't be cool otherwise", then it is time to re-evaluate your logic.

[An aside: of anyone on this list, with the exception of Greg Paul, *I*
should be the main protestor, having been one of the most public published
proponents of cursorial tyrannosaurs.  That I am not protesting too
vigorously might be a reason to stop for a moment and think rather than
react...].

As for Froude numbers: the whole POINT of dimensionless speed is to get
around the problems of size and shape.  It resolves to "ratio of centripetal
to gravitational forces".  One might be able to argue that the Fr of
neornithine birds should be calculated by the "functional limb length" of
Farlow et al. (2000) (i.e., the tibiotarsus + metatarsus, rather than femur
+ tibiotarsus + metatarsus).  However, as established by the papers of
Christiansen, Hutchinson, and Carrano over the last five years or so, the
hindlimb mechanics of the long bones non-avian dinos and of striding mammals
are very similar, even if the set of muscles that do the pulling are
different, and hence one can employ dynamic similarity models.

>From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
Michael Lovejoy
>
> Secondly, I'm not much good at equations ( what does the ^ mean? ).

"to the power of".  So, "^2" is squared, "^3" is cubed.  (Comes from the
fact that general text formats can't always handle superscripts).

> This one seems to imply
> that the higher the hip, and therefore the longer the leg, the
> harder it is to run.  Is this what it means or am I just reading it wrong
( wouldn't be the > first time )?

Actually, the longer the leg the faster you have to be before you run rather
than walk.  So a stumpy legged critter (say a hobbit) has to move from a
walk-to-jog transition before a longer legged one (a ranger); while the long
legged creature is still walking, the short legged creature may already be
running.

> Lastly, when calculating T-rex's required leg muscle mass, does it take
into account the
> tail-based caudofemoralis? This muscle is a big factor in theropod
locomotion.

Yes, yes, yes.

So, do Hutchinson & Garcia have The Answer (tm)?  No.  However, any
additional work to the contrary must find some way of demonstrating a flaw
in their model.  Some potentials include:
        * If inclusion of elastic structures in the model can somehow alleviate 
the
muscle mass necessary (although, as they are dealing specically at the ratio
of Ground Reaction Force to body weight on one leg at the mid-stance of fast
running, I am not certain that that will change too much there).
        * If someone finds an honest-to-goodness _T. rex_ trackway of an 
individual
of MOR 555's size or larger which is estimated at moving at Fr > 2.5 (note:
at this speed, you would need a BIG trackway...).
        * Redo their math and see if they miscalculated (it has happened 
before).
        * Something really cool that no one has mentioned yet (ditto).

And a final reminder:
According to their model, _T. rex_ was not a fast *runner*.  This doesn't
mean it was slow!  If MOR was doing 5 m/s, it could still probably catch
most of the people on this list if it were chasing us!

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