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Re: Acrocanthosaurus atokensis locomotion

<< Actually, birds of all types have very, very large ilia.
Many advanced quadrupedal dinosaurs, like titanosaurs and
ankylosaurs, have very large ilia, but are still not
considered swift runners.>>

True. But if you look at the overall lenght of the leg,
ankylosaurus has much, much shorter legs. Thus it might have
a "more advantaged" ilium/leg lenght ratio, but it would
demand MUCH more strides per second. Look at an elephant.
The ilium is about 0.65 the lenght of the Femur, while in
T-Rex, the ilia is about 1.05 the lenght of the Femur, and
on top of that, it has MUCH longer lower limbs and much more
flexible anckles.

Titanosaurs have very large iliums, true, but not in
relative to their sizes. It is very possible that large
theropods had truly exagerated muscle mass (As stated early,
% of body mass in leg muscles grows with size, and ostriches
still have ~20%). In order to move 5 m/s (like in the
study), T-Rex (Or at least MOR 555) would need ~29% of it's
mass in leg muscles, which is very high, but not impossible.

If you compair stoud legged quadrupeds and titanosaurs with
theropods, you see theropods have a truly great advantage
over them. Their longer legs allows much longer strides, add
that with my previous "t-rex is faster than an elephant" thing.

<< It is in some birds well over 30%, and such
osteological features that indicate mass in these taxa
indicate that similar values were present in cursorial
theropods, like *Tyrannosaurus*.>>

Heck, plus it increases with weight. Ulp!

<< I'm not sure which birds/data Jaime is citing, and whether
he is considering both legs and just extensor muscles (i.e.
those with an extensor moment arm about a joint ... and
especially those that are actively contracting at
mid-stance, which isn't all extensors necessarily). So I'd
have to say that I don't know of data that agree with his
statement. The ostrich data I've seen shows ~20% body mass
as valid extensor muscles (both legs total). More work does
need to be done on how much extensor muscles are in living
animals, and I am doing tons of that lately, but from what
I've seen so far in large and small animals; mammals, crocs,
lizards, birds, etc. 20% is toward an extreme end,
especially for bipeds.>>

Yes, but there is nothing alive today that resembles to a
bird which can even hope to come close to 6,000 kg ...

<< Horses _also_ frequently suffer fatal damage if they
fall while running, as far too many thorobred owners and
jockeys have discovered. And yet horses still run! In fact,
they're _specialized_ for it. Giraffes, as someone mentioned
yesterday, also gallop at great speed, and if one ever fell,
 from _that_ height, getting up and walking away would be
unlikely, to say the least.>>

How true. A monky can kill itself falling from a tree. What
does it do ? It climbs in the trees.

<< First, it has been pointed out repeatedly by Farlow et al.
1995, Carrano, and others that the Anderson et al. equation
likely overestimates body mass because it is based on femoral
circumference, which has been shown to scale differently
(more robust) in crown clade birds than in non-avian
theropods, and so it is inappropriate to apply bird
mass-circumference scaling data to much more basal
theropods. 6000kg is a lower end estimate for MOR 555 >>

Actually, from what Thomas Holtz told me, Anderson's method
"underestimates" weight. You will notist in Currie's 2000
_A.atokensis_ paper, he mentions that the Anderson weight
estimate for MOR 555 is 4,160 kg. Vollumetric estimates will
go as high as 6,000 kg, I think.

<< On the model, it's more complicated than "actual body
mass is irrelevant." Clearly body mass is relevant, as Fig 3
in our paper shows,>>

Thanks. I was trying to figure that one out ; Mass is
revelant ... Makes much more sense to me, I couldn't find
the quote Mallon pointed out ... So much contradiction.

Thomas Miller

Le journal des abonnés Caramail - http://www.carazine.com