The Torso Tilt and Stiff Tails .-- Function in Locomotion

["Jaime A. Headden" <qilongia@yahoo.com>]

Jura (archosaur@reptilis.net) wrote:

<Facultatively bipedal lizards with autotomized tails don't stop
running bipedally. They just compensate for their lost tails by
running holding their bodies more erect. Could a dinosaur that
attempted this tail loss strategy not do the same thing?>

  Most bipedal dinosaurs do, merely because the knees must be
flexed neurtrally and that the trunk and forequarters and neck
and head will weigh more than the tail itself, over balancing
the animal. To compensate the animal must bend the knees and
bring the center of gravity between the feet, somewhere behind
the shoulders in a level walking posture. Resting, or standing,
this point would shift caudally as the animal tilts upward in
the front.

  However, when running, the animal does not, as even birds will
"level" out when sprinting. The running tailed biped will have
to ability to compensate for the balance shift by placing one
foot in front of the other in an alternating pattern, resulting
in a side-to-side sway and a shift over each foot of the COG.
Thus the running animal is balanced. This is where a model is
needed to test a running animal with a shortened tail. To test
the ability to compensate. Ken doesn't want to do this and I
don't have the time or means to do it, but is there an attempt
at this approach before? Are there studies testing the long or
short of the problem? If so, I'm sure this debate will benefit
from it.

<Also, as HP Ken Kinman mentioned, we are not talking about a
heavy tail loss, just the tip or first 3rd. When one is running
from a predator, it is the tail that is the easiest thing to
grab a hold of. Even if it couldn't autotomize, it wouldn't
lower its chances of being the first to get snapped.>

  Ken wants to use a Triassic theropod for the test, then lets
do so: *Coelophysis*. As Rob Gay and Mickey Mortimer have
previously talked about, and which the former showed in Berkeley
earlier this month, *Coelophysis* has a tail that is in the
distal 1/5 or so an essentially locked rod -- it may even
suppoort a tuft of some sort at the end [more later]. The
stiffening of the tail, as in tetanurans, is a locomotory
feature: in increases the ability to turn by providing a stiff,
inflexible means of shift weight from one side of the body to
the other, in tandem with the step pattern. This sort of tail is
functional to locomotion, and to remove a part of it would
actually imperil the ability to turn at high speeds, and thus
imperil the animal's way of life (hunting, escape, etc.).

  Take another stiff-tailed animal: *Velociraptor*. Here we have
an animal that is more likely than not to have a fan of
something on its tail. Hartman, 2000 at SVP, Mexico City,
suggested that the tail had an implicit aerodynamic function in
aiding the turn, more so than in non-tufted tails, and this may
have driven the form of the tail distally. So it is likely that
the stiff tails of *Coelophysis* (also seen in *Elaphrosaurus*)
may have had a fan-like structure of something to aid in
turning, perhaps even a tuft of "dinofuzz" or modified scales
[not the same thing, if you've talked to Brush].

  Take an animal that needs it's tail distally for the ability
to run and turn, and thus hunt and escape, and chop off a bit of
it, and you will get a difference in its ability to do this. How
much of an ability is lost in not known without forming a model
[mechanical or mathematical]. So until then, I'd say this
discussion of diversionary tail nippers is moot.

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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