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Re: Ah ha! That's where therizinosaurs came from



Mike Keesey <keesey@gmail.com> wrote:

> On Mon, Aug 15, 2011 at 9:10 AM, Don Ohmes <d_ohmes@yahoo.com> wrote:
>
>> My resources are limited, but what I can glean indicates that the geo-record
>> shows powered flight as pre-dating the knee-walking pelvic limb condition
>> seen in modern birds, ratites (and phorusracoids, btw) included.
>
> Interesting observation. Are there other taxa with long legs and short
> femora? Also, didn't flight develop before the appearance of short
> femora in the avian line? (Even assuming a conservatively late
> appearance of flight, around Pygostylia rather than Eumaniraptora.)


A short femur (i.e., relative to the lower hindlimb) is associated
with avian-style "knee-walking", because the subhorizontal femur is
largely decoupled from stride generation, and the lower leg is
elongated to become the effective hindlimb.  Stride generation occurs
mostly via parasagittal rotation of the lower leg about the knee
joint.  This has been regarded as a flight-related feature, because of
the forward shift in the center of mass associated with the avian
bauplan (especially the much shorter tail, and expansion of the
pectoral skeleton and musculature for flight).  Thus, it was argued
that because _Caudipteryx_ had proportionally long hindlimbs, with a
hindlimb/trunk ratio comparable to modern birds, it adopted
avian-style locomotion, and was therefore secondarily flightless
(Jones et al., 1998).  This was rejected by Christiansen and Bonde
(2002), whose calculations showed a more posterior center of mass for
_Caudipteryx_ than found by Jones et al.  Further, Christiansen and
Bonde also found that the femora of birds tend not just to be short,
but also proportionally robust, with a thicker femoral diameter to
resist the increases in bending and torsional moments about the shaft.
 This was lacking in _Caudipteryx_.


Also, there may be other osteological correlates of avian-style
"knee-walking.  This is from Makovicky & Zanno (2011):

"Without a long tail to counterbalance the body
as in typical nonavian theropods, the last common
ancestor of _Confuciusornis_ and more derived birds
evolved a posture where the knee is permanently
angled to bring the center of mass above the foot
and offset the loss of a long counterbalanced tail.
An ossified kneecap, which is unknown in nonavian
dinosaurs, is present in _Confuciusornis_ and
more derived birds and serves to stabilize the bent
knee."

However, I don't know of the original source that describes an
ossified kneecap in _Confuciusornis_.


Finally, although it's tempting to associate avian-style locomotion
directly with powered flight, it is more correct to say that it's a
direct consequence of the anterior shift in the center of mass (as
mentioned above).  If this shift occurred *before* the origin of
powered flight - such as gliders favoring a more forward center of
mass - then long legs and short femora cease to be a flight-related
character.  If _Confuciusornis_ had avian-style "knee-walking", but
was incapable of powered flight, then "knee-walking" did precede
powered flight in avialan evolution.





Cheers

Tim