bird tracks versus theropod tracks

[Kendall Clements <k.clements@auckland.ac.nz>]

Re:     "It's not clear from the email whether all birds today use the
same walking stance that (probable) cretaceous shorebirds used.
How about the rattites? 
        The question is of interest if the walking stance of modern birds 
is an accidental result of an evolutionary bottleneck rather than an
adaptation to flight."

I don't know whether the question relates to morphology of the 
foot, ankle, or entire hindlimb here, but I think Stephen Gatesy's work 
is germaine. The following, which may or may not be helpful, was taken 
from his terrific 1990 paper: 

Gatesy, S.M. (1990)  Caudofemoral musculature and the evolution of 
theropod locomotion.  Paleobiology 16: 170-186

Living crocodiles and alligators have a large muscle called the 
caudofemoralis longus that originates on the tail and inserts on the 
femur.  The caudofemoralis is the main femoral retractor, and as such 
is the main propulsive muscle for the hind limb. In an alligator the 
caudofemoralis originates from caudal vertebrae 3-15, and fills the 
space created by the long transverse processes and chevrons on these 
vertebrae.  The major part of the caudofemoralis inserts via a thick 
tendon onto a structure on the femur called the fourth trochanter, 
while an auxiliary tendon arises from the main tendon and attaches to 
the knee and muscles of the lower leg.  Contraction of the 
caudofemoralis in crocodiles and lizards produces a large arc of 
femoral retraction, and the action of the auxiliary tendon also causes 
the femur to rotate.

In most birds, the caudofemoralis originates from the pygostyle (the 
remnant of the tail in birds) and the connective tissue of the tail 
base.  The caudofemoralis inserts on the outside of the rear edge of 
the femur, and there is no auxiliary tendon connecting the tail to the 
lower leg.  In some birds the caudofemoralis is absent altogether.  
Retraction of the caudofemoralis muscle in birds where present does not 
cause a large arc of femur retraction, and in fact in birds most foot 
displacement is caused by movements of the knee.

During the transition from archosaurs to birds there has been a change 
in the role of the caudofemoralis muscle in locomotion.  If we look at 
tail morphology in different groups of archosaurs we see that there has 
been a trend towards reduction in tail length, and also a trend towards 
the specialization of the posterior third of the tail in dinosaurs like 
ornithomimids and Deinonychus.  In many of these theropods the 
posterior tail was stiffened by bony ligaments, and consequently the 
posterior vertebrae are simple and lack structures for muscle 
attachment. The insertion site for the caudofemoralis on the femur, the 
fourth trochanter, is also reduced in size in the more avian theropod 
clades.  In ornithomimids there is an insertion scar near where the 
fourth trochanter is located in other dinosaurs, but no obvious fourth 
trochanter.  In Deinonychus  there is no fourth trochanter and no 
obvious scar.

If birds are compared to theropods, it's apparent that during each 
stride the femur moves through a much larger arc in the theropod.   The 
femur is held relatively horizontal during locomotion in birds.  Birds 
have more or less lost their tail, which means their centre of gravity 
is located well in front of the hip joint.  For balanced locomotion in 
a biped the weight of the body has to be centred over the legs.  To 
locate their feet under the centre of body mass birds have maintained a 
relatively horizontal femur.  Having a horizontal femur means that 
retraction of the femur only results in a very small arc of movement, 
and so femoral retraction is unsuitable as the major component of foot 
displacement in birds.  That's why birds mainly use knee flexion to 
move the foot symmetrically beneath the centre of mass during walking.

To summarise, in lizards the caudofemoralis retracts and rotates the 
femur during locomotion.  In crocodiles the caudofemoralis is still the 
main femoral retractor, but has less of a role in femur rotation than in
lizards.  In both crocodiles and lizards the caudofemoralis can serve to
rotate the femur and also wag the tail from side to side, because the 
femur projects away from the midline of the body (i.e. sprawling gait). 
In primitive theropods the substantial tail counterbalances the body, 
which means the femur can be oriented subvertically and can be retracted
by the caudofemoralis during locomotion.  However, retraction of the 
caudofemoralis does not result in rotation of the femur or wagging of 
the tail in theropods because the femur is located anteroventrally.  
Thus caudofemoralis retraction pulls the tail down rather than from side
to side in dinosaurs.  In birds the tail is reduced, the caudofemoralis 
is small or absent, the centre of mass is located more anteriorly, and 
the femur is oriented more horizontally.  Femoral retraction is 
therefore reduced, and is replaced as the primary means of propulsion by
knee flexion.  This means that although birds are close relatives of 
theropods, there are major differences between the two groups that have 
to be considered when assessing locomotor mechanics.

----------------------
Kendall Clements
k.clements@auckland.ac.nz