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bird tracks versus theropod tracks
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
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.