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Re: Caudofemoralis and theropod limb proportions



On Wed, 21 May Peter Buchholz wrote:

>Here's an idea that's been running through my head.  It is often claimed that
>dinosaurs had sub-standard running abilities comared to mammals and birds
>because they had disproportionately short metatarsals, but a thought occured
>to me.  Perhaps the unusually long metatarsals of modern birds and mammals
>are an evolutionary response to the reduction of the primitively huge
>caudofemoralis muscle seen in dinos and therapsids.

I think that you may be on to something, but that it is much more complex
than your generalization.  First, as far as theropods are concerned, the
metatarsals of non-birds may not be "disproportionately short" compared to
birds.  If the combined lengths of the femur, tibia (or tibiotarsus), and
metatarsals (or tarsometatarsus) are considered limb length, the
metatarsals account for anywhere from 13-45% of the limb in living and
extinct theropods.  These extremes turn out to be birds, spanning from
penguins to flamingos.  Non-avian theropods fit within the bird range,
running from 17% in Acrocanthosaurus to 28% in some ornithomimids.  Quite a
few ground-dwelling birds, including many galliformes have tarsometatarsi
less than 28% of limb length.  Archaeopteryx sits in the 22-26% range.  I'm
not pulling these numbers out of my head, but from a paper I did with Kevin
Middleton on theropod limb proportions; it's due out in JVP this summer.

Yet, Peter is right that many of the best cursors, like ostriches and rheas
at 30-40%, do have relatively longer tarsometatarsi than even the most
gracile ornithomimids.  I agree with his point that comparing birds to
non-birds might be an apples and oranges thing, even though oranges evolved
into apples in this case.  I pointed out this possibility in a study of
theropod bone scaling (1991, J. Morphology 209:83-96)

Nick Longrich followed with a message on the same day:

>What may be part of the explanation is found in Gill's _Ornithology_ -= if
>you are a bird and want to sit down, and you can't move your femur much,
>your tibiotarsus and tarsometatarsus had best be the same length- other
>wise, as you try to sit down your center of gravity is either going to be
>ahead of you or  behind you depending on how long the elements are to each
>other.

Unfortunately, this just-so story has been perpetuated in ornithology texts
for years.  Consider the data:  of all birds, only flamingos and rheas
approach a tibiotarsus/tarsometatarsus ratio of 1.  In all other birds and
all non-bird theropods the tibiotarsus is longer than the tarsometatarsus.
In penguins the tibiotarsus can be over four times the length of the
tarsometatarsus.  Sure, we can admit that penguins have higly derived legs
and don't sit like most other birds, but consider that Archaeopteryx has a
ratio of about 2.2, and that an "average" bird limb would have a ratio of
1.5.  The simple "collapsing Z" model of limb design can't be a very strict
rule.  One reason for this is that it ignores the toes,which can help even
out this length disparity.

I think that a much stronger constraint is limiting femur length in birds.
The lower limb elements are where proportions are freer to evolve.  If
birds are selected for long legs (including long running legs), the
tibiotarsus and tarsometatarsus elongate, whereas the femur remains more in
size with the body.  The result is an ostrich or flamingo or stilt that has
a tiny femur relative to the tarsometatarsus.

Nick then finished with:

>        Here's a question- if the avian center of gravity is roughly at
>the knees when the animal is on the ground, what about during flight? I
>would assume that you would want it at the shoulders. Does extension of
>the neck forward move it up to the shoulders?

Many birds don't extend their necks forward during flight, but the center
of gravity (or mass) doesn't need to move in most birds.  Recall that a
bird's knees are so far forward that they are well in front of the elbows
when the wings are folded.  During flight, the elbows are not fully
extended, but quite flexed.  The humerus is typically only abducted 45-60
degrees from the sagittal plane during flight in starlings, pigeons and
magpies (birds that have been viewed with high-speed x-ray films by Goslow,
Jenkins, Dial, Tobalske and colleagues).  Also remember that the wings'
center of lift is probably behind the shoulder closer to the elbow.  Even
if the center of lift is in front of the center of mass, most birds have
additional lift from the tail to compensate.  Ken Dial and I think this
tail lift was critical during the early evolution of flight before the
acquisition of a modern wingbeat cycle.

My overall read on this subject is that birds and quadrupedal mammals
differ from theropods in more than just the caudofemoral retraction system.
An equally, if not more, significant feature is that birds and quadrupedal
mammals have their center of mass well in front of the hip joint.

Sorry for the length.  Cheers,

steve


Stephen M. Gatesy
Dept. of Ecology & Evolutionary Biology
Box G-B209
Brown University
Providence, RI 02912
401-863-3770
401-863-7544 (fax)