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Re: Metatarsal fusion/nonfusion: birds/theropods



At 07:41 PM 3/15/97 -0800, Darren Tanke wrote:
> 1. All extant birds have fused tarsometatarsi (the "tibiotarsus").
> 2. All fossil birds have fused metatarsals, possibly except ARCHAEOPTERYX
>(although apparently the Maxberg specimen shows co-ossification).

Does the new Malagasy bird have fused metatarsals? The note in Natural
History seemed to imply otherwise.  Anyway, I believe that manipulation of
poultry chick embryos (not being home I can't check the reference) has
produced birds with unfused metatarsals (or at least embryos).

> 1. What structural and mechanical advantages are there to having a fused
>tarsometatarsus vs. unfused? Is the difference related to being a flier vs.
>ground bound?  Has anyone published on this?

Just guessing here:  first, as has been suggested, there may be no
advantage at all.  We may be seeing the results of genetic linkage relating
to fusion of carpal elements - though the fact that enantiornithines have a
different pattern of tarsal fusion makes me a bit skeptical of this.  If
two patterns arose independently in bird evolution it suggests that there
is an adaptive role here.

Secondly - if there is an adaptive role, what is it?  One thought (or
guess) I can offer is that a fused metatarsal may produce a thinner,
lighter leg that may either produce less drag in flight (if early birds
flew with legs dangling or extended) or be easier to tuck away in the
feathers.  Possibly there are tensile stresses involved as well, perhaps
relating to arboreal habit, long leaps, the ability to cling to
non-horizontal surfaces like trunks and limbs without exposing the leg to
shearing stresses, etc.  Or we could be dealing with an adaptation to
reduce the bulk of the skeleton for flight.  Or all of these - they are not
mutually exclusive.


--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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