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Metatarsal fusion/nonfusion: birds/theropods



 As I suffer through yet another in a series of massive head colds I was
soaking in a hot bath today and my swollen, clogged head and feverish mind
got to thinking about a topic that I rarely consider, and that is what makes
a bird and bird and a theropod dinosaur a dinosaur? Are they the same? Or
are they not? Where do you draw the line? Can you draw a line? This has been
a rather confusing, yet exciting topic lately what with the new feathered
dinosaur in China and all.
 
 Pondering this, I got to thinking about the feet of birds and theropod
dinosaurs. Without doing any heavy research, I've reached the following
general findings (correct me if I'm wrong):

 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).
 3. All theropod dinosaurs have unfused metatarsals (except Elmisaurids
which do fuse their metatarsals on the proximal end only and CERATOSAURUS,
which does not count here as the type specimen is pathologic).

 Why is it then that birds have fused tarsometatarsals and theropod
dinosaurs largely don't? Given the great skeletal similarities between both
groups this morphological difference is quite striking. Bird and theropod
feet appear so similar at first and yet not when the fusion/nonfusion
aspects are taken into consideration. Birds large and small have fused
tarsometatarsi yet theropods large and small don't (elmisaurids excepted). 

 Some questions:

 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?
 2. Is it related to flexibilty? For example the foot needs some "give"
during walking/running across the substrate? Could it be a mechanical
reason, related to weight-bearing?
 3. Is metatarsal fusion known in any other extinct/extant groups and if so
what reason/life behaviors are known/suggested for such animals?
 4. Are there any extant birds with unfused metartarsals? Embryologically,
do birds show development of a fully fused tarsometatarsus from the start,
or show some degree of metatarsal separation early on in embryogenesis with
full fusion manifesting later on?
 5. Why do birds have fused tarsometatarsi? I could see this being perhaps
advantageous for large ground birds, i.e. the Ostrich, but why so in small
perching birds?

 I'd like to open this up for discussion. Any ideas?

 Darren Tanke

 
Darren Tanke
Technician I, Dinosaur Research Program
Royal Tyrrell Museum of Palaeontology, Box 7500
Drumheller, Alberta, Canada. T0J 0Y0
             and
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