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Sauropod feet and ancestry ...
Keesey said (with response):
"> << Really, it appears to be a weight-related feature, but
hopefully this can
> be clarified by further study >>
> Don't think it's weight-related. Alxasaurus is a far smaller animal
> Tyrannosaurus, yet Alxasaurus shows the segnosaurian foot whereas
> Tyrannosaurus has a typically theropodan (albeit massively built,
> foot with highly reduced first toe. If it were purely a weight
> would expect tyrannosaurids and many other large theropods to show
> segnosaurian readaptation of the first toe.
Perhaps it's a graviportal issue?"
Well, remember that graviportal animals have a number of features
besides modifications in the feet. The limbs of graviportal animals,
like sauropods or elephants, are held as vertical columns during most
of the gait. This is because bone works best in compression, and
therefore the weight of the animal can be safely supported and moved
with this system. Also, the femur tends to be longer than the crus
(tibia and fibula).
I am not aware of the exact proportions of segnosaurs, but they do
not strike me as inherantly graviportal (maybe medioportal?). I have
to admit the feet of segnosaurs do look an awful lot like those of
prosauropods, but since other characters appear to unite them with
theropods, and since it was mentioned that the first digit does not
make contact with the tarsus (as in prosauropods), this foot condition
may be due to variation on the saurischian pes plan (speculation here,
to be sure).
Sauropod feet are unique in this way from other saurischians. Most
saurischians supported their weight with digits II, III, and IV. In
contrast, sauropods supported their weight with digits I, II, and III.
Their tarsus is also reduced to essentially the astragalus, although
the calcaneum was usually present as an ossified, fist-sized blob at
the end of the fibula. We don't know of any ossified distal tarsals,
so it may be that the metatarsals had an almost direct articulation
with the distal articular surfaces of the astragalus and calcaneum.
More on this later -- something came up. Like Star Wars, expect a
sequel (or a prequel?).