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more stuff



>Dear all,
>
>Why did the metatarsals of birds fuse together? Some time ago, some one
>(I'm sorry, I don't remember the name) posted onto this list that the
>fusion was an adaptation to hopping from branch to branch. I think I
>didn't get the logit. Would it be that the fusion made the
>tarso-metatarsus part of the limb stronger, and so less vulnerable to
>facture during rapid deceleration occuring when the bird "lands" on a
>branch.
>
>If you have looked into this matter, I'd like to hear from you.

        Somewhere I read that the most efficient way to bear a load for a
given mass of material is with a single beam. So if birds needed to
minimize the mass of bone needed to withstand a given stress in the
metatarsus, fusing all the bones into a single column might be the most
efficient way to do that. Fusion of bones in birds is of course a pretty
common thing (carpometacarpus, tarsometatarsus, synsacrum, skull, notarium)
as of course it was in pterosaurs as well (carpal blocks, synsacrum, skull,
notarium). I think Buhler has a paper called "light bones in birds" where
he talks about this.
        Cursors may be doing something similar in using a single-beam
construction to minimize weight. Just like your running shoes need to be as
light as possible so do your distal limbs; using a single toe and
metatarsal in horses, or fused metatarsals in things like deer, may be one
way of accomplishing this. Velocisaurus (probable ceratosaur, maybe even
abelisaurid?) seems to be doing something similar by emphasizing the
central metatarsal.
        There may be tradeoffs, which would explain why this construction
doesn't work for everything. A hollow cylinder is a far more efficient use
of bone than a solid one in resisting bending forces, for example, but is
far more susceptible to failure by buckling, which is what happens when you
bend a straw. Fusion may make some form of failure more likely, having
multiple metatarsals may create a useful redundancy- a fracture that would
cause failure of a fused metatarsus might merely break a single metatarsal
if they aren't part of the same unit. And of course things which aren't
obviously under weight-reduction stresses also fuse bones. Armadillos fuse
the tibia and fibula and there are xenarthrans with about 15 vertebrae in
their synsacrum. Given the amount of armor armadillos are already carrying
around, I suspect the weight savings that might be realized from a fused
hindlimb couldn't be that significant.


Jaime Headden wrote:
> you can
>easily arrive at a retroverted hallux by twisting the shaft of the
>first metatarsal, which retains a long contact with the second
>metatarsal, and the contact would in more advanced birds shift to the
>sole of the foot and further orient in line with the third toe.

        There's another way to get a reversed digit 1 and a good example is
within easy arm's reach. Manual digit I in humans may or may not have some
long-axis torsion (don't have any refs or cleaned specimens to examine
handy) but at least part of its opposability made possible by rotating the
bone on its long axis relative to the other metapodials. I've looked at a
lot of theropod first pedal digits- dromaeosaurs, troodontids,
caenagnathids, alvarezsaurids, and MT I from a therizinosaur, as well as
tyrannosaur, allosaur, and megalosaur MT I, and it is a possibility that
something analogous was going on in Maniraptora although probably not in
more primitive theropods. In addition, maniraptora have a much more
flexible joint between MT I and I-1; it may have allowed the first digit to
rotate relative to the metatarsal. A limited degree of facultative
reversability may therefore have been present in the first digit and may
have been how the digit began to develop opposability. Based on what I've
seen of them, the articulated dromaeosaur feet the AMNH has are not good
arguments for an unreversed hallux in the Dromaeosauridae. Anyways, some
birds have unreversed the first digit, e.g. swifts, and it is held more or
less medially (as opposed to reversed) in some other birds, so this stuff
about first digits not being able to unreverse from the opposed position is
inaccurate. Also, the first toes of dromaeosaurs, caenagnathids,
therizinosaurs and troodontids are all different, so it's dangerous to make
sweeping generalizations and assumptions about bird evolution without
sampling the whole range of variation, just as for example dromaeosaurs,
troodontids, oviraptorids,  Caudipteryx, and alvarezsaurids all had their
own unique tails, or pelves, or whatever so you can't just look at
specialized dromaeosaurs and draw all these conclusions about bird origins
like we've tended to in the past.

-n