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Re: 11th specimen of Archaeopteryx



Jason Brougham <jaseb@amnh.org> wrote:


> In discussing this we are discussing animals, perhaps one lineage of animals, 
> where evolutionary transitions were occurring (where one lineage gave rise to 
> proper birds). I am not an expert on
> evolutionary theory, but there must be a state that sometimes occurs in 
> evolutionary transitions where organisms have not yet adapted anatomically to 
> a new function or habitat, but are instead adapting
> by sometimes straining to the maximum of their physical abilities. This 
> strain, in turn, provides the natural selection that leads to the anatomical 
> adaptation. In other words, some small paravian or basal
> bird was in trees without a hallux, and this provided the selection pressure 
> for a hallux to evolve.


I understand what you're saying, and it's an excellent point.
Morphological transformations have to start somewhere.


Nevertheless, in feathered non-avian theropods and early birds, a
reversed hallux is conspicuous by its absence.  The selection pressure
for paravians to evolve a perching pes cannot have been very strong,
given that it is not until long after wings first appeared that we see
even incipient development of a reversed hallux (around the
confuciusornithid/sapeornithid level of avian evolution).


It appears that distal migration and elongation of the hallucal
occurred prior to retroversion.  The hallux of paravians is lower on
the foot than in the ancestral theropod condition.  This is most
apparent in _Epidendrosaurus_, in which the hallux is around the same
level as the other three toes - but still not reversed.
Unfortunately, we don't know enough to be confident whether the
condition in _Epidendrosaurus_ was directly ancestral to the avian
condition; or autapomorphic for _Epidendrosaurus_; or a juvenile trait
present in many or all paravians and later lost in ontogeny.



> There are also known cases where a group of animals perform a function which 
> is an important part of their biology and they simply never show an 
> anatomical adaptation to this function, it simply never
> arises. I now collect photos of turkeys and other basal neornithines brooding 
> their chicks in trees. Looking at these images the parents certainly look 
> awkward and we'd all feel a lot more comfortable
> about the situation if they had feet like Cracids. But they don't, they are 
> somewhat ill suited to this sub-habitat, and  future Paleontologists will 
> probably never ever be able to demonstrate that they
> behaved this way.


Turkeys have a reversed hallux that is fairly low on the foot.  The
first toe just isn't *as* long as in certain other birds (such as
cracids) because the first metatarsal is relatively shorter.  So
turkeys have a perching foot, complete with a hallux that is large,
low and reversed.  The hallux is not as specialized for perching as,
say, guans, because turkeys spend much more time on the ground, and a
very long hallux can be a nuisance when walking or running.


So the hallux of turkeys is completely adequate for perching.  The
hallux of _Archaeopteryx_ & co wasn't even adequate; it was utterly
useless.  Compared to the turkey hallux it was shorter, higher, and
not at all reversed.





Cheers

Tim