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Re: 11th specimen of Archaeopteryx



Don Ohmes <d_ohmes@yahoo.com> wrote:

> 1) Perching came prior to reversal (repeating Jason's point).


I agree - although with caveats. It appears that a larger, lower
hallux preceded reversal.  Some basal birds have a hallux that was
oriented medially or posteromedially, rather than fully posteriorly as
in modern perchers.  In these basal birds, the hallux is at around the
same level as the other three digits, and all toes (including the
claws) are enlarged.  However, the hallux of _Archaeopteryx_ was still
too high and too short to be useful for any kind of grasping.  There's
no evidence of even incipient grasping ability involving the hallux.


> 2) An animal that forages cursively has cursively foraging feet, so to speak
> -- one would not expect the basal pes condition to be rapidly re-arranged
> while a cursorial lifestyle was still conserving it -- especially just to
> get a good night's sleep. A slow transition to full-on arboreality, which is
> what reversal of the toe represents, is not surprising, any more than the
> gradual transition to the knee-walking condition is. Implied is that all
> modern cursorial birds are secondarily so -- which hardly seems
> controversial.


A slow transition to full-on arboreality is one thing.  However, in
_Archaeopteryx_ there is nothing at all to suggest that the hallux was
involved in grasping of any kind.  The turkey and the secretary bird
both forage on the ground.  In these modern birds, the hallux is
elevated such that it doesn't interfere with walking or running; but
it's still low enough and large enough that it's effective for
branch-grasping during roosting.  So it's difficult to argue that a
cursorial lifestyle was selecting against a longer, lower or reversed
hallux in _Archaeopteryx_.  (Modern ratites represent an extreme
example, in which superfluous toes have been lost altogether; but
ostriches, rheas, emus etc are highly specialized for cursoriality, as
were ornithomimids.  _Archaeopteryx_, dromaeosaurid, and
jeholornithids were not; the hindlimb proportions tell us they were
not the most speedy of theropods - far from it.)


If _Archaeopteryx_, microraptorines etc were roosting in trees,
there's simply no excuse for *not* having a hallux that was low enough
and large enough to grip branches - ideally combined with at least
some reversal of the hallux, for the pes to be capable of some kind of
opposable grip.  Invoking a cursorial/terrestrial lifestyle is not
reason enough because, as I said, the hallux can be enlarged without
being an impediment to a terrestrial lifestyle.


Other non-avian maniraptorans enlarged the hallux for various reasons,
like the dromaeosaurid _Balaur_ (for predation) and the therizinosaurs
(for weight support).  Why not _Archaeopteryx_ (for roosting)?


> 3) The difficulty an Archie-type animal would encounter
> not-falling-while-sleeping has been wildly exaggerated.


I disagree.  Roosting is not easy for a long-legged bipedal cursor.


> 4) The negative consequences of an occasional fall by a feathered animal
> that could likely fly at least a little and could certainly glide seem
> minimal -- much less negative than being stomped or eaten on a dark night.


And yet there were small, flightless, short-armed theropods that
presumably could not climb trees (never mind perch) throughout the
Cretaceous - such as _Mahakala_ and _Patagopteryx_.


> Looking at the generic theropod skeleton and wondering what modifications
> you could install to make the poor thing arboreal w/out starving him leads
> to this -- give him wings. Once he can fly, then you can mess around w/ his
> feet.


Again, I don't think having a pes adapted for perching or roosting
would interfere ("mess around") with the ability to forage on the
ground.  There are abundant examples in the modern world of birds that
spend their waking hours on the ground foraging and hunting, and
retreat to the trees at night to roost.  The hallux is large enough
(and reversed enough) to be useful in perching.





Cheers

Tim