Re: Protoavis?

[Jonathon Woolf <jwoolf@erinet.com>]

Jeffrey Martz wrote:

<snip>

>      Currie and Zhao do not explicitly state whether or not they
> consider _Protoavis_ to be a theropod, but one of thier opening statements
> are highly suggestive: "Although _Protoavis_ has characters suggesting
> avian affinities, most of these are also found IN THEROPODS [boldface
> mine], which considerably weakens the claim that it is a bird." (p.2243)
>      They later go on to say "Comparison between the braincases of
> _Troodon_ and _Protoavis_ does suggest that they are amazingly similar for
> animals seperated by 160 Ma."  They mention strong similarities between
> the brains; both have pretty big brains, an enlarged cerebellum that
> shifted the optic lobes ventrolaterally, and also has a large floccular
> lobe.  The inner ear is also pretty similar and bird-like in both: "both
> have marked differentiation of the canalicular systems and the cochlear
> process.  The vestibular region is relatively small and located in a
> ventral position to most of the anterior and posterior semicircular
> canals.  The anterior semicircular canal is significantly longer then the
> others.  The cochlear process is a relatively long, vertically oriented
> tube"(p.2244) , but also... "_Protoavis_ seems to be less bird-like then
> _Troodon_ in having only a single exit for the trigeminal."(p.2244)
>       There is a pretty extensive list of other (presumably bird-like)
> similarities between _Troodon_ and _Protoavis_, and the authors finish the
> section off with: "The braincase of _Protoavis_ is remarkably like that of
> a troodontid, and in our opinion has a few unique avian characters.  The
> latter would include the streptostylic quadrate and reduction in size of
> the olfactory lopes, but the characters are not robust enough to identify
> _Protoavis_ as a bird."  Draw your own conclusions, but there it is.
>       As far as _Unenlagia_ being more readily accepted then _Protoavis_
> in spite of its arguably more fragmentary condition, I don't consider this
> to be as unreasonable as you make out.  With the dinosaur-bird connection
> already very well established by many decent theropod specimens, another
> theropod with additional _Archaeopteryx_ like features is not as
> incongruous to the overall apparant pattern of bird evolution as
> _Protoavis_.

I don't have any specific comments on this, just some ramblings that the
above seemed a good jumping-off point for.  I'm no anatomist, so much of
what Currie and Zhao say might as well be in Esperanto.  However, the
more I hear about _Protoavis_, the more it puzzles me, on a variety of
levels.  A few specific bits for examples:

* _Protoavis_ is usually reconstructed with the forelimbs held up and
out, like a bird's.  However, if no details of the shoulder anatomy are
known, how do we know which way the arms actually articulated?  They
make sense either as unusually short wings or unusually long theropod
hands.  

* _Protoavis_ is always reconstructed as a biped.  A bipedal animal
older than six million years can be one of only three things: a
dinosaur, a bird, or some member of a group as yet unknown to science. 
Why is that third possibility never mentioned?  

* Lessem's book KINGS OF CREATION (1992) makes it sound as if Chatterjee
has never actually claimed this animal was a _flying_ _bird_.  At least
back then, Chatterjee apparently saw it as an early stage in avian
evolution.  Chatterjee has never claimed that it was a modern bird, or
even as advanced a bird as Cretaceous ichthyorniths.  As I understand
it, no one else has ever been able to make a detailed study of
_Protoavis_.  Chatterjee's opinion is (or until recently, was) the only
informed one that exists.  So how did the lines get drawn as "it's 100%
bird" versus "it's either a theropod or a chimaera"?  Is there no chance
that _Protoavis_ is, say, a very early scion of the line that led to
maniraptoran dinosaurs and through them to birds?

* The bones of _Protoavis_ were found jumbled into sandstone nodules,
completely disarticulated, which is why some people think it's a
chimaera.  What are the chances that it's reworked somehow from Jurassic
sediments?

My personal feeling on _Protoavis_ is that it might be a bird, a
theropod, a birdy theropod, or something else -- but if you want me to
accept it as a bird, find me one with feather imprints.  Nothing else is
going to clinch it.

-- JSW