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Re: Were some dinosaurs descended from birds?



At 02:12 AM 2/9/97 -0800, Ed Uber wrote:
>Flightless birds keep evolving, from times as early as the Cretaceous. 
>This process would be even easier for birds which still retained
>functional claws on their forelimbs.  It seems to me that the most avian
>of dinosaurs are very easily explained as being secondarily flightless. 

Although a number of people have suggested that some dinosaurs are
secondary flightless derivatives of birds, most notably for this list Greg
Paul, I confess to being skeptical about this. There is more to
flightlessness, or at least secondary flightlessness, than merely the loss
of flying ability. Storrs Olson has shown that most flightless birds today
have many features that are, in essence, chick-like. These include body
proportions and proportions of various skeletal elements, feather
structure, etc.. He suggests that paedomorphosis may be the mechanism by
which birds have become secondary flightless.  
 
Flightless can apparently be achieved quite rapidly in bird evolution.
There are, for instance, examples of modern birds for which some subspecies
are flightless and others are not, including the Brown Teal of New Zealand
and the White-throated Rail of Madagascar and surrounding islands. However,
in no flightless bird of which I am aware, either living or extinct, is
there any evidence that the forelimbs have reacquired their grasping
function (I am not knowledgeable about the details of the spurs or claws
found recently on the wings of phorusrhacid birds from South America, but
wing spurs are a feature of a number of flying birds today). The degree of
fusion and reorganization involved in transforming the wings of almost all
birds into flying structures is such that to return them to something
resembling the dinosaur forelimb may require a highly unlikely degree of
genetic change.  
 
I would therefore expect that any dinosaur that is put forward as secondary
flightless should exhibit the kinds of characters we see in flightless
birds today. The problem come of course, is that if a secondary flightless
dinosaur evolved from an animal like Archaeopteryx it might be impossible
to tell what had happened, because Archaeopteryx lacks many of the
modifications seen in later birds and very closely resembles a small
flightless dinosaur anyway. I am not certain how you would identify such a
derivative, unless you had enough forms available to do a cladistic or
phylogenetic analysis and show that for other reasons it could only have
been derived from flighted forms. I suspect that we are not in a position
to do so given the current fossil record, but palaeontologists on this list
may wish to correct me on this point.  
 
The animal most often put forward today as a flightless dinosaur-like
derivative of birds is Mononykus, but there has been considerable criticism
of this conclusion in the literature and I remain unconvinced that we are
able at this time to determine just what is animal really is. 

--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net