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Re: Ah ha! That's where therizinosaurs came from



Dr. Orenstein,

Your email was most thought provoking. It brought some clarity to my thoughts.

Please, all experts on the DML, critique what I write below. Have I got it 
figured out here?

It helped me realize that there is no single feature that is unambiguously 
associated with volant taxa. Not a furcula, not a triosseal canal, nothing. 
Rather it is more a matter of proportions between the elements: long arms, big 
pectoral girdles, etc.

Therefore we cannot prove that ratites had flying ancestors based on their 
skeletal morphology. Nothing in their skeleton proves that they don't retain 
the primitive condition of the avian lineage before they attained flight.

I believe that someone wrote on DML earlier, or else I am paraphrasing, that 
the only way to obtain evidence that an animal had a flying ancestor is to 
determine the most statistically likely phylogeny of that animal, which is to 
say its cladogram. Because so many birds that were more primitive than ratites 
had the flying proportions and specializations, it is more likely that ratites 
are secondarily flightless.

Indeed, I found a paper (Philips 2009) that used complete mitochondrial genomes 
to demonstrate that moa and tinamous are one another's closest relatives, 
suggesting that their last common ancestor was a flying paleognath.

But cladograms don't tell us the actual ancestors. They tell us the sister 
groups, and if you found an actual ancestor, as improbable as that is, it would 
show up in a  cladogram as a basal sister group. So, I guess, this debate is 
outside science. There is no rigorous or empirical scientific method that can 
demonstrate if an animal had a flying ancestor or not. 

So I guess we can now go back to speculating.

-Jason

> 
> On Aug 11, 2011, at 4:39 PM, Ronald Orenstein wrote:
> 
>> Jason Brougham wrote:
>> 
>> "If Mr. Paul points to all manner of biological similarities between ratites 
>> and dromaeosaurs as support for his hypothesis that both are secondarily 
>> flightless, as he does, then I may point to the differences as evidence 
>> against that hypothesis. In other words, if ratites can be compared to 
>> dromaeosaurs AT ALL (despite being different taxa in different geologic eras 
>> and periods), and if they can be compared in order to support Mr. Pauls's 
>> hypothesis (the "neoflightless" hypothesis for non -avian maniraptorans), 
>> then their differences can be weighed in the balance of evidence against 
>> that hypothesis."
>> 
>> I'm not the authority on this, but it would seem to me that similarities and 
>> differences aren't the point.  Similarities or differences between two taxa, 
>> on their own, may give you an idea of how closely related they are but they 
>> may say nothing about the direction of evolution with respect to them.  For 
>> that you need at least two things: outgroup comparisons, and, if you are 
>> dealing with directions of adaptation, some sort of developmental correlate. 
>>  Thus, if A is volant and B s flightless:
>> 
>> 1. Whichever of the two is more similar to the outgroup is, parsimoniously, 
>> the least derived. So if A gave rise to B it should share more 
>> plesiapomorphies with the outgroup than B does, and vice versa.   
>> 
>> 2. If A gave rise to B there should be no characteristics of A that can ONLY 
>> be explained as having been secondarily derived from a B-like ancestor [an 
>> example might be the early turtle Odontochelys, which appears to lack a 
>> fully-formed carapace but has features of the ribs and plastron that are 
>> hard to explain in any other way than as supports and connections for a 
>> carapace that was either present but not detectable or was lost in a recent 
>> ancestor - if this is correct the situation in Odontochelys must represent a 
>> secondary loss].  Storrs Olson has proposed that secondarily flightless 
>> birds achieve this condition through paedomorphosis, and that the evidence 
>> of this shows up in things like body proportions.  As paedomorphosis can 
>> occur for a variety of reasons, and as secondary flightlessness could arise 
>> in other ways, the absence of paedomorphic characteristics (eg juvenile 
>> allometries persisting in adults) would not prove that B's condition of 
>> flightlessness is not derived, but their presence might be highly suggestive 
>> for suggesting that B was secondarily, not primarily, flightless.
>> 
>> Of course it used to be assumed that hollow bones, air sacs, feathers etc 
>> fit this idea - anything that had them must have evolved from a flighted 
>> ancestor, or why would they be there?  We now have alternate explanations, 
>> though, for these features, so that argument falls to the ground - aside 
>> from the fact that these are structures whose absence might actually prevent 
>> powered flight, so they must have evolved at least to some degree before 
>> flight was achieved - which of course means that they must have appeared 
>> first in primarily flightless animals.  What we need is a character that is 
>> not needed for flight, but is a clear synapomorphy of flying forms ONLY 
>> whose continued presence, either fully-formed or as a remnant, can most 
>> parsimoniously be explained as an indicator that B evolved within the clade 
>> of flying organisms possessing it.  In other words, we're back around to 
>> outgroups again, because in this case the secondarily flightless bird and 
>> its immediate volant ancestor should have volant outgroup members.
>> 
>> That's the biggest problem with concluding that oviraptorids etc are 
>> secondarily flightless - where is the unquestionably volant clade into which 
>> they fit?  Of course it could turn up any day (and I assume Greg Paul will 
>> argue that it already has, but not everyone has tumbled to it yet.)
>> 
>> Does that make any sense?  I'm a bit rushed....
>> 
>> Ron
>> 
>>  
>> Ronald Orenstein
>> 1825 Shady Creek Court
>> Mississauga, ON L5L 3W2
>> Canada
>> ronorenstein.blogspot.com
> 
> Jason Brougham
> Senior Principal Preparator
> American Museum of Natural History
> jaseb@amnh.org
> (212) 496 3544
> 
> 

Jason Brougham
Senior Principal Preparator
American Museum of Natural History
jaseb@amnh.org
(212) 496 3544