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

["Thomas R. Holtz, Jr." <tholtz@umd.edu>]

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] 
> On Behalf Of Jason Brougham
>
> 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.

True. Plus, this is NOT a binary feature: volant or flightless. There is a 
gradation of volancy, and we would expect such a
gradation especially in the initial transition to flight.

> 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. 

Yeah, some of us have been saying that for years... :-)

> 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.

Correct.

> 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. 

Not entirely. We may not be able to specify actual ancetors (i.e., the very 
populations who represent the actual gametes gave rise
to a later lineage), but we can sometimes specify ancestral STATES. (I.e., it 
would be perverse in the extreme to specify that the
ancestor of all tetrapods was volant, or that the ancestor of all gnathostomes 
was terrestrial).

> There is no rigorous or empirical scientific method 
> that can demonstrate if an animal had a flying ancestor or not. 

See paragraph above.

That said, once you get to the base of Eumaniraptora, the data is fairly 
ambiguous. We cannot at present exclude aerial/aerodynamic
locomotion of some form in the common ancestor of Deinonychosauria, 
Archaeopterygidae, and Avialae (regardless of the resolution of
the three-taxon statement here: and just to let y'all know, a rather RADICAL 
topology for these should be seeing light in the
not-too-distant future), and indeed I would say the case that the common 
ancestor of these three used some form of
aerial/aerodynamic locomotion was pretty strong based on biomechanics. I would 
also say the case is very strong that modern avian
flight is NOT the form of locomotion found at the base of this clade.

As to whether the next generally-accepted node down the tree (the 
Oviraptorosauria-Paraves node) had some form of aerial/aerodynamic
locomotion: perhaps. Whereas we have a good sense of the ancestral morphology 
for Deinonychosauria, Archaeopteryidae, and Avialae
(given that basal members of these guys all pretty much look like Archie...), 
we are less secure on the likely morphology (including
size) of the Oviraptorosauria-Paraves ancestor. Still, some air-based 
locomotion there of some variety would not be terribly
surprising. (All of this predicated on Oviraptorosauria being outside of 
Eumaniraptora, of course.)

Below that, with Therizinosauria and/or Alvarezsauroidea joining in, I would 
say that at present the likely anatomical form of the
ancestors there do not at present support an aerial ancestor.


Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
http://www.geol.umd.edu/~tholtz/
Fax: 301-314-9661               

Faculty Director, Science & Global Change Program, College Park Scholars
http://www.geol.umd.edu/sgc
Fax: 301-314-9843

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA