Dinogeorge wrote:
> Well, I've been pushing this view for some 15 years now. There is indeed
> rampant convergence in dinosaur and theropod lineages as a result of secondary
> flightlessness, making cladistic analysis very problematic. Think of moas,
> giganornithids, phorusrachids, etc., etc. Forelimb reduction and hindlimb
> enlargement, as adjuncts of secondary flightlessness, occurred repeatedly in
> Cenozoic birds, why not also in Mesozoic birds?
OK, time for a reality check. What George wrote here is NOT what the vast majority of paleontologists would regard as accurate.
George has been pushing his view for around 15 years. A fellow named Gregory S. Paul has been pushing a very similar view for close to 20 years. It is Paul's hypothesis that certain ground-dwelling theropod dinosaurs (maniraptorans such as dromaeosaurs and oviraptorosaurs) are actually secondarily flightless birds. This view has sometimes been called the "2F hypothesis" (2F = secondary flightless), or the "neoflightless hypothesis" (Paul's own name for this hypothesis).
The "neoflightless hypothesis" certainly has its supporters (albeit not many). However, at the current time, most of these supporters are not professional paleontologists. Most paleontologists prefer the hypothesis that emerges from cladistic analysis: that maniraptoran theropods like dromaeosaurs and oviraptorosaurs lie CLOSE TO the origin of birds, but are NOT BIRDS themselves. These paleontologists include those who work from day to day studying these theropods, and who publish their work in peer-reviewed scientific journals. Their collective view is at odds to George's views.
When George says, "There is indeed rampant convergence in dinosaur and theropod lineages as a result of secondary flightlessness, making cladistic analysis very problematic", he is stating HIS OWN OPINION. This is not fact. George often confuses his own opinions for facts. George's own hypothesis is more extreme than Greg Paul's, in that it posits that dinosaurian evolution began in the trees, and therefore all lineages (ornithischian, sauropodomorph, theropod) are either secondarily terrestrial or secondarily flightless. George's hypothesis is very imaginative, but it has absolutely no support from the fossil record.
Now, to address the statement, "She [Dr. Frances James] also was able to point out several bird-specific (i.e. vestigial flight characters) features present in some maniraptors." It is true that theropods show many bird-like characters. However, this does not make these characters "bird-specific", and it certainly does not make these characters "vestigial". This is putting the evolutionary cart before the horse. It is better to say that birds inherited many theropod characters. It is teleological to say that all the flight-related characters seen in birds began as flight-related characters. Rather, the process of exaptation appears to have played a crucial role in the evolution of the avian flight apparatus. There are cogent explanations for why many flight-related characters (in modern birds) first appeared for reasons unassociated with flight (in non-avian theropods)... but this is too much information for one email
message.
Which brings me to this:
>>If anyone has time for this: How can you tell whether it was the ancestor
>> of the flightless bird that could fly, or it will be the descendants that would
>> become capable of flight. In other words, how can you tell the direction of
>> evolution from a fossil?
This is a good question. George replied:
> You can't. You can't even tell from a cladogram. You have to reason from
> physical principles, and from known evolutionary trends in modern organisms.
George is incorrect. A cladogram can indeed tell you the order in which individual characters arose. That is the very purpose of a cldogram. For example, a cladogram of the Dinosauria tells us that theropods inherited obligate bipedalism, and then went on to evolve (in the following order): furcula ("wishbone"); feathers; semilunate carpal ("swivel-wrist' - essential for execution of the flight stroke); elongated forelimbs; asymmetric vanes; and reversed hallux. This sequence of character acquisition contradicts George's own hypothesis, so he has a vested interest in distrusting what the cladogram is telling him.
As for "You have to reason from physical principles, and from known evolutionary trends in modern organisms", again this is George stating his opinion, not fact. Physical principles DO NOT mandate a neoflightless origin for the maniraptoran morphology. I have no clue where George is getting this from.
By "evolutionary trends" George is alluding to the prevalence of secondary flightlessness in Cenozoic bird lineages (ratites, gastornithids, phorusrhacids, dromornithids, dodos, penguins, flightless ducks, etc). However, this is irrelevant to maniraptorans - it merely demonstrates that, under certain conditions, individual avian lineages have abandoned flight for various reasons (cursorial locomotion; aquatic locomotion; insular habitat). It in no way supports George's contention that all terrestrial theropods must be secondarily flightless.
Also, for all these aforementioned lineages (ratites, gastornithids, phorusrhacids, dromornithids, dodos, penguins, flightless ducks, etc) cladograms tell us that all these groups evolved from flighted (volant) ancestors. These same cladograms (or at least the cladograms generated by the same methodology) also tell us that dromaeosaurs and oviraptorosaurs are primitively terrestrial and DID NOT evolve from flighted ancestors. Thus, they severely undermine George's hypothesis. In this example, George is cherry-picking his data. Tsk tsk, I say!
George may also try to tell you that his hypothesis is the only one that adequately explains the "trees-down" origin of flight, and that cladistics supports a "ground-up" origin of flight. This is also not true. In fact, recent cladograms appear to be swinging behind a "trees-down" origin of flight, especially with the discovery of small arboreal dromaeosaurs like _Microraptor_.
I apologize for the length of this message, but somebody has to remind the public that George does not represent the consensus view on theropod evolution (including the origin of flight). George is a maverick, and proud of it. For current well-supported views on the evolution of flight in theropods I would refer people to the scientific literature, to published work by paleontologists such as Dial, Chiappe, Xu, Norell, Makovicky, Holtz, Clarke, Sereno, Padian, Middleton, Prum, Naish, and others (I've probably forgotten some important people - I'm doing this off the top of my head). John Ostrom's work is also seminal, and he is the one who really got the ball rolling in the 1960's and 70's.