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



On Aug 9, 2011, at 3:27 PM, Jason Brougham wrote:

> 
> On Aug 9, 2011, at 2:41 PM, Ronald Orenstein wrote:
> 
>> I certainly would not want to pre-empt Greg Paul on this, but as an 
>> ornithologist I would like to offer a few comments on Question 1 (without 
>> expressing an opinion on the flightless-dinosaur issue - my remarks refer to 
>> crown birds only)..
>> 
>> First, I think that the recent distribution, abundance and variety of 
>> flightless birds is in part an artifact of human-caused extinctions, and may 
>> not be a good guide to what has happened in the past.  Thus we may not have 
>> a very good idea of the diversity of, especially, small fligtless birds (for 
>> instance, the Acanthisittids in New Zealand).  Also, if you consider New 
>> Zealand birds to be continental then so are the highly diverse and speciose, 
>> but extinct, moas.
> 
> If cenozoic flightless birds radiated widely - diversifying from one 
> flightlessness event - the record should till be there in the fossils. 
> Indeed, the Cenozoic fossil record is more complete than the Mesozoic one.
> 
>> 
>> Secondly, failure of any recent group of flightless birds to spread over the 
>> world is in part due to the recent positioning of the continents (ie there 
>> are oceans in the way).  Presumably flightless birds have only dispersed 
>> over a land connection (I can't think offhand of an example of a flightless 
>> bird spreading by, say, rafting).
> 
> Gondwana and Lauarsia were separated, but they shared several groups of 
> maniraptorans in common.
> 
>> 
>> Third, the fauna of mammalian predators and nest-robbers has been quite 
>> different since the K-T from what it was earlier, and the barriers to 
>> radiation of flightless birds on continents may not therefore be identical 
>> to what they might have been in the Mesozoic.
> 
> Different, certainly, but there is no reason to think the differences were 
> consequential.  Mammals were feeding on dinosaurs in the Mesozoic, they could 
> glide, they could swim, no reason to think they couldn't rob nests.
> 
>> 
>> Fourthly, the distribution of flightless rails (Rallidae) shows quite 
>> unequivocally that a volant stock can radiate and disperse worldwide and 
>> repeatedly evolve flightless forms from flying ancestors, presumably in very 
>> short order (in one case, for Dryolimnas cuvieri, there are volant and 
>> flightless subspecies of the same species).  Ducks are another example (the 
>> flightless Auckland Island and Campbell Island Teals are very close 
>> derivatives of the volant Brown Teal of New Zealand, and were once placed 
>> with it in the same species).  Thus though a flightless ancestral stock 
>> might not be able to disperse very widely, a volant stock with a propensity 
>> to evolve flightlessness could give rise to a range of secondarily 
>> flightless forms spread over a wide area in short order.  If the fossil 
>> record of such a radiation is poor with respect to the volant members 
>> (possibly to be expected as they would probably be smaller, slighter and 
>> less likely to fossilize well than larger, robust flightless forms) it might 
>> look as though only a few losses of flight were involved (eg if the only 
>> representatives of the Gallirallus philippensis complex known were the 
>> flightless ones it might not be immediately obvious that each one represents 
>> a separate loss of flight).
> 
> I agree. But, in the case of rails, the morphological changes associated with 
> flightlessness were so recent and superficial that the resulting species did 
> not move into new families, unlike with the ratites. In maniraptorans we had, 
> I guess, Troodontidae and Dromaeosauridae that Paul might say descended from 
> Archaepterygids, though he may group them all in the same family. 
>>  
>> Ronald Orenstein
>> 1825 Shady Creek Court
>> Mississauga, ON L5L 3W2
>> Canada
>> ronorenstein.blogspot.com
>> 
>> 
>> From: Jason Brougham <jaseb@amnh.org>
>> To: GSP1954@aol.com; Dinosaur Mailing List <dinosaur@usc.edu>
>> Sent: Tue, August 9, 2011 1:25:34 PM
>> Subject: Re: Ah ha! That's where therizinosaurs came from
>> 
>> 
>> 
>> > What the cladistics would
>> > have us believe is that all these predatory, omnivorous and herbivorous 
>> > early fliers
>> > were flitting about in the later half of the Mesozoic yet for some magical 
>> > reason were
>> > never spinning off reflightless forms that show up in the fossil record. 
>> > Really, that's what the 
>> > cladograms want us to take seriously.
>> 
>> 
>> Mr. Paul, I admit that the evolution of birds certainly has left a 
>> bewildering diversity of flightless forms in the fossil record, and you 
>> argue your case persuasively, as always. I take your hypotheses seriously 
>> and I hope I won't antagonize you when I note that I also take the other 
>> possible hypotheses seriously.
>> 
>> I have two questions for you, to help me be better informed in understanding 
>> your views.
>> 
>> Question 1:
>> 
>> Flightless families of crown group birds seem different from non - avian 
>> maniraptorans in their systematics. Namely, they don't seem as diverse, 
>> cosmopolitan, and geologically persistent as families of non-avian 
>> maniraptorans.
>> 
>> As you touched on in Dinosaurs of the Air, there are two modes of flightless 
>> birds today, the island types and the continental types.
>> 
>> The most diverse flightless crown group birds like rails evolved 
>> independently of one another on islands and other restricted, far - flung, 
>> habitats. Continental flightless birds such as the ostrich, kiwi, or Rhea, 
>> tend to have very few species, very low diversity within each family, and to 
>> be restricted to single continents. No lineage of flightless crown group 
>> birds seems to have gone on to form a broad, global, radiation of new forms 
>> over a hundred million years, as did several non - avian maniraptoran groups.
>> 
>> Dromaeosaurs, as one example, achieved great diversity of body size, limb 
>> proportion, and foot morphology, and they also enjoyed global distribution 
>> and invaded habitats from aeolian deserts to polar forests. Do you see them 
>> all as spinning off in separate convergences on flightlessness, from flying 
>> archaeopterygian ancestors all around the world, and thus are dromaeosaurs a 
>> paraphyletic group? Or did they evolve flightlessness once and then spread 
>> around the world and diversify, unlike any family of crown group flightless 
>> birds?
> 
> 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