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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
>> From: Jason Brougham <email@example.com>
>> To: GSP1954@aol.com; Dinosaur Mailing List <firstname.lastname@example.org>
>> 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
>> 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
> Jason Brougham
> Senior Principal Preparator
> American Museum of Natural History
> (212) 496 3544
Senior Principal Preparator
American Museum of Natural History
(212) 496 3544