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



I took the liberty of re-arranging this exchange in chronological sequence, for the sake of clarity. My comment is appended at the bottom, being the most recent.

>>> Jason Brougham wrote:
>>>
>>> <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.

>> Dr. Ronald Orenstein wrote:

>> Has anyone come up with an adaptive explanation for the degree of fusion
>> and finger reduction in the avian hand (including the hand of living
>> ratites) that does NOT involve supporting a more or less rigid feathered
>> wing? I suppose someone will point to alvarezsaurids, but the similarity
>> does not seem very great (and I suppose they could be secondarily
>> flightless too). This is not a question of reducing forelimbs (as seen in
>> tyrannosaurids), which I suppose can be explained as a consequence of a
>> shift in a bipedal animal from manipulating or catching food with the hand
>> to using the mouth - but of the very specific type of fusion seen in
>> modern birds.  I admit this point isn't proof of anything (it might be
>> characterized as an argument from incredulity) but surely it is at least
>> suggestive of secondary flightlessness in ratites.  Could the same thing
>> be said of the pygostyle, or could this be explained as an adaptation to
>> support and manipulate a
>>   display structure?

> On 8/13/2011 11:14 AM, Jason Brougham wrote:

> I think the point about the modified fingers is a good one, but ratites
> retain the claws, of course. Hoatzins either retain or reversed back to
> mobile finger joints. Ratites don't have pygostyles.
>
> I think there'd be more anatomical support for the idea that ratites had
> flying ancestors if ratites retained the large flat flanges on the first
> phalanx where the primaries anchor.
>
> We could write a scenario, and I think maybe someone (Thulborn?) has,
> where ratites evolve directly from, say, ornithomimids, and neognathous
> birds spun off from, say, oviraptorids much later. Ratites would still
> have the three fingers, at least as embryos, and claws, and the rest could > be explained by disuse. Ratite fingers aren't highly fused nor modified to
> support a rigid wing.
>
> Also a rigidly feathered wing, or pennibrachium after Sullivan et al.
> (2010), doesn't have to be used in flight.
>
> Perhaps it is the very notion of adaptive explanations that I'm skeptical
> of. I have more faith in sober surveys of the distribution of characters
> rather than coming up with speculative explanations for the apparent
> pattern. A lot of elegant adaptive explanations, ones that thousands and
> millions of words have been written about, have been proven false by later
> evidence. The idea that dinosaurs (they didn't say non-avian then) went
> extinct because they were cold blooded and the world got too cool for them
> is just one such doozy.
>
> But it is fun to speculate. I do appreciate the contributions of everyone
> on the DMl along these lines because the speculation is pretty fun, and
> jogs the imagination.

Don Ohmes wrote --

1) From an ecological standpoint, the ability to perform a leaping launch can be argued to be a powered-flight requirement for vertebrates, it's importance and difficulty increasing w/ size.

2) Can the short-femur-ed, knee-walking condition (i.e., femur moved toward the horizontal) as it appears in modern birds be shown to be theoretically launch-adaptive in bipeds?

3) Assuming the answer to #2 is "yes", then showing that ostriches are secondarily flightless birds seems uncomplicated, particularly if the initial transition from the non-avian theropod leg to the modern bird leg can be shown to mal-adaptive in a cursorial biped.

4) Assuming the transition from the basal non-avian theropod to the final modern bird leg condition is indeed theoretically mal-adaptive in a cursorial biped -- at least at some point in the process -- and also that knee-walking is leap-adaptive -- then it would follow that knee-walking, and also the ratio between the femur length and the combined vertical leg components, might appear in the geo-record as a uniquely flight-related characteristic in bipeds.

And don't get me started on the the whole tibula/fibula thing.

So, are there any flap-flying bipeds out there w/ vertical femurs? Or any bipedal knee-walkers that can't jump? Any really ancient knee-walkers?