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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
>>> 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
>> does not seem very great (and I suppose they could be secondarily
>> flightless too). This is not a question of reducing forelimbs (as
>> tyrannosaurids), which I suppose can be explained as a consequence of a
>> shift in a bipedal animal from manipulating or catching food with
>> 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
> be explained by disuse. Ratite fingers aren't highly fused nor
> 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
> 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
> 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?