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Re: New Jeholornis specimen



Gregory S. Paul wrote:

The shoulder glenoid of dromaeosaurs, troodonts, oviraptorosaurs shares about the same degree of lateral orientation as Archaeopteryx, so it does not provide evidence of relative grade of flight adaptation.

Or, I would say, it therefore does not provide direct evidence of flight at all. In the context of the rest of _Archaeopteryx_'s anatomy, it's sensible to regard the lateral glenoid as assisting in powered flight (i.e. execution of flight stroke). Yet, even in this context the lateral glenoid (as opposed to dorsolateral glenoid, as in more derived birds) precludes elevation of the wing above the horizontal.


As for the expanded finger base seen in all Jehol dromaeosaurs, no other predator has evolved it. It is such a clear flight adaptation that all other possibilities must be ranked as implausible unless really really good evidence shows up indicating otherwise.

As I've heard historians lament: the future contaminates the past. Features that incontrovertibly assist powered flight in modern birds may not necessarily have performed this same function in non-flighted ancestors. This is certainly true for the furcula; why not other pectoral and forelimb features?


What I find troubling is that, under your hypothesis, so many flight-related features in birds are held to have evolved specifically *for* flight. This leaves me to wonder what anatomical features pre-flight theropods had for evolution to work on. What preadapted these critters to powered flight?

Since basal dromaeosaurs had much greater central finger flattening than Arch, and stiffening too, and longer primaries relative to the hand, this is superb evidence of more, advanced flight. To argue otherwise is too dismiss powerful evidence on an arbitrary basis and is not really scientific.

I have yet to hear an argument why such adaptations could not have been employed for aerial gliding, rather than powered flight. For example, no gliding dromie wanted the primaries to detach during the aerial phase (especially if the long primaries "snagged" on branches during the glide) or during prey handling on the ground. One hypothesis is that basal dromies used the hands to wrestle with prey more so than _Archaeopteryx_. This favored a firmer and broader base for attachment of the primaries.


As for the proportionately longer primaries, this was to reduce wing loading in basal dromies - after all (with the exception of _Microraptor_) these were bigger and heavier animals than _Archaeopteryx_.

These competing hypotheses, I think, are scientific.

Cheers

Tim

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