The Evolution of Flight.... why are some birds grounded?

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We just don't know the size of, nor the relative front limb dimensions, of the bird ancestors. Dromaeosaurs aren't found in rocks older than Archaeoptery so we don't know what the so-called protodromaeosaur really looked like.

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Well... it's always possible that dromaeosaurs had an arboreal common
ancestor with birds. Anyway, what is the oldest known dromaeosaurid? And what group did dromaeosaurs evolved from?

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It doesn't always follow that bipedalism reduces the forelimbs, of course, us for example. Particularly if you are using your feathered arms for rapid changes of direction whilst prey chasing, to increase your acceleration/top speed. The flying fish managed surface up gliding without any musculature in the fins themselves.

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You forget that we humans descend from arboreal apes, hence our long limbs. Btw, are there any hint of the ecological setting of supposed bird ancestors?

Andya, I agree, but at the moment it is a hypothetical missing link, if fact that's all both sides have. The Maniraptorans. You miss the point, bipedalism did not significantly reduce the forelimbs.Mark

Mark24,Thanks for your reply, that answers my question nicely. I must take issue with this. The forelimbs of humans are massively less powerful than those of other apes. Most other apes are easily capable of pulling themselves up on one hand without apparent effort - a feat that is exceptionally rare in modern humans. Even top-class climbers can't match the ease with which large apes such as Orang-utans can pull themselves around using just their upper body strength.

Jack, But the fact remains we don't have tiny T-Rex forelimbs. They remained viable limbs, & could potentially be co-opted for other purposes, which they have alreasy been given we no longer use them for Tarzan like activities. The point I was arguing against was that bipedalism results in greatly reduced forelimbs that would be useless for co-option. It doesn't.Mark------------------
"I can't prove creationism, but they can't prove evolution. It is [also] a religion, so it should not be taught....Christians took over the school board and voted in creationism. That can be done in any school district anywhere, and it ought to be done." Says Kent "consistent" Hovind in "Unmasking the False Religion of Evolution Chapter 6."

A female orangutan's armspan gets upwards of 7 feet, while they're rarely much taller than 4 feet. Our armspan is typically roughly equivalent to our height. I'd say that a 40% reduction in forelimb length from a couple million year branch is quite significant. I'd additionally argue that one of the only reasons it's not worse than it is is that we are so dependent on dexterity compared to most animals.------------------
"Illuminant light,
illuminate me."

Rei,But they remain viable for co-option, which is the point, after all.Mark

In our case, that is true. ------------------
"Illuminant light,
illuminate me."

This should put the cat among the Archaeopteryx .From the excellent resource, Dinosauria On-Line.Dinosauria On-Line"Many Cretaceous theropods - dromaeosaurs, troodonts, oviraptors - are more bird-like than Archaeopteryx in many respects, and have a shoulder girdle that is similar to secondarily flightless birds. All this suggests that avian flight first evolved in arboreal theropods (where they developed big brains and forward facing eyes, features not found in flying insects and pterosaurs), and that some of the flying theropods lost flight. Not knowable at this time is whether Archaeopteryx was a member of the true bird clade, or was an independent experiment in flight, or where theropods end and birds begin, among other matters. "Mark[This message has been edited by mark24, 09-30-2003]

LOL, MarkIt will, one supposes, turn out to be, like other things, more complex that it first appeared.[This message has been edited by NosyNed, 10-01-2003]