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RE: Campbell's even crazier than a MANIAC? (archeopteryx climbing)




Erik Boehm wrote:


> See, I never bought the "insect trap" hypothesis, and I'm not sure I'd buy 
> the wave a bug into the mouth> (same thing?) hypothesis. 


I think Ostrom's insect-net model might have been refuted on aerodynamic 
grounds, but I can't recall the exact paper.  


> It gives "half a wing" some use before it can fly or glide, but then one must 
> ask what good "a fourth of a 
> wing" does - how would the above speculative feeding streategy evolve, 
> considering the predecessor 
> likely involves briging the arms forward, not backward to catch prey.


Not that I'm actually advocating this particular model, but Caple, Balda & 
Willis (1983) proposed that birds evolved from small, cursorial, insect-eating 
bipeds leaped into the air against fling insects, which they caught in their 
jaws.  Caple &c suggested that fringes of lift-giving feathers on the 
extremities conferred an advantage in insect-catching: "The addition of small 
increments of distal lift to the forelimbs and tail increased the animal's 
ability to control and maneuver its body axes during a jump.  This increased 
the predator's ability to capture prey."


> The "parachuting/gliding" hypothesis has no such problems - for small animals 
> than may fall from high 
> places, longer proto-feathers means more surface area, and a slower fall, 
> allowing falls from greater and> greater heights - there are many many 
> examples of this- from snakes that flatten their bellies, webbed 
> tree frogs, coluguos (flying "lemurs") - but all are arboreal and it has 
> never been shown to lead to flight 
> (although little is known about bat evolution, and bats are sometimes said to 
> group closest to primates, 
> and may have evolved from arboreal ancestors.)


This is all very well; but what were the theropods doing in these "high places" 
in the first place?  All the critters you mention are arboreal and use gliding 
or parachuting in order to commute through the air. 


> So if it is shown flight evolved from a therepod running on the ground, and 
> not falling/gliding from a tree -
> I wouldn't be surprised. but given the number of independant events that 
> acheive some form of 
> aerodynamic control, of arboreal origin, I wouldn't rule out an arboreal 
> origin of therepod flight, just 
> because archie couldn't perch in a tree like birds do today.


As Scott says, it's more than just an issue of perching ability (or lack of 
perching ability) in _Archaeopteryx_.  Its skeleton is manifestly ill-equipped 
for arboreality.  This also applies to any non-avain theropod you care to 
mention, microraptorans included.  The wrist and ankle joints are completely 
unsuited to climbing, given that the hindlimbs were adapted for 
cursorial/terrestrial locomotion.  The manus of maniraptorans actually shows a 
trend away from grasping ability (apart from maybe _Bambiraptor_), and the 
semilunate carpal reduced the flexibility of the wrist even further, 
constraining motion to a single plane.  Flight and perching both allowed birds 
to avoid climbing; but until that point was reached, arboreality (and even 
scansoriality) would have been very difficult for theropods.


Also, to reiterate... the idea that the origin of flight was either "ground-up" 
or "trees-down" has to be revised in light of recent evidence, which suggests 
that both "arboreal" and "cursorial" behaviors may have played a role in the 
incipient stages.  WAIR is one example; but there's also many studies on the 
morphology of the limb elements and claws which indicate that _Archaeopteryx_ 
and its relatives (avian and non-avian) may have divided their time between 
trees and the ground.



Cheers

Tim
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