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

[Tim Williams <twilliams_alpha@hotmail.com>]

Don Ohmes wrote:


> No telling how many times insects evolved flight, nor in how many ways not 
> available to verts, so I will 
> unilaterally exclude them :D.


The development of flapping flight in winged (pterygote) insects probably 
occurred only once.  The design of the wings is so similar across all 
pterygotes that it's difficult to believe (and more importantly, highly 
non-parsimonious) that it could arise by convergence.


There are several hypothetical scenarios for the origin of insect flight.  
Entomologists even have their own competing "up" versus "down" scenarios.  One 
hypothesis proposes that flight evolved from a silverfish-like insect that 
climbed up the stems of small vascular plants and used gliding for descents 
("stem-down").  Another (which I mentioned in a previous post) holds that 
flight evolved from a stonefly-like water skimmer ("pond-up").  AFAIK, 
"stem-down"/"pond-up" doesn't engender quite the degree of passion as the 
"trees-down" versus "ground-up" debate in the origin of avian flight.


> Given the presence of easy-to-climb trees (e.g., cycads), and good reasons to 
> climb them daily, but 
> perhaps not spend all day there, I do not see where the terrestrial ancestry 
> of birds necessarily speaks> to a ground-up scenario.


I agree with the first part of your statement, about theropods occasionally 
climbing and exploiting cycads, and then parachuting or gliding down.  But this 
"trees-down" step might have been a very mature stage in the development of 
powered flight.  The "ground-up" scenario could include all the preceding 
steps, and entail the development of many (most?) of the components of the 
avian flight apparatus: feathers, furcula, enlarged pectoral musculature, long 
forelimbs, decoupling of the tail from stride generation, maybe even the flight 
stroke itself (or its biomechanical precursor).


> Given an Earth-type planet populated w/ a generic terrestrial vertebrate 
> species, and a large block of 
> time, my wager will be that the majority of the eventual fliers (and the 
> first) will have taken a gravity-
> driven route, because it is "easier".


In the big scheme of things, what counts is not whether a certain route is 
"easier", but whether a given behavior or morphology lends itself to selection 
in favor of improved aerial ability.  Evolution doesn't make choices: it can 
only work on that which already exists.  That's why arguments about why a 
gravity-driven route is "easier" are so moot.


Besides, "easier" is a vague and loaded term anyway.  But if we use the term 
the way you are using it, I would ask which of the following is "easier" for a 
cursorial biped: (a) climbing a tree and leaping off it; or(b) or leaping into 
the air from the ground?  The question is essentially unanswerable - but why 
would you assume that (a) is "easier" than (b), or that (a) and (b) are 
incompatible?


August Haro wrote:


>Do you know of the paper where it is said? May it be: Middleton, K.M. (2001). 
>The morphological basis of 
> hallucal orientation inextant birds. J. Morphol. 250: 51–60.?


Yep, that's the one.  It dispells the myth that you can determine the 
orientation of the hallux in life simply by its position in death (and 
fossilization).  Whether the hallux was retroverted or not in life can only be 
determined by examination of the hallux itself, especially the morphology of 
the first metatarsal.


Cheers

Tim
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