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