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Re: bauplan convergence (long)




Ronald Orenstein wrote:

They also remind us that the earliest fossil winged insects had wings on
every thoracic and abdominal segment - just what you would expect, I
submit, if they evolved from gills of aquatic arthropods into rowing
structures (think of the banks of oars on a galley).

I like this idea, and it is probably correct that the first insect may have had a pair of "wings" (or proto-wings, since they were not originally used for flight but maybe for aquatic respiration) on each segment. Every segment also bore a pair of limbs - the mouthparts of insects (maxillae, mandibles, labium) are known to be highly modified limbs, as are the postantennal appendages (but not the antennae themselves, which have a different origin).


In the proto-insect, the gills perhaps came to be used for locomotion (used as, as Ronald states, as rowing structures, akin to the banks of oars of a galley), then subsequently for skipping on the _surface_ of the water. Fully-fledged wings came next - by which time the number of wings was consolidated to two pairs, one each on the second and third segments of the thorax. (Some winged Carboniferous insects apparently retain a pair of flaps on the first thoracic segment, which never went on to become true wings, and were soon lost in the course of evolution.)

That's one hypothesis for the origin of insect wings. For a contrary hypothesis (which favors the idea that insect wings evolved from flaps used for gliding) check out:

http://entmuseum9.ucr.edu/ent205/snodgras/snod10.html

What does this have to do with the price of fish? (or the evolution of flight in birds?). I think it ties in nicely with the idea that the wings of birds originally evolved for (or from) a purpose that had nothing to do with gliding or flight. (And I don't mean for grabbing prey and stuffing it into their mouths - I'm referring to the evolution of large, vaned feathers on the arms - not the evolution of arms themselves). Betty mentioned display, and I like the idea.

Look at _Caudipteryx_ - it has large, symmetrically-shafted feathers (pennaceous feathers) on its arms and tail. It also has more down-like (plumulaceous) feathers associated with other parts of the body, as do specimens of other nonavian theropods (_Sinornithosaurus_ etc). These probably evolved for insulation (the same purpose they have in modern birds, especially hatchlings). But the larger and more elaborate pennaceous structures - why did _Caudipteryx_ have these? The forelimbs were too short for the creature to fly (and besides _Caudipteryx_ was too big and heavy to fly). It could have evolved from a volant ancestor (a la BCF), but that conflicts with the favored phylogenetic position of _Caudipteryx_ as an oviraptorosaur, which suggests that it was NOT secondarily flightless.

_Caudipteryx_, like _Protarchaeopteryx_, may represent a stage of theropod evolution in which the arms (and the tail) served a secondary purpose for display. As Betty said, it's very prevalent in modern birds. And since the propagation of one's genes (and, in the bigger picture, the survival of one's species) depends on attracting a mate, the importance of display should not be underestimated. Nature is not always the red of tooth and claw, but the blue and green and iridescent beauty of feather and plumage.


Tim ________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com