[Taxacom] bird and feather evolution

Kenneth Kinman kennethkinman at webtv.net
Thu Jan 20 21:18:06 CST 2011


Dear All,
     Carl Zimmer has written an pretty good article on the evolution of
feathers (among the dinosaur ancestors of birds) in the most February
issue of National Geographic.  It is an excellent summary, and the only
thing that bothered me was the implication that the dinosaurian
ancestors of birds perhaps used striped tail feathers to confuse their
prey.   
      I actually have argued that although feathers did originally
evolve on the tails of dinosaurs to confuse their prey, I see no reason
at all to assume that they were striped.  On the contrary, they may have
been brightly mono-colored and increasingly long (slowly extending far
beyond the end of the tail).  Much like modern lizards with brightly
colored tails, such feathers could have confused predators toward the
expendable tail-end of the animal.  However, it could have been even
more effective if the predator just got a mouthful of colorful
protofeathers, and thus not even a piece of the tail at all.  Detachable
tails of lizards (using as similar ploy) may have arisen earlier or
later than this ploy.  Who knows?  In any case, Zimmer's comparison with
zebra stripes was the weakest part of his otherwise reasonably
arguments.     
      My prediction has been that feathers started out as simple
bristles on the tail of some dinosaur group (perhaps pre-theropod) in
which stripes would have been of no conceivalbe use.  In fact, it seems
more likely that a bushy bunch of bristles (brightly colored or not) at
the end of the tail looked like a "pseudo"-head and distracted predators
to attack it instead of the valuable real head of the dinosaur.  Some
later dinosaurs (like the bristled-tailed Psittacosaurus) would have had
just a remnant of such bristles.  
       Only later (late Triassic?) would have such bristles and or even
branched-feathers expanded anteriorly to the actual body of the
dinosaur, first on the butt-end to incubate eggs, but later even more
anteriorly to warm the whole animal's body (adapting to cooler
environments).  So I agree with Zimmer that it was a series of
exaptations.  And of course, we totally agree that flight feathers were
a VERY MUCH later exaptation (presumably some time in the Jurassic).     
      In any case, the later evolution of dinosaur arms and shoulders
(and advanced feathers) capable of true powerful flight is still a
subject that is extremely controversial.  So "powered flight" (however
you define "powered") is also subjective and controversial, i.e. words
did Archaeopteryx possess true "powered" flight).  So asymmetical
feathers (not as easily fossilized) do not seem as important as earlier
exaptations in the arms and shoulder structure which allowed such
asymmetical feathers to later perfect increasingly powered flight.  It's
much like the ear bones being more important in mammal evolution than
hair.  Hard structures in a long series of fossils tend to be a more
reliable indicator of phylogeny than structures (like hair or feathers)
that are less commonly preserved.        
      We have yet to tackle the transition from dinosaurs to birds in
the same way that we have tackled the relatively quick transition of
bones from the posterior jaws of therapsid "reptiles" to the inner ear
bones of true mammals.  Those who simplistically say that birds ARE just
dinosaurs are more fixated on the similarities between the forms than
they are in documenting fast-evolving skeletal structures which would
show how anagenetically important such evolution can differentiate
reptiles (as a whole) from a Class Aves (birds).  We certainly cannot
use "feathers" (however you might want to define them) to define Class
Aves.   Anyway, below is a weblink to Zimmer's article (which is largely
very helpful, although the zebra analogy for predator evasion seems
totally inappropriate in this case, from my point of view).  Confusing
Mesozoic predators from the dinosaurian ancestors of birds is almost
certainly not an obvious analogy to zebras using their stripes to
confuse Cenozoic mammalian carnivores.  Such an analogy really misses
the reality in my opinion, although it might seem appealing on a
superficial level.  Otherwise, it seems a nice summary of the whole
subject of feather evolution.       
        --------Ken Kinman

 
http://ngm.nationalgeographic.com/print/2011/02/feathers/zimmer-text 





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