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more evolution of flight
I want to thank everyone for their very excellent reponses to my sauropod
stuff. Don't be concerned about being too critical, spare the criticism
and spoil the scientist is my motto. Keep the comments coming, I will have
some follow-up soon.
Good news and bad news. I managed to get the Discover magazine article on
dinosaur skin thru interlibrary loan. The xerox is awful and shows me less
than I already knew, but I got it. [sob]
Meanwhile, I want to get back to the flight question for a moment. I was
looking at the archives and I've noticed that a lot of remarks about the
evolution of feathers and flight seem to assume that Archaeopteryx is
somewhere near the very beginning ot this story. Many seem to look at this
animal, its morphology, its habitat, etc., and try to relate these to the
evolution of flight and even feathers.
I take it as fairly well established that Archaeopteryx flew well. I won't
go into the various pieces of evidence that establish this here. But
taking this as an assumption, Archaeopteryx is the product of the evolution
of feathers and flight, not a snapshot of the process. Admittedly flight
was refined after Archaeopteryx, but this is irrelevant to what I'm saying.
Feathers evolved before flight, and flight in this line evolved before
It has been suggested that feathers are rather elaborate structures to have
evolved for insulation, and that fur-like structures are better for this
purpose. Such elaborate structures are more adaptive for display, so the
argument goes, and again the feather distribution of Archaeopteryx is cited
as evidence against the insulation hypothesis. But if this is so, why
hasn't any terrestrial ectotherm evolved such elaborate structures? Many
ectotherms have display structures and adaptations. There are all kinds of
ways to make yourself look bigger without resorting to highly complex
epidermal structures, and needless to say many ectotherms manage to be
brightly colored without feathers. When I see an animal with a fully-erect
posture that has feathers, I am VERY suspicious that they evolved (at least
partly) for thermoregulation.
This does not eliminate the display hypothesis. On the contrary, I have
already forwarded my hypothesis about flight evolving in the context of
territoriality. It is very easy for me to envision a little feathered
dinosaur evolving long forelimb and tail feathers for both display and for
increasing the speed and efficiency of moving from perch to perch within
its territory. Long before it showed any significant adaptations for
powered flight, such an animal would have been using air resistance to
increase the efficiency of its patrols.
The tricky part here is that energy efficiency has to now DECREASE if we
want to evolve powered flight. But this scenario has the little protobird
displaying, then launching off its perch, gliding as far as possible,
climbing up the next perch, and repeating the process. From this point
powered flight will give a tremendous advantage. The animal will be able
to move rapidly across the breadth of its territory, directly from perch to
perch, bypassing terrestrial predators altogether and enabling it to patrol
its territory very rapidly.
I have to say I am not impressed with scenarios that have these protobirds
using their forelimb feathers to help them catch prey. I have seen
mockingbirds using their wings to apparently scare up insects, but modern
birds do not have forelimb claws (with few exceptions), and I just cannot
envision long feathers evolving strictly for the purpose of prey
acquisition in an animal that already has well-developed forelimb claws.
Where exactly do feathers first appear in the fossil record, what type of
feathers are they, and what is the pattern of feather abundance by type
over the course of the Jurassic?
How many skin impressions are available for pre-Cretaceous dinosaurs? What
is the current consensus about the Triassic trackway that purports to show
forelimb feathers? And can anyone name an Early-Mid Jurassic predatory
dinosaur <2 m in total length as an adult? The only candidate I have is
Iliosuchus incognitus, and it is by no means clear to me that the material
is that of an adult.