Re: half-evolved feather pt 2

Stephen Jones (
Tue, 14 Apr 98 05:55:25 +0800


On Wed, 08 Apr 1998 20:38:10 -0500, Glenn Morton wrote:

GM>I have chased a bit back into the literature looking for info on
>the Longisquama feather-like scales. What I found is really
>fascinating. "Longisquama insignis is a remarkable, tiny (some 50
>millimeters[2 in] long) presumed thecodont from the late Triassic
>of Turkestan and the only known reptile to possess scales that
>show a possible intermediacy with feathers. The tiny specimen is
>preserved in its entirety, crushed on a slab. it exhibits an antorbital
>fenestra, a well-developed furcula, and long keeled, overlapping
>body scales." Alan Feduccia, The Origin and Evolution of Birds,
>1996, p. 87

What does "a possible intermediacy with feathers" mean? Feathers
grow from follicles deep *under* the skin. Scales grow *outside* the
skin. From the description above these are just large scales.

GM>The figure on page 88 has the following caption says that it
>possessed a unique gliding mechanism of "a double series of long
>scalelike structures that were unfolded in butterfly fashion to form a
>gliding wing." P. 88 the scales are as long as the animal and each
>one is separate from the other scales and looks like a feather.

No doubt a 2-inch scale could "look" *superficially* "like a feather".
But *is* it a "feather"? It sounds to me like a scale that has
converged to perform a similar *function* to feathers, ie. it is
*analogous* not homologous.

GM>Now, the above should go some distance towards answering
>what Kenyon and Davis ask of the fossil record,
>"If only we could find a fossil showing scales developing the
>properties of feathers

answering what Kenyon and Davis ask of the fossil record", in
respect of "scales developing the properties of feathers". Here we
have convergent, highly specialised and apparent dead-end scales
which have enabled a very light reptile to glide, in much the same way
as some small mammals have converged separately to gliding. There
is *no* evidence that these scales were turning into feathers, just as
there is no evidence that gliding mammals are turning into birds!

GM>or lungs that were intermediate between the very different
>reptilian and avian lungs, then we would ahve more to go on. But
>the fossil record gives no evidence for such changes." Percival Davis
>and Dean H. Kenyon, Of Pandas and People, Dallas Haughton
>Publishing, 1993, p. 106

You just ignore the "reptilian and avian lungs" part! If "Longisquama
insignis" was indeed transitional between reptiles and birds it would
need to have these developing as well. Johnson ("Darwin on Trial,"
1993, p36) cites Denton's conclusion regarding the twin problems of
the feather and avian lung:

"Denton describes the exquisitely functional avian feather, with its
interlocking hooks and other intricate features that make it suitable
for flight and quite distinct from any form of feather used only for
warmth. Bird feathers must have evolved from reptilian scales if
Darwinism is true, but once again the intermediates are hard to
imagine. Still more difficult a problem is presented by the distinctive
avian lung, which is quite different in structure than that of any
conceivable evolutionary ancestor. According to Denton,

`Just how such a different respiratory system could have evolved
gradually from the standard vertebrate design is fantastically difficult
to envisage, especially bearing in mind that the maintenance of
respiratory function is absolutely vital to the life of an organism to the
extent that the slightest malfunction leads to death within minutes.
Just as the feather cannot function as an organ of flight until the
hooks and barbules are coadapted to fit together perfectly, so the
avian lung cannot function as an organ of respiration until the
parabronchi system which permeates it and the air sac system which
guarantees the parabronchi their air supply are both highly developed
and able to function together in a perfectly integrated manner.'
(Denton M., "Evolution: A Theory in Crisis", 1985, pp211-212)

In any event the gliding to flapping flight scenario is not thought
to be feasible:

"Ostrom, as well as Padian (1985) and Caple, Balda, and Willis
(1983) point out that specializations for gliding and flapping flight are
antithetical and that a gliding ancestry, as characterized by "flying"
squirrels and "flying lemurs" (see Chapter 20), is very unlikely for
birds or pterosaurs. In gliders, the wing membrane is attached to the
body and extends between the front and rear limbs. The proximal,
rather than the distal, elements of the limbs are elongated. The
membrane of squirrels and flying lemurs have no internal support. In
pterosaurs and birds, the distal elements, which would be most
effective in flapping flight, are enlarged, but the proximal elements are
not; the hind limbs are not involved in support of the flight
membrane. The structure of the rear limbs of Archaeopteryx and
most later birds is clearly an adaptation to rapid terrestrial
locomotion. There is no evidence for arboreal specialization, although
the claw structure does not preclude climbing in trees. The great
length of the distal elements of the forelimbs indicates that flapping
was important for Archaeopteryx, whereas gliders would have had
shorter limbs if the membrane continued all along the trunk." (Carroll,
R.L., "Vertebrate Paleontology and Evolution", 1988, p341)

GM>Since the first english language report on Longisquama appeard
>in 1972 (P. F. A. Maderson, Am. Natu. 106:424 (1972)) 17 years
>prior to the 1st edition, and 21 years prior to the publication of the
>second edition of Of Pandas and People, one must wonder why the
>authors could claim that the fossil record showed no evidence of
>scales turning into feathers.

You have mentioned that "the first english language report on
Longisquama appeard in 1972". When was the *latest* journal article
on it?

If this obscure reference to "Longisquamais" which is "21 years prior
to the publication of the second edition of Of Pandas and People", is
the best you can do, then you have answered your own question! It
obviously is not considered relevant by the scientific community itself.
If it was we would have heard about it more prominently before now.
For example, the latest Scientific American article on "The Origin of
Birds and their Flight" doesn't mention "Longisquamais", even though
it mentions both feathers and gliding:

"As one supposedly uniquely avian trait after another has been
identified in nonavian dinosaurs, feathers have continued to stand out
as a prominent feature belonging to birds alone." (Padian K. &
Chiappe L.M., "The Origin of Birds and Their Flight," Scientific
American, Vol. 278, No. 2, February 1998, p35)

"Traditionally, two opposing scenarios have been put forward. The
"arboreal" hypothesis holds that bird ancestors began to fly by
climbing trees and gliding down from branches with the help of
incipient feathers. The height of trees provides a good starting place
for launching flight, especially through gliding. As feathers became
larger over time, flapping flight evolved, and birds finally became fully
airborne. This hypothesis makes intuitive sense, but certain aspects
are troubling. Archaeopteryx and its maniraptoran cousins have no
obviously arboreal adaptations, such as feet fully adapted for
perching. Perhaps some of them could climb trees, but no convincing
analysis has demonstrated how Archaeopteryx would have climbed
and flown with its forelimbs, and there were no plants taller than a
few meters in the environments where Archaeopteryx fossils have
been found. Even if the animals could climb trees, this ability is not
synonymous with arboreal habits or gliding ability. Most small
animals, and even some goats and kangaroos, can climb trees, but
that does not make them tree dwellers. Besides, Archaeopteryx
shows no obvious features of gliders, such as a broad membrane
connecting forelimbs and hind limbs." (Padian K. & Chiappe L.M.,
"The Origin of Birds and Their Flight," Scientific American, Vol. 278,
No. 2, February 1998, p35)


Stephen E (Steve) Jones ,--_|\
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