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Sci. Am. - present.



Thoughts on reading Sci. Am. Feb '98:
"The Origin of Birds and Their Flight"
by Kevin Padian & Luis M. Chiappe

Their paper subscribes to the Birds Are Dinosaur Descendants theory,
"ground up" variety.  (Other theories include: BADD (Trees Down); Birds and
Dinos are Unconnected; and the most modern, elegant, unembarrassed by
evidence and supported by the writer: 'Birds' Came First.)

Maybe the authors did their best with a bad hand, but it wasn't good
enough.  They cannot explain why Archy's near ancestors (strictly
non-arboreal and non-flying or gliding, they say) lengthened the arm while
numerous other lines shortened it.  The semi-lunate wrist bone which lets
the wrist twist out sideways allowing birds to fold their wings, they say
allowed the hand to "be rotated round and whipped forward suddenly to catch
prey".  Unfortunately this movement on its own moves the claws sideways,
and would have to end with a conventional downwards movement for a strike. 
The prey of animals which flick something out, is always taken by surprise
while within range of the extension: insects caught by chameleons, frogs
and mantids, and fish caught by squids' telescopic arms are seldom fleeing
at full speed.  Dromeosaurs could not hunt by creeping up to within range
of such a flick; for one thing it would bump its nose against the prey
first.

As usual we see Greg Paul's Velociraptor skeleton laughing her head off at
the folly around her - or maybe she's just panting in an effort to get past
the Archaeopteryx in front and take up her proper place on the cladogram. 
Why must a media superstar like her always suffer the indignity of being
placed ancestral to a creature 70 million years earlier?  Because too much
faith has been placed on the ability of cladistics to deal with unprepared
input.  Objectivity is nice in science,  but some traits input into
cladograms really are more equal than others, and can easily be swamped by
multiple less significant features. The dozens of flight-dependent
characteristics such as arm length and feather symmetry change in a flash
upon loss of flight and are way too labile to be used as a guide for
relationships.  They will always group fliers together.  By priming the
cladistic process erroneously, for example by placing special emphasis on
"flying" and "nectar-drinking", we can show bees are related to
hummingbirds, or even to shunting engines if we choose "humming" and "black
and yellow markings" instead.  The first toe in larger non-arboreal
theropods was probably used for scratching (or preening), so it usually
pointed forwards.  In smaller arboreal forms when reversed, it could help
with stability on branches.  Since we KNOW we have to use some common sense
with cladistic input, we must surely give features such as uncinate ribs,
more weight.  Surely a feature such as this is more significant than the
highly variable 1st toe orientation?

A guiding principle in science is - look for the simplest explanation. 
Most people accept that bats and pterosaurs attained flight through an
arboreal gliding stage. Gliding lizards ancient and modern are known, along
with frogs and snakes but for some bizarre reason large numbers of people
have found this route to flight in birds hard to accept.  Despite earlier,
famous supporters now having changed their minds, Padian and Chiappe still
think all those pre-adaptations miraculously suited to flight simply
accumulated in flightless animals over 70 million years.  Here's how they
express their mistrust of the trees-down hypothesis:

"[It] 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 metres 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."

That paragraph, which is unworthy of a professor or a respectable
publication, nonetheless contains a couple of interesting points. First,
there were rather a lot of palm-tree-shaped plants in the Jurassic, and
only conifer branches to perch on, so trunk-climbing ability was relatively
more important than branch expertise.  Secondly if Archy or its near
ancestors had learned to fly in one environment, there would be no reason
to stay there forever.  Many pterosaurs are thought by most to have been
marine, so they didn't stay near trees all the time.  Just seven million
years ago our ancestors lived in trees yet a billion humans have lived and
died without ever seeing a forest.

In summary:  Misuse of the cladistic process, total failure to explain the
uncanny homing in of flightless forms towards bird-hood over most of the
Cretaceous, reliance on unjustified beliefs such as "the immediate theropod
ancestors of birds were terrestrial", and the apparent ignorance of the
elegant and untroubled BCF theory, go together to make a pretty dreadful
piece of science writing.

John V Jackson    jjackson@interalpha.co.uk

(Wannabeasaur  beecee-effia - & proud of it!)