Re: Gliders to Fliers?

["Jaime A. Headden" <qilongia@yahoo.com>]

Dinogeorge wrote:

<If you look at the earlier, less derived theropods,
such as ceratosaurs and dilophosaurs, you find the
forelimbs retain a grasping function, which is greatly
diminished in the more advanced and birdlike
theropods. Sam Welles once told me, very emphatically,
that _Dilophosaurus_ most definitely had an opposable
pollex digit on the hand that when used with the other
two large digits could grasp and hold things. I could
never understand why theropods would have >lost< this
marvelous and useful ability until I realized that the
hands of the more advanced theropods are derived from
the fairly good wings of their volant ancestral forms.
The grasping ability gave way to a probably more
useful aerial function.>

  Am still relatively perplexed over the idea that the
tiny, flexible manus of ceratosaurians (I guess
*Podokesaurus*, Coelophysoidea, and Ceratosauroidea)
with their semi-rigid shoulders, tiny arms, and nearly
absent fourth and fifth fingers, can be hypothesized
as having derived from a more rigid, winglike
structure. This perception (my perplexity) is based on
the assumption that all animals requiring their arms
to achieve flight or the comparative function
(pterosaurs, bats, birds, flying lemurs, or squirrels
or sugar gliders) have virtually inflexible supportive
structures, with a possible exception of bats, given
their digit-controlled flight membrane. What I do not
see is the possible avian-form wing-system being
exapted into a non-supporting structure, then
developing into a wing.

  However, and not to say I "follow the crowd," there
does in fact seem to be a precise evolutionary series
from crocodiles, to lagosuchids (reduced digits), to
ceratosaurians, to stem-tetanurans (reduced digits),
to maniraptorans (swivel wrist, bowed ulna, longer
arm, etc.), to avians (etc., etc., blah-blah, you
_all_ know the transformations here). Now, while the
reduced arms in ceratosaurs are not precisely an
impairment to climbing, and the flexible hand (I do
agree on this point) makes a good grasping tool, the
consistent shortness of the arm in archosaurs until
maniraptorans does not appear to allow for a
avian-style wing until the mid- to late Jurassic, with
the split of dromaeosaurs and avialaeans. The
hypothesized intermediate between ceratosaur-style and
bird-style appears in dromaeosaurs and *Unenlagia,* as
early as 140mya (Barremian, EC), so what room is there
for a similar intermediate (or rather, stage) in the
mid Triassic, around the divergence of the lagosuchids
and probable advent of Dinosauria?


=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

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