Re: Gliders to Fliers?

[dbensen <dbensen@gotnet.net>]

>>  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?


<<
Why not have the grasping arm having evolved for some reason other than
climbing and then primitive, small ceratosaurs using that adaptations to
get into the trees.  These squirly ceratosaurs might have evolved flight
the same way that flying squirrels are.

Dan

"Jaime A. Headden" wrote:

> 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."
>
> Qilong---is temporarily out of service.
> Check back soon.
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