Gliders to Fliers?

["Matthew Bonnan" <mbonnan@hotmail.com>]

Dinogeorge said:

> Nobody has claimed that these animals lived in trees! (Where do these 
> notions
> come from???)

To quote you: "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."

Since I was going by the idea that Ceratosaurs are the sister group to 
Tetanurans (Padian et al., 1999), I figured they might be the ancestral, 
volant group of which you spoke.  I'm confused now, so please clear this up 
for me: are you saying that each major group (clade, grade, what-have-you) 
of dinosaurs descended in progressive order from arboreal archosaurs which 
eventually became the birds?  For instance, the Ceratosaurs came down first, 
followed by the early Tetanurans, then the Maniraptorans, etc.?

> The tree-dwelling ancestors of Herrerasaurus, Dilophosaurus,
> and so forth would have been small Middle Triassic animals a foot or two 
> long
> (with tail).

Would these small, Middle Triassic animals have the adaptations I described 
for tree-climbing mammals, or is there a different character suite?  Which 
small, Middle Triassic animals if you don't mind?

> Their large cursorial, terrestrial descendants had abandoned
> tree-dwelling for millions of years and evolved a suite of cursorial,
> terrestrial adaptations that changed the shapes of their feet and hands 
> (for
> example).

Surely, if the terrestrial theropods' hands and feet changed so much that we 
would have difficulty recognizing them as arboreal descendants, how would 
you know that they were?

> Can you see any arboreal adaptations in the skeleton of an ostrich?
> Yet most ornithologists will claim that it had an arboreal ancestral form.

This isn't a fair question.  It is a false analogy.  You're implying that if 
you can't see the arboreal adaptations in an animal we know came from the 
trees, how can you say the same for dinosaurs?  This is not fair because we 
know that the ostrich is a bird and is a descendant of once volant forms.  
We do not, on the other hand, know that dinosaurs are all terrestrial 
descendants of once volant forms: this assumes we already know the answer to 
the question we're asking.

But, to answer you question: yes, the limbs, while somewhat modified, are 
like those of all birds, with a spalyed, horizontally placed femur - 
something we see as well in all the volant forms.  And, the ostrich has 
wings which are now vestigal, but were most probably once used ancestrally 
for flying and getting up into trees.  The rigid backbone, the modified 
supracoracoideus, the still large pectoral muscles, etc., being yet other 
characters of once flying, and hence arboreal, ancestors.

Before we go further, let me say again that these questions stem from my 
giving your hypothesis a chance and are in no way to be taken personally.  I 
may not end up agreeing with you, or you with me, but I'm trying to keep us 
focused on function and bone structure, things that leave little room for 
"perhaps" and "maybe."  I really do want to hear what you have to say about 
the Middle Triassic archosaurs and their limbs, but you have to be ready for 
me to ask more questions and question some of your answers.  You'd only do 
the same to me, right? =)

Matt Bonnan

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