Re: New Article in Experimental Zoology

[ekaterina A <a_ekaterina@yahoo.com>]

--- Dinogeorge@aol.com wrote:
> bypassed dinosaurs in 
> evolving from small arboreal archosaurs. Rather,
> birds and dinosaurs both 
> evolved from small feathered arboreal archosaurs,
> with different kinds of 
> dinosaurs (= giant, flightless dino-birds) evolving
> repeatedly from different 
> kinds of arboreal archosaurs throughout the Mesozoic

Better preserved fossils of forms like Scleromochlus
would definitely help in clarifying if you hypothesis
stands a chance. Still we have quite a few primitive
dinosaurs and dinosauromorphs:
Marasuchus, Lagerpeton, Lagosuchus, Eoraptor,
Aliwalia, Staurikosaurids, Pisanosaurus. In not one of
them do we see any special arboreal adaptations.Your
hypothesis would demand that we see at least a few
signs of them. Even in the predinosauromorph
archosaurs we see very little evidence for major
arboreality. So the most parsimonious explanation is
that the cursorial primitive archosaur ancestors gave
rise to cursorial early dinosauromorphs. Why mutliply
the hypothetical entities when there is no evidence
for them.

Prum's work shows that the feathers develop through a
pathway that is mirrored in evolution by structures
seen in coelurosaurs. Thus the emergence of feathers
and their orthologs are quite independent  of flight.
It may be that the feather ancestors began emerging in
the common ancestor of all dinosaurs or their ancestor
with the pterosaurs (as I believe). But why do we need
these ancestors to be arboreal from the begining.

Birds are the closest sister group of the
deinonychosaurs so even the term BCF is basically not
a very accurate description. I guess you do not imply
that birds represent the ancestral dinosaurian state.
Merely that the ancestral dinosaurian morphologically
resembled birds in a few features linked to tree-life.

> lineages that must have 
> appeared: note intense mosaic evolution among
> Liaoning forms, for example). 
> Liaoning finds are very significant not just in
> showing feathered dinosaurs 

But mosaicism is seen elsewhere in evolution: even the
early tetrapods show rampant mosaicism of characters
linked with the advanced tetrapods. If I understand
you right all these mosaicisms are mere reversals from
an already derived ancestor. But there is another
evolutionary pathway that can entirely account for it:
rapid evolutionary diversification that accompanies
opening of new niches.

> My guess is that mammal competition during the later
> Cretaceous made strong 
> inroads into terrestrial/arboreal dino-bird
> diversity, so that when the 
> asteroid impact destroyed the big dinosaurs, mammals

But then even today it is the avifauna that dominates
the arboreal and aerial niches. Every day I see more
dinosaurs than mammals in elevated places. So where
was the mammalian challenge? There seems to be more
nocturnal mammal life in the trees than anything
else...
-EA

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