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Re: semilunate carpal



Dinogeorge wrote:

<< All that's missing in this intuitively attractive, well-thought-out,
 explicit hypothesis is the fossil evidence to back it up. >>

This is true of >all< the hypotheses of bird origins. The cladograms used by
the ground-up theory apparently ignore the fact that there are millions of
years of evolution between all the known early theropods and their common
ancestors with birds.

We are not talking about *all* the known early theropods - just the ones that seem closest to the origin of birds. And there are quite a number of small maniraptorans in close phylogenetic and temporal proximity to _Archaeopteryx_. (_Microraptor_, _Sinornithosaurus_...).



Why would anyone think that the common ancestor of,
say, Coelophysis and birds had to look more like Coelophysis than a
primitive, archaeopteryx-like bird?

Back again we go to the concept of phylogeny. Why? Because an analysis of anatomical characters indicates that the common ancestor of _Coelophysis_ and birds was phylogenetically closer to known basal ceratosaurs than any known flying maniraptoran. So, this undiscovered common ancestor probably looked more like a coelophysid than an archaeopterygid.


Of course, you could counter that we don't have enough fossils to *know* what the common ancestor of _Coelophysis_ and birds might have looked like. I would say (1) that's why we have cladistics; and (2) if you start plugging holes in intuition-driven phylogenies with a plethora of hypothetical and undiscovered flying birdy-things from the Triassic, then I think we've entered the realm of Feducciary phylogeny. Game over.

Anyway, the common ancestor of birds and velociraptorines *did* look a lot like _Archaeopteryx_. Osteologically, _Archaeopteryx_ looks a lot like a dromaeosaurid. This is hardly controversial.


<< Velociraptorines and birds share a common ancestor, and that ancestor used
the predatory stroke (sensu Gauthier and Padian) - the lunging,
forward-and-downward motion of the forelimb and the inward movement of the
manus. >>


You don't know this any more than I know that the common ancestor had a wing.

I contend that the common ancestor of birds and velociraptorines did *not* have a wing in the sense that it was used in sustained flapping flight. The forelimb was used to grasp prey and, within the ancestors of birds, it became exapted for aerial locomotion.



We have no direct fossil evidence of the common ancestor, so it could have
been either birdlike or velociraptorlike or both or neither. You are
inferring the appearance of the common ancestor via your acceptance of the
ground-up hypothesis of bird origins.

Whoooaaaaahh!! Ease up there. I never said I favored a "ground-up" theory for the origin of bird flight. I actually prefer a "trees-down" theory, among semi-arboreal ambush predators.


To me, the simple mechanical reasons
for discarding the ground-up hypothesis are sufficient, so the birdlike
nature of the common ancestor is the favored hypothesis over the
velociraptorlike nature.

IMHO, a "trees-down" theory is completely compatible with a maniraptoran origin of birds.



Tim


---------------------------------------------------------------------

Timothy J. Williams

USDA-ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163



From: Dinogeorge@aol.com
Reply-To: Dinogeorge@aol.com
To: dinosaur@usc.edu
Subject: Re: semilunate carpal
Date: Wed, 8 Aug 2001 16:31:29 EDT



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