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Re: Say this slowly: theropoda is paraphylitic, theropoda is paraphlyitic.....




ELurio@aol.com wrote:

<< I think what he is arguing is that the theropod bauplan evolved several
times independently in nondinosaurian archosaurs, and that we're lumping all
these lineages into Theropoda because of their homoplasies. This isn't BCF,
since BCF assumes dinosaurian and theropod monophyly. >>


Bingo!!!!!!!! Thanks for the help.


Hmmm... With enemies like me you hardly need friends. My response was actually in *opposition* to your viewpoint. So you're not left with any lingering misconceptions, I'll set the record straight.

The available fossil evidence from the Triassic, Jurassic, and Cretaceous overwhelmingly favors the view that:
(1) The Theropoda is monophyletic (not polyphyletic, or as you put it, 'paraphyletic').
(2) The birds (Aves) are a lineage of theropods, which evolved from ancestors close to known maniraptoriforms, like dromaeosaurids and troodontids.


On the other hand, the evidence that _Longisquama_ is related to the ancestors of birds is weak, and relies almost entirely on the existence of a true furcula and whether its plume-like appendages are indeed feathers. The evidence for this is highly suspect.

According to Richard Prum, Curator of Ornithology (NOT a dinosaur paleontologist) at the University of Kansas, the "feather"-like appendages of _Longisquama_ appear more like ribbed membranes.
(1) Two of these appendages overlap, and the one on top clearly shows an imprint of the one underneath.
(2) The appendages do not fray at the edges the way bird feathers do.
(3) The "barbs" of the "feathers" curve away from the tip and toward the body, not toward the tip as in bird feathers. They also lack barbules.


All these features are very un-feather-like. (1) and (2) are more consistent with the interpretation that they are ribbed membranes. Alexander Sharov certainly thought they were scales, and I see no reason to disagree with him.

To add to that, there is no evidence that _Longisquama_ is an archosaur. The acrodont (rather than thecodont) dentition, was pointed out by Sharov back in 1970. Tom Holtz noted the presence of only six neck vertebrae in _Longisquama_, rather than at least eight as in archosaurs (including birds). _Longisquama_ looks more comfortable as a basal diapsid.

One thing I would add is that the "feathers" of _Longisquama_ are *huge* compared to the animal's body. The series of long scales down the dorsal spine must each be longer than the animal's body. (It is impossible to know exactly because the entire back half of the animal is missing - but a good part of the front half is preserved).


Tim


References

Stokstad, E. (2000) Feathers, or Flight of Fancy?. _Science_ 288: 2124-2125. (in News Focus)

Sharov, A.G. (1970).  [Title in Russian] Paleontological Zhurnal 1: 127-130.

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