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Re: [Fwd: MEGALANCOSAUR notes from Ruben.



John Ruben (through Betty Cunningham) wrote:

>
> Unfortunately, it's become an increasingly untrendy animal for serious

> discussion because as soon as the genus is mentioned, many
> "dinos-are-birds" zealots automatically react as if it were being
> proposed as the avian ancestor,....

I suppose as one of those who has spoken up about this, I should explain
why
we sometimes react the way we do.

First, some authors (though not necessarily John Ruben) have, indeed,
suggested that Megalancosaurus is a possible "ancestor" or "sister
taxon" or
whatever of birds.  Some references include:

Tarsitano, S.  1991.  Archaeopteryx: Quo Vadis? In H.-P. Schultze and L.

Trueb (eds.), Origins of the Higher Groups of Tetrapods. pp. 541-576.
Cornell University Press, Ithaca.

Feduccia, A. and R. Wild.  1993.  Birdlike characters in the Triassic
archosaur Megalancosaurus.  Naturwissenschaften 80:564-566.

So when, in 1998, John suggested in his SVP abstract that
"Megalancosaurus
may well provide valuable insights into intermediate stages leading to
the
origin of Aves," (JVP, 18:73A),  it was simplest to assume that he meant

"Megalancosaurus may be a close relative of birds."

Basically, with respect to this debate, we can approach Megalancosaurus
in
two ways:

1.  Megalancosaurus is a derived archosauromorph that shows strange
adaptations that could, possibly, be interpreted as adaptations for
gliding.  It might thus be relevant to discussions of the origin of
gliding.

2.  Megalancosaurus is a "thecodont" that may have been a close enough
relative of birds to say something about the origin of Aves.

Either Megalancosaurus is a close relative of birds (and thus relevant
to
discussions of the origin of the taxon), or it is not (and thus silent
about
this).  One could argue about Megalancosaurus' relevance to the origin
of
gliding behavior, but John's abstract addressed the origin of a clade
(Aves), not the origin of a behavior or function (gliding).

I suspect John was actually trying to say "birds must have had an
arboreal
gliding ancestor, and maybe it looked something like Megalancosaurus."
(John - if I'm misinterpreting you, I apologize.)  This would still be
objectionable from a phylogenetic perspective, because it assumes a
priori
something about the evolutionary process that we should be recovering
from
the phylogenetic pattern.



chris




--
----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

voice: 312-665-7633  (NEW)
fax: 312-665-7641 (NEW)
electronic:  cbrochu@fmppr.fmnh.org