[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Ornithoms, Parrots, and others

At 03:14 PM 11/12/98 EST, you wrote:
>In a message dated 11/12/98 8:44:18 AM EST, th81@umail.umd.edu writes:
><<  A large radiation of
> small, tree-climbing non-avian coelurosaurs is fully compatible with the
> standard model. >>
>But it isn't compatible with the "ground up" cursorial scenario of the origin
>of flight.

[*bingbingbingbing*]  The bells and whistles go off...

Yes, it isn't.

They are (now get this, everyone) TWO DIFFERENT THINGS.

There are a couple different arguments here, with both sides are guilty of

I) What is the phylogenetic position of _Archaeopteryx_ with regards to
Neornithes, other Mesozoic non-neornithine birds, and the various non-avian
dinosaur groups?

II) How many times was flight gained in Mesozoic theropod history?
     IIA) How many times was flight lost in Mesozoic theropod history?

III) From what behavior (trees down, ground up, trees up (Greg Paul's
model), etc.) did bird flight evolve?  (Note, if the answer to II is >1,
then different models might apply to different origins).

Answers to any one question may illuminate the others, but they are
different questions.

>In the "ground up" scenario, birds evolve flight as ground-dwelling
>animals by accumulating the anatomical features associated with flight for
>non-flight reasons. They become arboreal only >after< flying makes trees
>accessible. If the standard model admits tree-climbing non-avian coelurians,
>then it is virtually indistinguishable from BCF, and I can get off my soapbox.

As the standard model does admit tree-climbing non-avian coelurosaurs, it
might be indistinguishable from your model, true.

However, there is another component (that which is argued by Greg Paul, and
more recently by John Jackson): that several of the known lineages of
(traditionally) non-avian theropods (dromaeosaurs, troodontids,
oviraptorosaurs, etc.) are secondarily flightless, rather than secondarily
arboreal.  As framed by these workers, this concerns question I from the
above, and is independant of any particular answer to question III.

(That is that the hypothesis that dromaeosaurs and oviraptorosaurs are
_Archaeopteryx_ (or "archaeopterygian") descendants asks and answers
different questions than the question of bird flight origins, just as the
hypothesis that whales form the sister taxon to hippos is independant of the
question of the origin of ungulate feeding adaptations).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661