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Re: Doesn't George have a point?



On Thu, 29 Jun 2000, Larry Dunn wrote:

> What I'm asking is really simple.  Is it in line with
> prevailing standards of scientific analysis of
> prehistoric life to hypothesize that birds are
> dinosaur descendants when the dinosaurs the birds are
> said to have descended from come after the first birds
> in the known fossil record?  I'm certainly not saying
> that the answer is "no"; I'm just asking.  

Birds are not said to have descended from any Cretaceous dinosaur. Rather,
they are proposed to share common ancestry with lineages, e.g.

    LJ-->EK--------->LK--------->Tertiary

-+-----------========----======== Ornithomimosauria
 |-----------------------==--==== Tyrannosauroidea
 |-----== _Coelurus_
 |-----== _Ornitholestes_ 
 |-----==------== Compsognathidae
 `-+-+---------==------==----==== Oviraptorosauria (and relatives)
   | `---------==--==--------==== Therizinosauria
   `-+---------======--------==== Deinonychosauria (?incl. Troodontidae)
     `-==------=======================> Avialae

(The equals sign represents times where unambiguous fossils are known
from. *Very* rough, but should give some idea of the gaps involved.)

> And if the answer is "yes" (as is obviously strongly
> suggested by the work of many people on this list),
> what is the thinking that makes it so in this
> particular case?

The fossil record is incomplete. Proposing Deinonychosauria as the sister
group of Avialae is no more inconsistent with stratigraphy than proposing
Monotremata (platypuses, echidnas, etc.) as the sister group of Theria
(marsupials & placentals), or _Pan_ (chimpanzees) as the sister group of
Hominini (humans and "australopithecines").
 
> One respondant said that it's ok, as we've found that other lineages
> go back further than we had anticipated, and that may very well turn
> out to be true of Dromaeosaurs as well.  But as of right now, we don't
> have such evidence.  This is really quite a process-oriented question.

Basically the fossil record is so incomplete for most types of organisms
that stratigraphy is next to useless when trying to determine phylogenetic
relationships. Certainly we can say that birds didn't evolve from any
particular genus of Cretaceous Coelurosauria, but we can't rule out sister
group relationships based on stratigraphy. Arguments must be made based on
physical characteristics.
____________________________________________________________________________
T. Michael Keesey <tmk@dinosauricon.com> | AIM <Ric Blayze> | ICQ <77314901>
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