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

RE: Terrestriality is a bias

> -----Original Message-----
> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> dbensen
> Sent: Tuesday, June 06, 2000 3:43 PM
> To: tholtz@geol.umd.edu
> Cc: gbabcock@best.com; tlford@ix.netcom.com; Dinonet
> Subject: Re: Terrestriality is a bias
> >> Well, almost NOBODY claims that dromaeosaurs are the ancestors of
> birds!
>         Sister group, yes, but not ancestors.
>         (I said "almost" because some recent evidence suggests that
> "Dromaeosauridae" as
>         traditionally defined is paraphyletic with regards to birds, in
> which case the ancestors
>         of birds (and thus birds themselves) would indeed be
> dromaeosaurs.  Will have to wait
>         and see on that one!)<<
> What about GS Paul's theory that dromaeosaurs are flightless early
> birds?  Does anyone still hold that theory to be valid (especialy in the
> light of the new little dromaeosaurs)?

Well, you have to separate this hypothesis into its two components:
1) the relative phylogenetic positions of _Archaeopteryx_, Dromaeosauridae
(or its constituent taxa), and pygostylian birds;
2) the number of times flight evolved in Theropoda.

As for 1:

At present, none of the phylogenetic analyses I know of result in a
cladogram consistent with Greg Paul's specific hypothesis: that is,
(Dromaeosauridae + _Archaeopteryx) + (?one or more other maniraptoran
groups? + pygostylian birds).  Similarly, none has yet yielded other
"dromaeosaurs as early bird" results: for example, _Rahonavis_ +
_Archaeopteryx + (Dromaeosauridae + pygostylian birds).  These WOULD be a
potential result from at least some data matrices (the Forster et al.
_Rahonavis_ study, or my Gaia & Ostrom Symposium matricies, for example),
although others (such as Sereno's 1999 Science matrix, or the Makovicky &
Sues 1998 one, or my old 1994 one) are technically incapable of yeilding
this result, as _Archaeopteryx_ is included with other birds as a single
operational taxonomic unit.

Although this result has not yet found, it is not an unreasonable position
given the similar anatomy of the various dromaeosaurids, _Archaeopteryx_,
_Rahonavis_, etc.

So, for 2:

Given that Paul's hypothesis is not currently yielded by present studies,
bird flight is most simply resovled as evolving once (in the common ancestor
of _Rahonavis_, _Archaeopteryx_, and  pygostylian birds).  However, should
some future study show _Archaeopteryx_ is the sister group to
dromaeosaurids, and this clade is the sister group to all other birds, then
we are faced with two choices: flight evolved in the common ancestry of all
of these, or evolved independantly in _Archaeopteryx_ and in other "birds".
However, should dromaeosaurids be unambiguously nested WITHIN known (or
pretty well supported) fliers (for example: _Archaeopteryx_ + (_Rahonavis_ +
(Dromaeosauridae + pygostylians))), then the idea that dromaeosaurids were
secondarily flightless would have strong phylogenetic support.

Incidentally, to address a point raised by Tracy earlier: tree-climbing
theropods are not "laughed at" by either myself or Phil Currie.  I think it
is quite reasonable to expect at least some of these guys, in particular
smaller long armed maniraptorans, spent some time in the trees.  However,
that is different from saying that we have good hard morphological or
morphometrical evidence supporting this habit.  The evidence might well be
there, but hasn't been synthesized or tested yet.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843