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RE: Tarsitano's challenges



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Ken Kinman
>
>       I am still trying to play "catch up" on the theropod-bird
> controversy.

Make sure you read the follow-up news article in Science by Stokstad.  It
pretty well sums up the "contraversy": there is the majority of workers who
use a repeatable, independantly verifiable (run the matrix yourself),
explicit methodology on the one hand; on the other are a few people who use
a non-verifiable, non-explicit (so, is _Longisquama_ closer to birds than
coelurosaurs?  Note that the article does not say yes or no), scenario-based
almost-hypothesis.  I say "almost-hypothesis" because, in such cases as the
_Longisquama_ paper, the authors make absolutely no commitment to a
particular position.  They state a) these integumentary elements may have
something to do with feather origins and b) this animal may have something
to do somewhat with the origin of birds.  How can you disprove either of
these vauge generalities?

(In point of fact, outside of this paper these authors have only been
offering an antithesis, not an hypothesis: they offer the statement
"dinosaurs are not bird-ancestors", but do not offer "taxon x is more
closely related to birds than are coelurosaurs".  They have implied that
various "avimorph thecodonts" are possible models for bird ancestors, but
have not committed to any one (or more) of this diverse group as a
provisional ancestor).

>    I am wondering which of the following problems posed by
> Tarsitano in 1991
> (Archaeopteryx: Quo Vadis) are still problematic, and which have been
> resolved by newer coelurosaur fossils:
>       "The following morphological problems involving the
> theropod ancestry
> of birds must be addressed by those who favor a coelurosaurian
> ancestry or
> sister-group position for birds:
>       (1) the absence of a siphonium and pneumatic quadrate in all
> coelurosaurs;

Pneumatic quadrates are confirmed in oviraptorids, tyrannosaurids,
ornithomimids, and troodontids.  The non-pneumatic nature of other
coelurosaur quadrates has not been confirmed at present.

(Of course, Tarsitano was surprised at SICB 1999 to learn that
tyrannosaurids were coelurosaurs.  I, he, Dodson, Farlow (I think), Ruben,
and a few others were at a table, and somehow the subject of tyrannosaurids
as coelurosaurs came up.  Tarsitano said "who ever said that".  I can't
remember if Dodson said, pointing to me "He did" or I said "I did" first: it
was pretty close to simultaneous).

>       (2) the lack of clear evidence of a fenestra rotundum in
> coelurosaurs;

The fenestra rotundum (actually the fenestra pseudorotunda or fenestra
cochleae: it is not homologous to the mammalian fen. rotunda) is present in
troodontids, oviraptorids, therizinosauroids, and dromaeosaurids (at least).

>       (3) nonavian tooth morphology in coelurosaurs;

Tooth implantation of _Archaeopteryx_ is similar to that in troodontids.
Furthermore, there is no single "tooth morphology" of coelurosaurs: instead,
coelurosaurs (even independant of birds) have in gross terms the most
diverse set of teeth of the major theropod clades.

>       (4) reduced coracoids in all coelurosaurs;

Complete and utter crap.  Coracoids of dromaeosaurids, troodontids,
oviraptorids, and therizinosauroids (at least) are large; those of
tyrannosaurids and ornithomimids are comparable in proportion to more basal
theropods.

>       (5) the presence of a massive and specialized pelvis and hindlimbs
> that, biomechanically, seem to argue against the devlopment of flight;

Massive?  Large, yes: indeed, the largest pelves among archosaurs are in
(cue music) birds!  So, that's not worth much.

Specialized?  Sure thing, coelurosaurs have varying degree of
specializations in their pelves.  Indeed, dromaeosaurid pelves are so
specialized they are nearly identical to those of basal birds
(_Archaeopteryx_, _Rahonavis_).

>       (6) the presence of the middle temporal arch in
> coelurosaurs, but its
> absence in birds;

The middle temporal arch is present in the basal bird _Confusciusornis_;
evidence in _Archaeopteryx_ is ambiguous.  No one ever said that evolution
stopped when birds first diverged from other dinosaurs...

>       (7) the presence of lateral basicranial sinus openings in
> coelurosaurs, but their absence in birds.

Yep, birds have lost the lateral basicranial sinus openings.  Evolution
happens.

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