Re: Ornithoms, Parrots, and others

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 07:53 PM 11/11/98 EST, Diogeorge wrote:
>In a message dated 11/11/98 7:33:15 PM EST, A.Yates@zoo.latrobe.edu.au writes:
>
><< In conclusion I'd say that Tom is backing a winner. >>
>
>Tom is also backing BCF, perhaps unwittingly. Both BCF and BADD predict pre-
>archaeopterygid dromaeosaurids and other maniraptorans. This alone is not a
>criterion that will distinguish between the two scenarios.

True enough.

>A preponderance of
>unquestionably ground-dwelling, cursorial pre-archaeopterygid maniraptorans
>might support BADD over BCF. BADD predicts virtual absence of small, arboreal
>maniraptorans (tough to establish >absence< when fossil record is so poor!)
>until just before appearance of _Archaeopteryx_;

No, it doesn't.  Sheesh, but could folks actually try to understand what
others argue?

The standard model (BADD, BAMM, whatever little acronym you want) predicts
that _Archaeopteryx_ lies closer to modern birds in terms of phylogeny than
do other maniraptoriform lineages: dromaeosaurids, oviraptorosaurs,
troodontids, etc.  That's it.

The timing of the event is a separate issue: the divergence between Avialae
and other maniraptoriforms is minimally immediately pre-Tithonian, but could
have occurred at any point between that date and the divergence of
maniraptoriforms from other coelurousaurs.  This might be in the Middle
Jurassic, Early Jurassic (Maniraptoriformes might well be this old if the
Yunnan therizinosauroid really is a therizinosauroid), Late Triassic, or
theoretically even earlier (although since no one has discovered definite
pre-Late Triassic dinosaurs, this is unlikely.

The behaviors associated with the transition is ALSO a separate issue.  This
is where the BCFers seem to make there biggest errors: they conflate the
*phylogenetic pattern* with the *behavioral pattern*.  (They are not the
only ones guilty of this, of course).  If, for example, someone could
demonstrate anatomical features in basalmost birds birds, or dromaeosaurs +
birds, or oviraptorosaurs + (dromaeosaurs + birds), or..., which are
strongly correlated with scansorial behavior, then this would support an
scansorial lifestyle for the lineage leading to birds.  A large radiation of
small, tree-climbing non-avian coelurosaurs is fully compatible with the
standard model.

>BCF predicts >lots< of small
>(bird-size), arboreal maniraptorans throughout Jurassic and perhaps even as
>far back as Late Triassic.

However, the unique and necessary prediction of BCF (or at least some of its
incarnations) is this: that dromaeosaurids, oviraptorosaurs, and possibly
some other forms are secondarily *flightless*: that is, that they lie
phylogenetically within a clade of volant animals.  This is a fully testable
hypothesis, which is why boiling it down to this essence is the main thing
going for it.

At present, no one has been able to produce a most parsimonious tree from
explicit data sets which support this conclusion.  However, there is nothing
in the analyses which would prevent such a result, and indeed future studies
may well support it.

Hoping to clear things up.

P.S.  For some non-avian theropod fun, stay tuned to the news tonight.

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