Re: T-Tip

["Mickey Mortimer" <mickey_mortimer@email.msn.com>]

Dan Bensen wrote-

>They do, however, look a lot like _baby_ tyrannosaurids (especialy the
>ornithomimes). Imagine a primitive small predator (probably a lot like
>compsognathus) which, at the end of the Jurassic, diversified into three
kinds.
>One, was a heron-like runner, the other, a predator of small animals that
used >its hands, the third was a predator of large animals that used its
mouth and not its
>hands.  All of the superficial adaptations of the three groups, the long
necks of
>the ornithomimes, the sickle-claws of the troodonts, and the two fingers of
the
>tyrannosaurs, are all reflections of those three lifestyles.


I know several people (including Tom Holtz) believe tyrannosaurs,
ornithomimids and troodontids form a monophyletic group.  Try as I may,
however, I just can't find many synapomorphies to link them together.  Sure,
an arctometatarsalian metatarsus is shared by all three, but is absent in
the Thai ornithomimid and probably Harpymimus (not to mention the
Tonouchisaurus rumors).  In addition, it obviously convergently developed in
caenagnathids, mononykines and Protoavis (whatever it/they is/are).  Other
characters listed by Holtz (1994) (the only published study I know of that
supports this hypothesis) are also dubious:
large surangular foremon- not so in ornithomimids, also present in
dromaeosaurids.
semicircular proximolateral ischial scar- I've never seen any proof of this
in ornithomimids or troodontids (correct me if anyone knows otherwise).
iliac blades contact along dorsal edge- also present in oviraptorosaurs
(according to Makovicky and Sues, 1998) and mononykines.
In my analyses, tyrannosaurs are always basal coelurosaurs, ornithomimids
are up a bit higher and troodontids are avialans.  I see a few potential
troodontid-ornithomimid synapomorphies and a few tyrannosaur-ornithomimid
synapomorphies, but never any for all three.  So, does anyone know of any
good characters to unite these dissimilar groups?

Mickey Mortimer