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Re: Ornithomimids and Ratites (was Re: Big Dinosaur Prints Found)



Tim Williams wrote-

> I don't know if this has been published or not (I heard it word-of-mouth),
> but _Timimus hermani_ (Dinosaur Cove, Victoria; Aptian) is now regarded as
> Theropoda _incertae sedis_.  I know Pat and Tom Rich originally described
> _Timimus_ as an ornithomimosaur, based on the slender proportions of the
> femora.  It *might* be an ornithomimosaur, it's just that based on the
> available material (two femora), there is not enough evidence to warrant
> such a referral.  I'm not even sure if _Timimus_ can be confidently
regarded
> as a coelurosaur, since the femur also bears an uncanny resemblance to
that
> of _Elaphrosaurus_.

Timimus shows an inclined femoral head, suggesting it is a tetanuran (and
distinguishing it from Elaphrosaurus).  In addition, the anterior trochantor
reaches above the head, which is only known in some coelurosaurs.  While the
original description cites this character as being an ornithomimosaur
synapomorphy, it is only known in tyrannosaurs and maniraptorans.
Ornithomimosaurs (eg. Archaeornithomimus, Gallimimus) and some basal taxa
(Deltadromeus, Scipionyx, Nedcolbertia, etc.) have lower anterior
trochantors, more comparable to carnosaurs.  Also, the authors diagnose
Timimus solely on the basis of lacking an extensor groove.  This is also
true of Archaeornithomimus among ornithomimids.  Most non-tyrannosaur
coelurosaurs do lack extensor grooves (Nedcolbertia, Nqwebasaurus,
Archaeornithomimus, Microvenator, mononykines, Avimimus, Saurornithoides,
Achillobator, Velociraptor).  The only non-tyrannosaur coelurosaurs with
extensor grooves that I know of are Gallimimus, Bagaraatan and Patagonykus.
The first has an oddly pinched closed groove that is unique as far as I
know, while the other two have very slight concavities.  The authors state
the absence of an extensor groove is primitive, but it seems derived within
Tetanurae (extensor grooves are present in tyrannosaurs, carnosaurs,
megalosaurs, spinosaurs, Fukuiraptor, etc.).  Even the abelisauroid
Elaphrosaurus has an extensor groove.  Only Dilophosaurus and coelophysoids
among basal theropods lack extensor grooves.  To summarize, the inclined
femoral head suggests it is a tetanuran, the tall anterior trochantor
suggests it is either a tyrannosaur or a maniraptoran, and the lack of an
extensor groove shows it is a non-tyrannosaurian coelurosaur.  This
indicates Timimus is probably a maniraptoran.  The well-separated anterior
and greater trochantors are not found in eumaniraptorans.  Thus, perhaps
Timimus is a segnosaur, oviraptorosaur, avimimid or alvarezsaurid.
Segnosaurs are as of yet unreported from Australia and seem to have more
robust femora, judging by Beipiaosaurus.  Oviraptorosaurs have been reported
from Australia (Currie, Vickers-Rich and Rich, 1996), so may be good
candidates for Timimus' identity.  Microvenator is roughly similar, but has
more divergent, smaller distal condyles with a wider flexor groove.
Caudipteryx and Nomingia are derived in having only a small separation
between their trochantors.  Chirostenotes has a more divergent medial
condyle and a less divergent lateral condyle, both of which are much
smaller.  It is more robust and not sigmoidal.  Ingenia's femur is also more
robust, with lower anterior and greater trochantors, and a narrower, more
laterodistally projected lateral condyle.  Kakuru kujani has been referred
to the Avimimidae, so this family may have been present in Australia as
well, although the taxon needs further study.  Avimimus has a more slender
femur with a more medially placed greater trochantor and a strong posterior
trochantor not seen in Timimus.  The lateral condyle is more divergent, both
condyles are smaller, and the flexor groove is wider.  The manual phalanx
named Rapator has been referred to the Alvarezsauridae, showing the family
was present in Australia.  Among alvarezsaurids, only the basal taxon
Alvarezsaurus has well-separated anterior and greater trochantors.  It's
femur is only known from a partial proximal portion, so is not very
comparable to Timimus.  It does have a much more elongate head than the
latter taxon however.  In conclusion, although Timimus is probably a
non-eumaniraptoran maniraptoran, it cannot be placed more exactly with the
description and figures currently available.  If a posterior trochantor is
present, oviraptorosaurs can probably be excluded as a possibility.
Patagonykus and mononykines share some distal femoral characters that if
identified in Timimus, could show it is an alvarezsaurid (Timimus certainly
lacks one- medial condyle proximodistally compressed).  Now that Timimus is
probably not ornithomimosaurian, it would be interesting to examine the
Australian pubis and unguals to see if evidence of aussie ornithomimids does
exist.

Mickey Mortimer