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Re: Therizinosauroids (was Re: Giant Birds)



I wrote:
 
<<14 characters exclude *Caenagnathasia* from
Oviraptoroidea, on the basis of shared postcranial and
cranial characters of Caenagnathidae and
Oviraptoridae.>>

<How could shared postcranial characters exclude a
taxon known only from cranial fragments? Or has more
been discovered of Caenagnathasia besides the original
dentaries?>

  Sorry: 6 of the 14 that join caenas + ovis exclude
*Caenagnathasia,* 1 ambiguously, whereas only 4 group
them, and 1 is ambiguous as well. I'd say these are
good odds. Most are also autosynapomorphic, with the
exclusion of "edentulous dentary" which is also found
in ornithomimids
 
<<both Caudi and theriz's have a notch in the pubic
pedicle, suggesting the disarticulated pubis in Caudi
was retroverted.>>

<I know Caudipteryx has this and Segnosaurus would
appear to also, but it would seem to be absent in
Beipiaosaurus, Alxasaurus, Nanshiungosaurus and
Enigmosaurus.>

  Present in Beipiao, incomplete in Alxa to my
knowledge, and in Nanshiung and Enigmo, the region is
fused over, obscuring sutures in the latter given
given figures (Barsbold and Perle, 1976; Barsbold,
1983).
 
<<All have a fairly short ischium, a feature I see a
_lot_ in "mesopubic" maniraptorans/maniraptoriforms
including "dinobirds" such as *Rahonavis* and
*Unenlagia* (even *Sinornithosaurus* has the requisite
features of a mesopube even though the pubic pedicle
is deeper than the ischiadic one);>>

<Ingenia (the only oviraptorid for which I have this
information) has a plesiomorphically elongate ischium,
as do all therizinosauroids that can be measured
(Alxa, Segno, Enigmo, Nanshiungo).>

  Perhaps I should have extemporized: the ischium in
oviraptorids (indeed, in all maniraptorans) has a
distally displaced obturator prong. In oviraptoroids
[="caenagnathoids"] the ischium is rather large, but
the obturator prong is very distally directed,
originating in a distal 'half' shorter by almsot 30%
than the proximal 'half'. Therizinosauroids have a
distally displaced obturator prong that is so far down
the shaft (or plate) that there is no distal half: the
prong's at the end of the bone, from which the
proximal half is elongated, an autosynapomorphy of
therizinosauroids in my opinion.

<I thought that only Rahonavis, among
non-pygostylians, had a mobile scapulocoracoideal
joint. Or perhaps you're speaking of something else.>

  And my pardon again, for setting off a mini-thread.
There are three conditions I recognized in the
scapulocoracoids of dinosaurs: open&mobile,
open&locked, and fused; or mobile, locked, and fused,
for short. The mobile coracoid has been discussed to
some detail, so I'll jump to the other two: a locked
coracoid is one in which a "hinge" is not present, but
the particular elements still possess an open
connection. Fused speaks for itself. As Nick Longrich
explained, ontogeny of the scapulocoracoid in
maniraptorans appears to progress from locked to
fused, with possibly a mobile form originally, which
*Rahonavis* may have paedomorphically retained into
ontogeny -- that is, it kept the "baby" condition.

  Anyway, there is no mobile scap.cor. in
oviraptorosaurs, but a fairly developed
*Chirostenotes* (RTMP 79.20.1; about 80% full grown
given larger but identical specimens) has a locked
form, whereas all known oviraptorid scap.cor.s I've
seen (admittedly, not all of 'em, but quite a few,
including from many different taxa) are fused. A
definate ontogenetic series is lacking in
oviraptorids, but this should be resolved soon and may
clear some of my confusion.


=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

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