Therizinosauroids and the Question of the "Segnosaur" Pelvis [long!]

["Jaime A. Headden" <qilongia@yahoo.com>]

Everyone, whoops!

  I did mean mt I when I wrote the last message, and George caught me
on that, so thanks. On to the discussion.

I wrote:

<<The pelvis hardly asserts a relationship with ornithschians, becuase
the the obturator prong in the segnosaurs occludes the pubis, which it
does not in any other group except birds.>>

Dinogeorge wrote:
 
<Presence of obturator on ischium in segnosaurs is autapomorphy (note
obturator is absent in primitive ornithischians). So this is by no
means necessarily an avian(!) condition. Or are you arguing that
segnosaurs are more closely related to >birds< than to any other
theropods?>

  No such thing. I did realize later that the obturator prong on
segnos was autapomorphous, but I'd sent the message already, without
even proofing it! Again, my appologies.

<<The form of the ilium is nearly identical to caenagnathids,
*Caudipteryx*, and *Rinchenia* except for the lateral deflection of
the anterior blade in which case it resembles some sauropods,
pachyderms, and giant ground sloths. An apparent feeding adaptation,
if you ask me.>>
 
<Form of segnosaur ilium is totally unique to dinosaurs and not
"nearly identical" by any stretch of imagination to ilium of
caenagnathids, oviraptorids, and so forth. Likewise sacrum. Study
segnosaur ilium in dorsal view, not just lateral view, which can be
misleading. Segnosaur ilium has hypertrophied anterior ramus, already
well developed in advanced prosauropods. Many other features also
autapomorphic. Vague resemblances of a few scattered theropod ilia
representing several different groups to segnosaur ilia are quite
clearly convergent, since ilia of other theropods from same groups
quite unlike segnosaur ilia.>

  I have examined these ilia in dorsal aspect, having Perle, 1976, but
I need to get my hands on Dong, 1976, which this message reminded me
sorely that I needed. And the one which describes *Nanshiungosaurus
bohlini*.

  Here's something I'd prepared for something else, but it applies
here as well, in truncated form (pardon for the jargon, and full refs
are not given within, but all refs of "1990" are in _Dinosauria_ for
simplicity's sake, and the others are found on my site):

  The pelvis of therizinosauroids is very unique, but qualities
suggest relationship to other members of Dinosauria, including:
sauropodomorphans, hypsilophodontian grade and pachycephalosaurian
ornithschians, and maniraptoriform (especially maniraptoran)
theropods. The shape of the ilium is nearly identical to that of the
oviraptorosaurians *Chirostenotes* and *Rinchenia* (Currie and
Russell, 1988; Barsbold, Maryanska, and Osmolska, 1990) and
brachiosaurid and titanosaurid grade sauropodans (McIntosh, 1990) in
that the ilium has a longer anterior blade than a posterior one, and
that the posterior blade is very highly abbreviated. Only in
*Chirostenotes* and sauropodans, though, does the ilium match the
shape of the segnosaur ilium is being tapered to a point (Currie and
Russell, 1988; McIntosh, 1990), as well as an indistinct incomplete
sacrum from the Santana Formation of Brazil which has been compared to
oviraptorosaurs (Frey and Martill, 1995).

  The width of the pelvis is suggestive of an enlarged gut, and the
wider posterior width is similar to that of pachycephalosaurians,
especially *Homalocephale* (Maryanska, 1990) but the sacral ribs are
much more robust and wider in therizinosauroids than in
pachycephalosaurians (Maryanska, 1990), hypsilophodontians (Sues and
Norman, 1990), and are more slender than in sauropodomorphs (Galton,
1990a; McIntosh, 1990). Lateral deflection of the anterior blades
occur in therizinosauroids, sauropodans, and genusaurian ornithschians
among the Dinosauria, and elephantoids, megalonychids, and bradypodids
among the Mammalia; an opisthopubic pelvis is known for Ornithschia,
dromaeosauridae, and Aves among the Dinosauria (the pubis is turned
medially and vertically in mammals, but the gut is not confined by the
bone, so it is not analogous), all of whom an herbivorous diet is
known, suggesting that segnosaurs may have had such a diet. A similar
diet is cause for convergence in different animals, so the
opisthopubic and wide pelvis are considered herbivorous adaptations,
and may have no bearing on evolution; the fusion and/or occlusion of
the pubic and ischiadic elements at the obturator prong suggest that
great anteroposterior pressure was placed on the bones, such as that
of a gut increased in size and the major axis of the animal (the
dorsal vertebral column) was tilted into a vertical or diagonal
position, another feeding strategy employed by elephantoids and
pressumably in megalonychids and bradypodids. Ornithschians have an
opisthopubic pelvis, but generally have a longer ischium than pubis,
and all possess an obturator prong that does not occlude the pubis and
does, in fact, insert medial to the pubic shaft (thyreophorans lack
such a feature (Galton, 1990b; Gauthier, 1986; Sereno, 1986) ).

  The prescence of a pubic boot indicates theropodan relationship
(Gauthier, 1986), as no other group of dinosaurians possess the
feature, and support of the gut is not considered a purpose for the
development of the feature. Gastralia are reported for *Segnosaurus*
(Perle, 1979), which only occur otherwise in Saurischia.

<I'll believe segnosaurs are theropods and cheerfully admit grave
error when someone finally exhibits clear series of fossils showing
intermediate stages between [write your favorite theropod group here]
and segnosaurs. Good start: exhibit clear ornithomimosaur with
opisthopubic pelvis and/or functionally tetradactyl pes;>

  Why the pes? *Harpymimus* already has an ilium of similar form to
that of segnosaurs and a longer pubis peduncle than ischiadic peduncle
of the ilium; the form of the ischium is almost identical to that of
*Enigmosaurus* and *Segnosaurus*. The manual claws are laterally
compressed and not flat ventrally (Barsbold and Perle, 1984; Barsbold
and Osmolska, 1990). It could be a basal maniraptoriform, but I cannot
see it as an ornithomimosaur.

<or clear segnosaur with arctomet foot.>

  Why arctomet? if these were related to oviraptorosaurs, or
maniraptora, an arctomet pes would only place it near birds or
ornithos and tyrannos and troos.

  I can't wait to get my hands on the rest of this data, that of
segnosaurs and ornithomimosaurs and troodontids, which will help
considerably; but I do know that most of the characters linking segnos
and ornithoms are cranial, which cannot be discounted on a basis of
the pes and pelvis.

==
- Often, it is the man who is brought
  down the path to the end who does
  not see his own steps. -

Jaime A. Headden

Qilong, the website, at:
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